PHOTEC - O2web.telecom.cz/interconti/pdf/enthone/Photec Technical Process... · marketer of PHOTEC dry film resists ... the nomenclature “aqueous type resist ... The monomers may

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PHOTEC

Dry Film PhotoresistsTechnical Process Guide

Troubleshoot ing Guide

Enthone

www.cooksonelectronics.com

Issued: 11/02Supercedes: 10/992002 Enthone Inc.TPG-Photec: Europe

*Trademark used under license from Hitachi Chemical Co., Ltd.

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PHOTEC*Dry Film Photoresists

Table of Contents

Section Pages

Introduction

Technology

Substrate Preparation

Lamination

Exposure

Development

Etching

Resist Stripping

Questions and Anwers

Appendices

2-3

4-9

10-22

23-28

29-33

34-41

42-50

51-53

54-62

63-73

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Introduction

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INTRODUCTION

This technical process guide will provide detailed instruction in the use andapplication of PHOTEC* dry film photoresists, as well an overview on resisttechnology.

Historical BackgroundDry film photoresists (“resists”) were first patented in 1968 by Mr J Celeste ofDUPONT. Since that time they have been used extensively in the primaryimage formation on printed wiring boards.

At the time of introduction dry film resists were processed with solvents. It wasnot until the mid 1970’s that the aqueous (water-based) processing wasintroduced.

Hitachi Chemical creates the resists. Enthone delivers the results.

As noted by the table below, Hitachi Chemical Co., Ltd. has been a leader indry film resists for 25 years.

1974 First PHOTEC products released in Japan1975 First aqueous PHOTEC product released1985 PHOTEC released to Europe

In 1985, Enthone and Hitachi Chemical Company, Ltd., formed a strongpartnership. For fifteen years, Enthone has been the exclusive distributor andmarketer of PHOTEC dry film resists throughout Europe. Backed byunmatched technical support, PHOTEC dry film rolls are custom slit atEnthone’s ISO 14001 certified facility in The Netherlands.

The dry film resist as supplied consists of a three-layer construction. Theresist is supported on a polyester film and protected on the other side by apolyethylene-separating layer. This polyethylene-separating layer, which isremoved prior to use, is so that it can be rolled ready for application by thelaminator. Polyester is used as the support film for two reasons:

• It has good mechanical and thermal properties so that there is nodistortion or wrinkling when heat and tension is applied at thelamination stage.

• Polyester has low permeability to oxygen. If oxygen were to permeatethrough this protective layer to the resist in the unexposed state, freeradicals could be generated which would lead to polymerisation of theresist.

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Technology

• Acrylic Polymer

• Reactive Monomer

• Imaging Agent

• Photo sensitisers and Photo initiators

• Resist: General Formulation Principles

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TECHNOLOGY

A dry film photoresist is a complex mixture of components, each having a veryimportant role in the function of the resist. A typical dry film resist contains:

• Acrylic Polymer• Photo-reactive monomer• Photo sensitisers• Photo initiators• Imaging agent• Fillers and adhesion promoters• Plasticisers, etc.

Dry Film Resist Component Properties

Acrylic Oligomer (Monomer) • Resolution• Resist Profile• Sensitivity• Stripping (Size of stripped flake)

Photoinitiator • Resist Profile• Sensitivity• Resolution

Imaging Reagent • Latent Image (Contrast)

Additives • Adhesion• Resist stability

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Acrylic Polymer (Binder Polymer)

The polymer is the backbone of the resist and gives physical strength to theresist. The main chemical composition is a copolymer of acrylic or methacrylicesters with acrylic or methacrylic acid. The molecular weight of the polymer isa maximum of 250,000.

H H H H I I I I

- CH2 – C – CH2 – C – CH2 – C – CH2 – C – I I I I O CO CO CO I I I I OR OH OR OH n

R =Alkyl group n= Polymer chain length

The ratio COOH groups to the alkyl groups determines the ease ofdevelopment and stripping. It will also determine the chemical resistance andin particular the resistance to alkaline etchants.

The carboxylic groups within the polymer chain will react with either SodiumHydroxide (NaOH) or Potassium Hydroxide (KOH) to make the polymerwater-soluble. Hence, the nomenclature “aqueous type resist”.

Reactive Monomer (Oligomer)

The monomers may be a mixture of mono or polyfunctional acrylates. Themolecular weight of the monomer will affect the odour of the resist, the rate ofpolymerisation (i.e. the speed of exposure), and other physical properties ofthe resist.

These monomers are dispersed within the binder polymer mixture such thatwhen the reaction takes place it forms a highly interconnected threedimensional network of polymers. This then forms a very strong and flexibleresist.

After exposure up to 30-40 % of this monomer is left unpolymerised. In certaincases this may form stickiness of the resist after development and if thepanels are stacked on top of each other they may stick together.

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Imaging Agent

The blue or green colours are the most commonly used colours as theunaided eye judges these to provide the best contrast to the pinkish colour ofthe copper surface. Normally a triarylmethane dye such as Crystal Violet orMalachite green is used. These products are used in the resist formulation intheir “Leuco “ or white form.

The Leuco form oxidises when exposed to ultraviolet radiation to form acoloured compound and gives the printout image which is essential inassuring correct registration of the phototool to the drilled holes.

Dyes that provide a latent image when exposed are referred to as photo-chromic. These dyes must have a low absorption of light in the spectral rangeof sensitivity of the dry film in order not to reduce the radiation energyavailable for resist polymerisation. In some formulation of resists the dyesused work with a synergistic effect to help initiate the resist polymerisationthrough electron transfer mechanism. Dyes of this form include xanthene andacridinium compounds. The peak spectral absorbency of PHOTEC dry filmresist is 365 nano-metre.

Photo sensitisers and Photo Initiators

In order to polymerise the photoresist must contain some light sensitivemolecules that are able to convert the absorbed energy into a form thatenables the functional monomers to polymerise.

There are two basic types of photoreactive chemicals used in photoresists.

Photo sensitiser: The photo sensitiser absorbs the energy andtransfers it to another molecule, which forms the primary reactivecomponent. The photo sensitiser is not consumed or structurallyaltered and could be considered a photo catalyst.

Photo initiator: A photo initiator is the additive present whichfacilitates the initiation reaction. In the case of PHOTEC, the photoinitiator absorbs the radiation emitted by the high pressure mercurylamp or light in the wavelength of about 365 N.M. and then changes toform the reactive species - "free radicals". The photo initiator isconsumed in the polymerisation reaction.

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Mercury lamps were the first types of lamps to be used in exposure systems.The wavelength of 365 N.M. was originally used because it is the principalemission of the lamp. Molecules known to respond to radiation of thiswavelength are of many structures and numerous variations. However, themolecules in common commercial use are relatively few. This is becausesome of the important considerations for obtaining efficient photo initiation arewave length sensitivity, relative absorption coefficients, quantum yields, otherreacting species and any side reaction products that do not stoppolymerisation. Typical molecules that are used include:

4,4' Bis (Dimethylamino) bezophenone (Michler’s Ketone)

N- aryl - α - amino acids

Phenyl imidazolyl dimers

Benzophenone derivatives (no longer used)

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Resist: General Formulation Principles

Increase resolution by…

• Reducing the molecular weight of polymer.

• Increasing the number of carboxylic acid groups on the polymerchain.

Enhance resist adhesion and provide a wider operating window by…

• Increasing the glass transition temperature (Tg) of polymer.

• Increasing the cross-link density at polymerisation.

Increase conformance by…

• Decreasing molecular weight of polymer.

Low contamination in the electrolytic solutions and a smaller “resist foot,” isachieved by…

• Decreasing molecular weight of polymer.

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Substrate Preparation

• Drilling

• Deburring

• General

• Base Copper Laminate

• Electroless Copper

• Electro Deposited Copper

• Direct Metallisation

• Base Copper Laminate

• Electroless Copper

• Electro-deposited Copper

• Direct Metallisation

• Pretreatment Methods

• Abrasive Brush

• Brush Pressure, Brush ‘ Footprint ‘

• Surface Condition of Brushes

• Water Placement onto Brushes

• Pumice Brush

• Spray Pumice

• Chemical Clean

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SUBSTRATE PREPARATION

Prior to the final surface preparation and lamination, drilling and deburringmay impact the properties of the dry film resist.

DRILLING

Burrs or holes with jagged edges are caused by drilling and may effect thetenting property of the resist. Burrs are caused by three major factors:

• An incorrect drill speed/feed ratio. (The penetration of the drill into thebase material is too fast for the speed of rotation of the drill bit.)

• The drill bits are not sharp or have chips in the cutting edges.• Choice of the wrong type of support board on the exit side of the drill

stack.

DEBURRING

Deburring is an operation that is necessary when the drilling operation lackscomplete control.

Deburring is a mechanical operation. If not carefully controlled, deburring maycause deep scratches on the substrate’s surface and lead to resistconformance issues. The periphery of a drilled hole may be damaged giving adish down on one side of the hole that may lead to conformance or tentingbreakdown.

Substrate preparation is a critical stage prior to the lamination of thephotoresist. The photoresist requires a clean micro roughened surface with ahigh surface energy in order to obtain good adhesion. Unless the coppersurface is correctly prepared, there is a great potential for poor photoresistadhesion.

Mechanical and chemical bonding are two adhesion mechanisms that areimportant in obtaining the optimum properties of the resist. Immediately afterlamination the initial adhesion mechanism is mechanical; after about fiveminutes the resist starts to form a chemical bond.

PHOTEC resists experience a covalent chemical bond that is formed betweena hydrogen atom from the acrylate and the clean copper surface. There areresists on the market that contain either a sulphur or nitrogen compound toincrease the adhesive forces. In the case of sulphur compound compoundssuch as benzotriazole are used.

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To obtain the ideal adhesion values of the dry film resist to the substrate, athree dimensional, highly micro roughened surface must be generated duringthe pre-cleaning process. This surface allows for the maximum number ofanchorage points required for mechanical interlocking of the photoresist to thecopper substrate. The surface must have a high surface energy for the correctchemical bond to form between the resist and the substrate.

Checks should be made after pretreatment to ensure that the preparedsurface supports a film of water for at least 30 seconds. This is also known asthe “water break test,” as failure to hold the water on the surface indicates thatoils or grease remain and should be cleaned. Good water rinsing and dryingafter the pretreatment operation is essential. This will ensure completeremoval of any chemical, pumice or copper dust left on the surface and in theholes. If water remains in the holes during lamination this water tends to formsteam which can cause a pressure build-up in the hole resulting insubsequent tent failures.

The pH of the water used for rinsing should be controlled so that it is eitherneutral or very slightly acidic, pH 6-7. Dry film adhesion is affected by thesurface pH of the substrate. An alkaline surface will give rise to less adhesionforces and may result in lifting of the resist. A slightly acidic surface givesideal adhesion forces. If the substrate surface is too acidic then resist lock-inmay possibly occur.

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GENERAL

All panels should be free from grease, oil, fingerprints and uncontrolledoxidation. Panels should be handled using clean, lint-free gloves that arechanged regularly. (See Appendix 3 for specification of panel surface.)

There are basically four types of copper surfaces that are used in circuitformation.

• Base copper laminate• Electroless copper• Electro deposited copper• Direct metallisation

Base Copper Laminate

A base copper laminate is an electro deposited copper foil that has beendeposited without additives and therefore the crystal structure of the copper iscolumnar. The surface is protected from oxidation by an anti-tarnish, usually achromate layer.

Electroless Copper

Electroless copper is normally fine grained but is dependent on thecomplexing agent used in the chemical formulation of the deposition process.

Electro Deposited Copper

Electro deposited copper is either a “lock–in” plate of about 5 microns inthickness or a full deposit of about 25–30 microns. These deposits can beeither fine grained equiaxed or lamellar in structure dependent on thechemical composition of the deposition process and the equipment used forelectro deposition. Electro deposition processes are available to deposit fine-grained matte deposits which are ideal for direct lamination of dry film resists.

Direct Metallisation

The surface characteristics are dependent upon the direct metallisationprocess being used. There are several different types of direct metallisation,including:

• Conductive Polymer• Carbon or Graphite• Palladium Chloride• Palladium Sulphide

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BASE COPPER LAMINATE

Base copper laminate is used for innerlayers of multilayer and single-sidedboards or double-sided non-plated through boards. As delivered, the copperfoil is often protected by a chromate or zinc passivation to prevent copperoxidation. To obtain optimum adhesion this passivation film must becompletely removed.

Accepted pretreatment methods include:• Abrasive Brush• Pumice Brush• Spray Pumice• Acid clean• Chemical clean followed by a microetch or combined

cleaner/microetch.

Care should be taken since the thickness of the substrate may be as low as0,1mm. Any bending or creasing of the substrate will give rise to rejects dueto poor conformance of the dry film at the lamination stage.

The pretreatment method should also take into consideration the possiblestretching of the substrate under the influence of mechanical stressesproduced by abrasive brushing etc. Any stretching of the substrate will giveregistration problems at the exposure stage. (See Section on pretreatmentmethods for guidance on these processes.)

Electroless Copper

The type of electroless copper process will influence the adhesion of thephotoresist. It will also depend upon whether anti-tarnish is used after theelectroless copper.

In general, surfaces with EDTA-based electroless coppers are somewhatmore difficult to achieve good post-lamination adhesion than Quadrol-based(Ethylene dinitrilo-tetra-2 propanol) electroless coppers. This is due to thedifferent crystal structures obtained from the two different processes. TheEDTA-based process provides a finer grained, higher crystal densitystructure.

In many electroless copper processing lines an anti-tarnish stage is used toprevent unwanted oxidation of the copper. Alkaline anti-tarnish formulationshave a tendency to provide worse adhesion of the photoresist than thosebased on mineral or organic acids. This is due to the fact that resists adherebetter to a slightly acid surface than an alkaline surface, even after thoroughrinsing the surface pH may follow the pH of the last processing stage.Although dry film resists will adhere to an electroless copper surface it isnormal for these surfaces to be either pumice brushed or chemically treatedprior to resist lamination.

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ELECTRO DEPOSITED COPPER

If either full panel plating or “lock in“ plate is carried out prior to laminationthree problem areas have been identified.

• Uneven deposit (i.e. greater deposit thickness in the high currentdensity areas), provides unequal pressure across the board at thelamination stage, resulting in poor adhesion in the centre of the panel.

• Unless the boards are pre-treated effectively prior to lamination thesurfaces may be too smooth to obtain good initial, post laminationadhesion.

• Under exceptional circumstances the electrolytic copper deposit maycontain a high concentration of grain refining additives.

Grain refining additives contain sulphur and nitrogen compounds similar tothose used in certain dry film resists imparting adhesion to the substrate. Theelectrolytic copper additives may give rise to two side effects.

• The “high” concentration of additives may interfere with the resist-to-substrate adhesion mechanism.

• The additives may chemically bond with the dry film resist and increasethe adhesion to such an extent that stripping of the resist may becomea problem.

The pretreatment used for these panels must take the above points intoconsideration.

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DIRECT METALLISATION

Each type of direct metallisation requires different pretreatment methods.Carbon and graphite processes require that the carbon or graphite beremoved by an aggressive microetch and the surface protected by an organicanti-tarnish prior to resist lamination. Unless the carbon or graphite iscompletely removed, resist adhesion failures may occur. (Note: Carbonproducts are used as lubricants in other industries.) The anti-tarnish normallybased on benzotriazole is used not only to prevent oxidation of the copperafter the microetch but also to assist with adhesion of the photoresist.

As it’s final chemical stage in the metallisation sequence palladium sulphide-based processes include a microetch which removes the palladium sulphidefrom the copper surfaces. Therefore, it is essential that no additional etchingor mechanical methods are used in the pretreatment process prior tolamination. Additional microetching may cause “ring voids “at the final copperelectrodeposition stage.

Conductive Polymer

The surfaces that have been treated with direct metallisation using aconductive polymer, do not normally require more surface preparation otherthan a chemical clean prior to lamination. If, however, the surface is oxidised itmay be treated with a combined cleaner/microetch. The conductive polymer isonly adherent to the dielectric substrate and not the copper surfaces.Therefore, the copper surfaces may be microetched with no problems.

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PRETREATMENT METHODS

The four major methods used for surface preparation require differentequipment. Although each method provides the basic requirement for resistadhesion, each yields different surface topography that may affect the finalcircuit requirements.

Abrasive Brush

The abrasive brush is one of the oldest methods of cleaning copper prior toresist application. It is a simple method that uses a rotating abrasive brush tomechanically remove oxides from the copper surface. However, successfulpretreatment requires that the following control parameters be taken intoconsideration.

• Grade of abrasive used for the brush• Brush pressure and the resulting “brush footprint“• Surface condition of the brush• Placement of water spray onto the brush

Abrasive brushes are by definition brushes that remove surface imperfectionsand the base copper by mechanical means. The result is that the surface ofthe copper will be degraded to an extent that depends on the grade ofabrasive used. Abrasives used in the brush manufacture are normally basedon silicon carbide that has excellent abrading power and long life. To obtainthe surface finish required for resist application, the particle size of the siliconcarbide must be chosen carefully.

To achieve good resist conformance, a controlled surface topography isrequired. This enables excellent adhesion without problems of non-conformance.

The grades of brush recommended are a combination of brushes with twodifferent particle sizes. For the first stage it is recommended that a 320 gritsize is used to remove all oxides, surface imperfections etc., followed by a600 or 800 grit brush to smooth the surface to the final roughnessrecommendations.

By removing surface imperfections, a surface finish with controlledtopography is provided with a surface energy that enables the resist to haveinitially good mechanical adhesion. The resist may then form a chemical bondto the surface.

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Brush Pressure, Brush “Footprint”

The actual pressure applied to the brushes is dependent on the thickness ofthe panel and also the width of this panel. The pressure is normally controlledby a torque metre fitted to the motor that turns the brush. Actual pressureapplied is normally controlled by the “ brush footprint“, which is set at between10 and 15 mm. This brush footprint is the area of the brush that is in contactwith the surface of the panel at any time.

Surface Condition of Brushes

The brushes will wear with the number of panels processed and must beinspected for “evenness of wear”. It is recommended to position the panelsalternatively across the total length of the brush to ensure even wear over thebrush. If this is not carried out then uneven wear will result in a possiblepretreatment difference across the panel being processed.

Although copper is considered a relatively soft metal it becomes much harderas it is subjected to mechanical forces. Any copper particles that remain in thesurface of the brush will result in a deep scratch in the panel being processed.

Water Placement onto Brushes

During use the brushes are sprayed with water to both keep the temperatureconstant and to remove copper particles from the surface. It is essential thatthe water be sprayed onto the surface of the panel at the point where theactual brushing action takes place.

Abrasive Brush Pretreatment

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Pumice Brush

Pumice brush is a means of cleaning a copper surface by using a suspensionof pumice in water which is brushed onto the surface by means of a nylonbrush. If properly controlled, it does give a three-dimensional structure on thecopper surface that is suitable for resist application.

The “pumice brush stage” may be one of three different combinations.Specifically, there are three abrasive media that are used in combination witha nylon brush: normal pumice, white pumice, and an aluminium oxide that isused as a replacement to pumice. Normal pumice is a grey/cream colour andcomes in various particle sizes. Typically 3 ON grade are used but this canvary between ON and 6N.

Pumice is a natural ore product that is ground and sieved with different meshsizes to produce the different products. Due to the fact that it is a naturalproduct, the particles are of random shape and with different angularconfiguration. It must be noted that the grade of pumice is normally controlledby passing through different sieve sizes. Checks should be made todetermine the minimum particle size and concentration contained in theavailable product. A high concentration of small particles does not give therequired surface topography.

Pumice is relatively soft and during use the angular shapes are converted to arounded structure. Changes in angular structure impacts its effectiveness asan abrasive medium. The action of the brushes with the abrasive particlesremove any inorganic contamination from the copper surface and leaves thecopper in an active state with a surface that is ideal for resist lamination.

In practice pumice is used in 12–18 % weight/volume pumice-to-water ratio.This pumice concentration must be checked at least every working shift andadjusted if necessary to the correct concentration. Since copper is removedduring the brush/pumice operation, the copper concentration in the pumiceincreases with use. This copper concentration must be controlled and thepumice changed regularly to ensure constant quality. Commercial equipmentis available to reduce the copper particles in the pumice mixture.

The white pumice or “B“ grade is a harder material with a more consistentparticle size.

Pumice in a particle form may be trapped in through-holes. This entrappedpumice must be removed by either high pressure water rinsing or by the useof ultrasonics. It must be remembered that high-pressure water is onlyeffective on the surface of the panel, actual pressure of the water in anythrough-hole is much lower than the force on the surface.

If pumice brush is used it is essential to remove all particulate matter from thepanel surface prior to lamination otherwise conformance of the resist willresult. Poor resist conformance will give rise to either open or short circuitsdependent on which part of the circuit the pumice particle is trapped or thedifferent type of circuit produced.

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As with all mechanical methods, only inorganic contaminants will be removedfrom the copper surface. The method will not remove oils or grease from thesurface.

Aluminium oxide is an ideal replacement for pumice. It is harder with a moreangular particle. Being harder it has a longer working life. The majordrawback, however, is that aluminium oxide is more expensive and does givemore wear on the equipment. Spray nozzles must be changed to allow thismaterial to be used. The same guidelines presented for pumice should benoted for aluminium oxide.

Pumice Brush Pretreatment

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Spray Pumice

Spray pumice a common method used in the PWB industry. The methodemploys pumice that is sprayed under pressure onto the copper surface. Themechanical action of the pumice under pressure removes all inorganicimpurities and also copper from the surface. Nozzles must be checked toensure even application of the pumice over the entire surface otherwise resistadhesion may become a problem. All guidelines presented for thepumice/brush operation should be followed for the spray pumice.

Chemical Clean

Provided that the chemistry is well controlled, the chemical cleaning method isthe preferred method of cleaning the copper surface prior to resist application.This method eliminates the possibility of particulate matter on the surfacewhich may become entrapped under the laminated resist.

There are two different chemical cleaning options:

• A two-stage process (cleaner, followed by microetch)• A combined cleaner/microetch

Either option will achieve a clean micro-roughened surface.

The thickness of copper removed during the etching process must becontrolled for consistent performance. Although the actual amount of copperremoved during processing is dependent on the process chemistry, it isnormal to remove about 0,6–1,0 micron of copper. Technical data suppliedfrom the process supplier must be observed for optimum surface topography.

The temperature of the process and the dissolved copper content determinesthe actual crystal structure of the copper surface. As the copper concentrationincreases in the microetch, the crystal structure changes from an angularstructure to a more polished rounded crystal structure. It is the angular form ofthe crystal that is required for optimum resist adhesion. Good water rinsing isrequired to remove all chemical products from the surface before drying andresist application.

In practice there are three different oxidising chemicals used for the microetch

• Hydrogen peroxide• Sodium or potassium persulphate• Sodium or potassium monopersulphate

Hydrogen peroxide is the least expensive material and has the highest weightper volume of oxidising power, however unless carefully controlled it does notproduce the ideal crystal structure. Various additives are available to controlthe stability of the hydrogen peroxide and each may have different effects onthe crystal structure produced.

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Normal persulphate is the most commonly used material for the microetchformulation. However, monopersulphate is preferred for the ideal surfacetopography.

Chemical Clean Pretreatment

Both spray pumice and cleaner/microetch preparation prior to resistapplication produces a homogeneous three-dimensional microroughendsurface. For high density fine line circuitry a properly controlled chemicalcleaning method is the preferred method of pretreatment since it doeseliminate the possibility of particulate matter from the copper surface.

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Lamination

• Preheat Temperature

• Rollers

• LaminatingTemperature

• Pressure

• Speed

• Hold Time after Lamination

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Lamination

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LAMINATION

Lamination is the application of the dry film resist to a properly preparedsubstrate.

The lamination process must be carefully controlled to ensure that therequired mechanical adhesion of the resist to the substrate is obtained byflowing the resist into the surface irregularities. The resist should not flow toomuch into any drilled holes or slots. Over flowing of the resist will causethinning along the periphery of holes and result in tent breakage. Therefore, acorrect balance of all the lamination parameters is crucial to ensure optimumperformance of the resist.

There are different types of proprietary laminators on the market ranging frommanual to fully automatic and from hot rollers being heated by resistanceheaters within the roller to indirect heating of the rollers by infrared. In allcases the recommendations of the manufacturer must be followed. The basicprinciple of the operation is to preheat the resist to a temperature of 110 +/-100C to lower the viscosity of the resist just prior to application, underpressure, to the substrate.

A normal sequence of operation is: -• Preheat substrate (40-500C)• Heat photoresist (110 +/- 100C)• Apply resist by roller pressure to the substrate. Typically a

pressure of 2-4 Kgf/cm2 is applied.• Lamination speed 1,0-3,0 meters per minute

Although preheating the substrate is not essential, it does ensure that the coldsubstrate does not act as a heat sink and thus reduce the actual temperatureof resist at the lamination stage. If it were to do so it would affect the adhesionand conformance of the resist to the substrate.

There must be a correct balance of lamination temperature, pressure speedand resist tension to ensure that maximum resist adhesion and tenting abilityfrom the photoresist is obtained.

During lamination the resist is heated on one side and is laminated onto thecooler substrate surface, thus a temperature gradient exists through theresist. The lower molecular weight and lower boiling point fraction of thechemicals within the resist will migrate to the cooler surfaces. A hold time afterlamination prior to further processing is to ensure that mobile chemicalsequilibrate with the higher molecular weight fraction.

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Preheat Temperature

The actual preheat temperature required is dependent upon both thethickness of the dielectric and the copper on the surface. As the thickness ofeither or both increase, the greater the thermal heat sink. In order to obtain asurface temperature of 40–500C, the preheat temperature must be adjustedaccordingly.

If the preheat temperature is too over 550C, wrinkles may occur duringlamination. When using an automatic cut sheet laminator, wrinkling isparticularly predominant at the edges of the panel. Thinning of the resist at theperiphery of holes or slots can occur if lamination speed is too low or thelamination pressure too high.

If the preheat temperature is too low poor resist adhesion immediately afterlamination will result. Resist conformity to the substrate, especially in deepand narrow areas, will be imperfect at best.

Rollers

The condition of the rubber on the rollers is important to ensure a constantpressure over the entire panel. Any imperfections in or on the surface willappear as defects on the laminated resist. A cut or a piece of rubber removedfrom the roller will give a lower pressure at that spot during lamination and willin severe cases show up as a blister on the surface of the laminated resist.

The hardness of the rubber should be about 65 Shore hardness. If it is harderthan this then the resist will be pushed into the holes or slots on the panel. Ifthe hardness is too soft poor conformance will result. The thickness of rubberon the rollers should be as recommended by the equipment manufacturer.

Care should be taken when re-coating any rollers since the removal of therubber is normally mechanical and at this stage the steel shaft is reduced indiameter. To obtain the required outside diameter of the roller, a thickerrubber will be applied. Rubber is a poor conductor of heat and therefore, theactual transfer of heat from the heating elements inside the roller will be lessthan normal.

During lamination of a batch of boards the roller may not be able to maintainthe correct lamination temperature. On most laminators the heating isaccomplished by a resistance heater located within the steel core of the roller.To ensure that the heat is transferred from this element to the actual roller aheat transfer gel is used. Unless this gel is evenly coated around the heatingelement, irregular heat transfer will occur along the length of the core andaround the diameter. All rollers should be checked whenever they arechanged and also on a regular basis. If there are major temperaturedifferences on the roller defects will occur. The two heated rollers should bechecked periodically to ensure that they are parallel with each other.

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If pressure is applied by air at both ends of the rollers and narrow widthpanels are laminated frequently, a “bow“ may form on the rollers. If thiscondition exists, pressure applied to the centre of the panel will be less thanthat applied to the centre. In severe cases there is a possibility that pooradhesion of the resist to the substrate will result.

Laminating Temperature

During lamination the resist must be heated above its glass transitiontemperature to make it semi-liquid and hence be in a state to be pressed intosubstrate defects by roller pressure. Although the glass transition temperatureis about 350C, the resist must be heated to a higher temperature to accountfor cool air and the heat sink effect of the substrate. Heat must pass throughthe polyester support film prior to heating the resist. It has been shown thatthe roller temperature, measured by a contact temperature-measuring probe,should be 105-110 0C to provide optimum flow characteristics of the resist. Onmany laminators the hot rollers are heated by resistance elements in the coreof the roller. If the contact gel used to transmit the heat from these elements(firstly to the steel core and the rubber coating) is not operative, the heatingwill be much slower and the rollers may not return to the set point betweenlamination of subsequent panels.

The temperature indicated on the read out on the laminator is via a contactprobe situated at one end of the roller. If this contact point is dirty or loose itwill lead to an incorrect read out temperature.

Pressure

The dry film resist is heated to about 1100 C so that it becomes semi-liquidand will flow under the influence of pressure. The pressure that is applied tothe rollers is to ensure that the dry film resist is forced into the microroughness and surface defects that are present on the copper surface.Unless the pressure is sufficient to enable this action to take place, the resistwill not have the necessary physical properties to withstand subsequentprocessing.

If insufficient pressure is applied, poor conformance of the resist to thesubstrate irregularities will result. Development, etching or electroplatingchemicals will penetrate under the resist resulting in rejects being produced. Ifthe applied pressure is too high, the resist will be forced into any holes thatrequire tenting and the tent strength will not be sufficient to withstandsubsequent processing.

Pressure must be sufficiently high to enable the resist to flow into the macroroughness of the panel formed by the different thickness of glass fibres usedin the construction of the dielectric substrate. A normal pressure range of 3-5bars is used depending on the type of laminator.

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Speed

The resist requires a finite time to flow into the irregularities of the substrateunder the influence of temperature and pressure. The actual speed oflamination determines the time of applied pressure.

The lamination speed is adjusted to give optimum conformance and at thesame time the productivity that is required. Lamination speed is 1,0 to 3,0metres per minute.

The overall lamination parameters are a balance of temperature, pressure,speed and substrate thickness. Substrate thickness is important since this willimpact the amount of heat required to obtain the correct temperature on thepanel.

If the speed is too high then the resist will not have sufficient time to flow intosurface irregularities and hence poor conformance. If the speed is too low andall other parameters are at optimum, the resist will flow into tented holescausing tenting failures.

Hold Time After Lamination

After lamination the temperature of the laminated substrate must cool to roomtemperature as quickly as possible.

• The resist is made semi-liquid at the laminating stage and as such willflow into surface irregularities. Once laminated the resist must be cooledquickly to prevent continual flow into holes that require tenting. If theresist continues to flow into the hole the resist thickness around theperiphery of the hole will thin and may not have sufficient mechanicalstrength to withstand subsequent processing. A tent failure will result.

• The resist must stabilise prior to subsequent processing. The photosensitisers and photo initiators move during the time that the resist isexposed to ultraviolet light and continue to move for a period after thelight is switched off. The polymerisation of the resist will stop once themolecules are at rest. The higher the temperature of the resist, thefurther the molecules will travel. This is why the resist should be cooledas quickly as possible.

• The minimum hold time after lamination is the time taken for the resist tocool to room temperature.

• The maximum hold time after lamination is normally four days. However,to hold the boards for an extended time the panels should be covered inblack opaque plastic. Yellow light does contain an element of visual lightthat dry film resists are sensitive. In the worse case, polymerisation willoccur and prevent development of the resist.

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Exposure

• Equipment

• Lamp Type

• Illumination Intensity

• Exposure Energy

• Phototool Quality

• Degree of Collimation

• Vacuum Delay

• Exposure Unit Temperature

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EXPOSURE

The exposure process is the polymerisation of the oligomers/monomers in theresist chemistry. Polymerisation is accomplished by exposing the resist to aknown amount of ultra violet radiation.

The following sections detail eight critical areas that should be taken intoconsideration to ensure successful exposure.

Equipment

There are several different types of exposure units used in the industry whichrange from single-sided exposure manual printers to double-sided exposurefully automated types. Different types of light reflectors are used with varyingefficiency of light transfer. The height of these reflectors above or below thepanel to be exposed will determine the light energy reaching the panel. Thiswill directly impact the rate of exposure. The quality of the reflectors willdetermine the ENERGY DISTRIBUTION across the exposure frame.

Light will not be reflected evenly across the frame leading to resist exposuredifference across the imaged panel. On some older manual exposure unitsthe difference in light energy across the exposure frame can be as high as40%. This does mean that we can have a difference of up to one step on a 21Step tablet across the frame.

Lamp Type

PHOTEC photoresists have a peak spectral absorption of about 360manometers. Therefore, the lamp used must have a peak spectral output atthe same wavelength. If there is a mismatch between output of the lamp andthe absorption characteristics of the resist there may be incorrect exposure,even if the required number of millijoules is applied. In general, the type oflamp used is a high pressure mercury lamp. To obtain the requiredwavelength output the mercury is normally “doped” with small quantities ofiron.

Lamps should be changed regularly as the efficiency of the lamp changeswith age. Regulating the current that is applied to the lamp controls the lightoutput. In addition, spectral output may also change. It is common practice tochange the lamps after 1000 hours of use. A meter is normally incorporatedin the machine to record the time that the lamp is used.

Some lamps are cooled with a water jacket around the lamp. This coolingdoes prevent heat build up within the exposure machine but it also reducesthe amount of energy transmitted. In severe cases this may increase thelength of exposure time to reach the desired number of millijoules.

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Illumination Intensity

A short, high illumination intensity is preferred to a longer, low intensityillumination. The optical density of the resist changes during exposure. Sincethe resists are of the surface polymerisable type, the surface layer of theresist will consume energy, resulting in reduced energy passing to the lowerlevels within the resist. Therefore, unless there is sufficient energy intensity,most of the energy will be consumed as it passes through the resist and theresist at the copper interface will not be exposed sufficiently.

During normal exposure we will see an exposure difference between theresist on the surface and that at the base of the resist. The difference inexposure can be two steps on a 21 Step Density tablet. Two steps are areduction of 50% of the light energy.

Exposure intensity is measured in milliwatts.

The actual time of exposure in seconds is millijoules. milliwatts Millijoules = milliwatts x time

Exposure Energy

Each resist requires a certain amount of energy to reach the optimumexposure state. The amount of exposure will determine the actual chemicalproperties of the resist. In all cases the technical data sheet of the resistshould be followed.

If the exposure energy is too low, then the resist will be attacked by both thedeveloping solution and the subsequent processing chemicals. The adhesionof the resist can be affected leading to open circuits after innerlayer etchingand short circuits on electroplated circuits.

If the resist is exposed with too high an exposure energy, the image on thephototool will not be transferred in a 1:1 ratio. This means that the exposedtraces will be wider than on the phototool.

Phototool Quality

Phototool quality is extremely important for high circuit density products.

Image density ( Dmax ) should be in excess of Dmax 3,5. Edge definition of theopaque areas should be sharp and well defined. Diffused image edges canlead to variation in track widths after development or incorrectly exposededges which may cause problems in the development, electroplating oretching processes.

Background clarity of the clear areas of the phototools is essential to ensureshort exposure times. A high background density may increase exposuretimes by up to 100%. If not corrected to compensate for high backgrounddensity, poor resolution and resist side wall profile may occur.

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The use of phototool emulsion protection systems extends the life of thephototool. When setting exposure times the thickness of the protective coating(normally 3-5 microns) must be considered to ensure accurate line widthreproduction. This thickness increases the off-contact distance from the resistand thus makes the beam collimation of the exposure unit more important. Ifthe exposure unit has poor beam collimation, the use of a phototool protectivesystem will be dependent upon the permitted deviation from the normal 1:1reproduction of the phototool.

Degree of Collimation

To obtain accurate phototool reproduction, the exposing energy (light) whichis hitting the panel at right angels to the resist is required. This light should beevenly distributed across the exposure frame.

Collimated light is defined as light that is very close (90 degrees) to the panelsurface. Normally the angle of declination is about 0,5 degrees. It must benoted that the higher the degree of collimation, the more dirt and scratcheswill have on the phototool. These defects will be reproduced on the exposedpanel.

There are several factors that can influence the angle at which the light hitsthe panel being exposed. The light emitted by the exposure lamp passesthrough several different layers before it actually reaches the photoresistThese include the glass or polyester vacuum frame, phototool, phototoolprotective layer and the polyester cover sheet on the resist. All of these layerscan give diffraction of the light.

Any air trapped between any of the layers will also cause light scatter. This airgap can lead to “off contact“ printing. The actual print out image will bediffused and lead to rejects.

Vacuum Delay

A vacuum delay is necessary to allow time for all the air between the variouslayers to be removed. We need the closest contact possible between thephototool and the photo-resist, the vacuum delay allows the time for the air tobe removed and for the phototool to pull down onto the resist surface. Atypical vacuum delay is about 10 seconds but for high circuit density boards orhigh-resolution boards then the vacuum delay is increased up to 30 secondsin severe cases.

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Exposure Unit Temperature

Heat is a key factor during exposure. Polymerisation reactions started by ultraviolet light can be continued by heat energy and continue after the lightenergy has been stopped. This may result in over exposure of the resist evenif the number of millijoules applied is correct. The actual degree of overexposure caused by the heat in the exposure unit will depend on the actualtemperature and the inhibitors used in the formulation of the resist.

The exposure unit should have sufficient airflow within the unit to remove theheat emitted by the lamp and to maintain the temperature within the unit at ornear room temperature.

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Development

• pH concentration

• Resist Loading

• Temperature

• Development Time (breakpoint)

• Spray Nozzles

• Antifoam

• Filtration

• Water Rinsing

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Development

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DEVELOPMENT

Development is the removal of unexposed portions of the negative workingresist. The development stage is critical as it determines the quality of theresist remaining on the surface in terms of track profile, adhesion, etc.

As circuit density increases, the track width becomes smaller and moreclosely packed. As such, the development process becomes more important.On exposure the resist is polymerised and this alters the dissolution kineticsbetween the exposed and unexposed resist.

The development mechanism is a diffusion controlled process, that is thedeveloping solution penetrates the unexposed resist and partially removes theresist in the form of a colloid of binder polymer carboxylate salts. This layermust then be removed by mass transfer of developing solution on the resistsurface and mechanical action by spray pressure before the next layer can beattacked.

Eight key factors must be taken into consideration to achieve the correctdevelopment action, cleanliness of the developed surface and optimum trackprofile.

• pH concentration of the developing solution

• Resist loading within the developing solution

• Temperature of the developing solution

• Development Time (breakpoint): removal of the resist

• Spray nozzles (type, volume of solution, spray impact)

• Antifoam type and quantity

• Filtration

• Water rinsing (time and temperature)

The above factors demonstrate how the mechanical aspects of thedevelopment machine and the chemical control of the process are extremelyimportant. Equipment design is very important and often the dry film resistsupplier has to use equipment that is already installed. However, the resistsupplier must ensure the following points so that resist performance is notjeopardised.

• The spray from the nozzles is effective and even over the entire boardsurface. Development is even on the upper and lower board surfaces.

• The spay impact pressure is sufficiently high to remove unexposed resistfrom within fine lines and spaces and yet is not too high to break tents,particularly those over oblong holes.

• There is no puddling on the upper surfaces of the panel.

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• If more than one chamber is incorporated in the development part of themachine, the space between the chambers should be clean and theconveyor mechanism should remain wet. This can be achieved by using“misting “nozzles.

• Rinsing is an integral part of the development mechanism. Ideally, therinsing time should be 50 % of the development time.

• Drying reduces swelling of the resist that occurs in the development stage.If wet resist passes directly to a cupric chloride etching solution, increasedorganic contamination may occur in the etchant.

The development mechanism is a diffusion controlled reaction, The sodiumcarbonate reacts with the carboxylic (acid) radicals in the unexposed resistand solubilises the resist. This carbonate resist mixture must be removed withfresh sodium carbonate before the reaction may proceed.

Dry film resist is not dissolved in the developing solution, but is held insuspension in a colloid form. Any mechanism by which this colloid is brokendown can lead to scum formation on the panel surface. It may also appear asan “oily“ substance on the solution surface. These mechanisms includemechanical stress created by sheer forces in the pumps, spray nozzles, anti-foams, dry film resist load in the development solution, etc.

If the colloid is destroyed then a scum will deposit in the developing solutionand an oily substance may be seen floating on the surface of the developingsolution.

COOH COONa

COOH + Na2CO3 COONa

COOH COONa

Carboxylated Polymer Sodium Salt (Colloid)

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pH Concentration

The concentration of sodium carbonate used for development must be withinthe range specified for each dry film resist. If the concentration is too low theunexposed resist will not be completely removed and a scum remaining onthe surface will lead to etching or electroplating problems. If the concentrationis too high the resist will be attacked at the resist substrate interface leading topoor track profile, and in severe cases, lifting of the resist. This in turn will leadto electroplating deposits under the resist or track width reduction duringetching.

The dry film resist has acid radicals within its chemical structure. As the resistreacts with the sodium carbonate, the acid radical is neutralised and the pH ofthe developing solution will fall. If the pH of the developing solution falls belowa specified pH, the developing mechanism will cease. Therefore, it is essentialto replenish the developing solution by either pH control, conductivity, or areaof resists developed etc.

Resist Loading

The quantity of dry film resist within the developing solution is defined asresist loading. As resist loading increases, the dissolution kinetics is changedand therefore the speed of development changes. There is also a greatertendency for the colloid resist particles to become unstable and precipitateback onto the panel. This leads to open circuits in the case of pattern platingand short circuits in the case of innerlayer production or circuits produced bythe tent and etch technique.

The specific recommendation for resist loading will depend on the type ofcircuit being produced. Circuits with lines and spaces greater than 150microns a loading of 0,4 square metres of 40 micron thick resist per litre ofdeveloping solution is recommended. As the lines and spaces becomesmaller, the resist loading becomes correspondingly lower. For lines andspaces of 100 microns and lower, the resist loading should be less than 0,1square metres per litre of developing solution. (See Appendix 1 for method ofanalysis for resist loading in developing solutions.)

The dry film resist has acid radicals within the chemical structure. As the resistis dissolved in the sodium carbonate solution this acid radical is neutralisedand the pH of the developing solution will fall. A 10-gpl Sodium Carbonatesolution will have a pH of about 11,2.

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Temperature

Most dry film resist specifications require that the temperature of thedeveloping solution be controlled within close limits. Normally this is theoptimum temperature plus or minus two degrees Celsius.

Thermostatically controlled heaters are required to control the temperature. Ifthe action of the pump mechanism generates heat the developing solutiontemperature will rise and could exceed that set on the heater thermostat. Inthis case cooling coils are also necessary to remove this excess heat. Ifpersistent problems arise always check the actual temperature of the solutionwith a thermometer; never solely rely on the temperature indicated on thedevelopment machine.

When the developing solution temperature is too high the speed ofdevelopment is greater and will lead to over development unless the totaldevelopment time is reduced. It will also give rise to a poor resist side wallprofile. A high temperature may lead to breakdown of the colloid and give riseto scum on the panel surface. If the temperature of the developing solution istoo low then under development can occur and leave resist residues on thepanel surface and also give a larger “ foot “ to the resist profile.

Development Time (Breakpoint)

The minimum development time is the time taken for the resist to be removedfrom the copper surface. Actual time in seconds will depend on a number offactors including concentration and temperature of the developing solution,volume of solution being sprayed onto the surface, type of nozzle, height ofthe nozzle above the surface, etc.

For PHOTEC dry film photoresists, a breakpoint is required when the resist is50-60% developed through the total effective spray length of the developingmachine. Note: The effective spray length may not be the overall length ofthe developing machine. It is essential to inspect the machine and check theactual length where the developing solution is sprayed onto the panel.

If the breakpoint is less than 50% of the total effective spray length overdevelopment will occur. This in turn will lead to a poor resist sidewall profileand undercut at the resist to copper interface. This results in under plating inthe case of pattern plating or over etching when innerlayers are produced orthe tent and etch technique is used.

If the breakpoint is greater than 60% then under development will occur.Under development will lead to track width reduction in the case of patternplating and in severe cases will lead to plating inhibition or plating adhesionfailures.

In the case of etching innerlayers under development will lead to a track widthincrease. In severe cases this may also lead to organic contamination in theetching solution. Organic contamination in the etching solution will give smallareas of copper not etched on the surface (copper spots).

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Spray Nozzles

Development is a diffusion-controlled mechanism. This means that thedeveloping solution must diffuse through the unexposed resist and throughthe interaction of the sodium ion in the sodium carbonate solution react withthe carboxylic acid groups within the resist to form a colloidal particle. Thesecolloidal particles are then washed from the resist surface by fresh developingsolution. As the colloids are removed it leaves the surface in a condition formore sodium carbonate to diffuse into the resist surface. This mechanism isrepeated until all the unexposed resist is removed.

Because resist removal is a diffusion controlled mechanism, a large volume oflow pressure sodium carbonate to reach the resist surface is required duringthe first stage of development. To achieve, spray nozzles are used in the firstpart of the development machine that spray a cone shaped pattern onto thesurface (i.e. cone nozzles). These cone nozzles give a low pressure on theresist surface. It is recommended that each nozzle deliver in excess of 4 litresper minute of developing solution. (A typical cone shaped nozzle will have aspray impact of 0,4-0,5 Bar on the panel surface.)

To ensure the complete removal of resist at the base of fine lines and spacesa higher pressure is required so that the developing solution will reach thebase of resist at the resist to copper interface. In fluid dynamic terms this is adeep recess. A nozzle that delivers a fan shaped spray pattern is used forthis purpose.

The nozzle with a fan shaped spray is referred to as a high impact nozzle andwill have a typical spray impact of 8-9 Bar on the panel surface. Equipmentdesign is important to achieve the correct development of the resist across theentire panel. Nozzle height and the angle of the nozzle jet must be such thatall the entire panel is sprayed at equal volume and pressure and that nooverlapping of the sprayed solution occurs. Any overlapping of the sprays willreduce the effective pressure that impinges onto the surface. Prevention ofdevelopment solution “puddling” on the top of the panel must be prevented,otherwise this will affect the replenishment of fresh solution on the resistsurface.

To even out the differences in solution replenishment on the top and lowerpanel surfaces different spray pressures are used between top and bottom.Normally a 0,2 - 0,3 Bar higher pressure on the top spray bar is used. Typicalspray pressures for development are Top 1,5 Bar and Lower 1,3 Bar.However, the exact spray pressure is equipment related and should bearrived at experimentally.

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Antifoam

The type and quantity of any antifoam used in the development solution willdetermine the stability of the colloid formed. If the incorrect antifoam is usedprecipitation within the development solution will occur. This may lead to scumformation on the panel surface that will interfere with the subsequent platingor etching operations. To eliminate these problems, an antifoam that is apolyethylene oxide or polypropylene oxide block copolymer based should beused.

Siloxane based antifoams are known to cause instability of the colloids in thedevelopment solution and give rise to scum or sludge. Silicone basedantifoams should not be used as they may provide problems in subsequentprocessing operations such as electroplating. Trials should be made with allnew antifoams to check for colloid stability prior to use on production.

The concentration of antifoam used should be minimised to limit the foambuild-up in the development solution. Actual concentration used should bewithin the range recommended by the supplier.

Filtration

To prevent the possibility of resists particles or large colloidal particles beingre-deposited back onto the panel it is essential to filter the solution. The filtershould be in line between the pump and the spray bar manifold and equippedwith filter to remove particles greater than 30 microns. The filters should bechanged regularly to prevent loss of developer solution volume being suppliedto the spray nozzles. Pressure indicators should be fitted into the spraymanifold after the filters; this will give a more accurate indication of the spraypressure.

Water Rinsing

Water rinsing after development is an integral part of the process. The type,volume and temperature of the water are important to ensure dry film resistperformance. Water with a hardness of 8 - 12 on the DIN scale should beused. (See Appendix 4 for water hardness conversion.)

The reason for using slightly hard water for rinsing is that the divalentcautions. Calcium and magnesium converts the soluble sodium form of thepolymers present on the resist sidewalls after development into less solublecarboxylate salts. This effectively stops further development while improvingboth resolution and resist sidewall profile.

Soft water or softened water is not an effective rinsing medium. The pH of softwater is easily affected by drag-in of developer solution and in severecircumstances may continue the development action.

The pH of the rinse water should be less than pH 9,0 and the temperature ofthis rinse water should be less than 30 0C to stop development action andprevent attack on the resist surface.

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Etching

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Photoresists are employed as an etch resist in the print and etch technologyfor the production of innerlayers or in the "tent and etch" method of final circuitproduction. The etchant used in these technologies is primarily cupric chlorideor ferric chloride. Following the etching process defects can remain on thesubstrate in the form of copper spots or fine shorts.

This section details the chemistry involved with the etching process and someof the reasons for defects. In addition, means of preventing these defects willbe provided. It is essential that care be taken to identify the cause of theproblem to enable the corrective action to be taken so that the problem maybe eliminated.

Factors Affecting Copper Etching

• Substrate and Substrate Preparation• Dry film photoresist exposure and development• Cupric chloride etching solution

Substrate and Substrate Preparation

The innerlayer copper foil or the electroplated copper used in the tent andetch process must be checked for the defects listed below.

• Epoxy resin through pinholes in the copper foil during lamination of thesubstrate

• Copper nodules on the plated via hole boards or tent and etch panels• Excessive “shiny” spots on the surface• Heavy oxidation of the copper surface• Handling defects such as scratches, dents or fingerprints• Incomplete removal of any anti-tarnish coating

The standard practice of using 18 micron copper foil for innerlayers and theincreasing use of 9 micron copper has highlighted the problems with pores inthese thin copper foils.

Epoxy resin

During the substrate manufacture the epoxy resin of the dielectric flowsthrough pin holes forming circular spots of epoxide resin on the surface.These epoxide resin spots are not removed during normal surface preparationof the copper prior to resist application. These spots may change from circularin appearance to an elongated form after any brushing operation. In generalresin spots should not exceed one spot per square metre.

Corrective Action: Incoming inspection should be able to detect this problemwithout difficulty.

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Copper Nodules

The increasing requirement for buried via holes and panel plated copper fortent and etch technology has necessitated drilling and metallisation (oftendirect metallisation) followed by electrolytic copper plating. This three stepsequence may result in both nodules and gas pits on the surface prior tolamination. Copper nodules may result in copper remaining on the etchedsurface. Both copper nodules and gas pits may result in poor conformance ofthe resist at the lamination stage and allow the development or etchingchemistry to penetrate beneath the resist and lead to open circuits.

Corrective Action: Inspect the panels after electroplating prior to imaging. Themost probable causes of nodules after electroplating include:

• Thick electroless deposits in localised areas are caused by insufficientcontrol of the pre-activation and activation stages prior to electrolesscopper deposition. These nodules appear after electroless copper andproduce localised high current density areas in the electrolytic copperstage producing higher than average copper deposit thickness.

• Particulate material in the electrolytic copper electrolyte is co-depositedwith the copper. These particles may arise from edges of the panelsparticularly if an excessive desmear operation has been carried out onan in-line processing sequence.

Gas pits may be caused by insufficient control of the additive system in theelectrolytic copper electrolyte. Localised over concentration of these additivesmay result in passivation which reduces the cathode efficiency of thedeposition process and result in hydrogen evolution and a localised reductionin copper thickness.

Excessive "Shiny" Spots

The excessive shiny spots on the copper foil, as received from the laminatesupplier, are generally caused by the glass cloth used for the construction ofthe dielectric. A high localised thickness where the weft and the warp of theglass fibres cross may lead to shiny spots (depending on the diameter of theglass fibre used in the construction) being formed after pressing the copperfoil onto the dielectric. The excessive pressure on these high spots during thepressing operation to produce the laminate changes the structure andhardness of the copper in these localised areas. Any differential hardness orstructural differences in the copper cause changes in etch rates, often slowingthe etch rate such that reduced copper thickness spots are left on the surfaceafter etching.

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Corrective Action

• After laminate preparation, check to see if pumice, pumice brush,abrasive brush or chemical pretreatment have removed these spots.

• Check sprays or brush pressure. Increase pressure if necessary.

• Ensure that the pumice concentration is correct; increase pumiceconcentration to 20 volume percent.

• Ensure pumice suspension is acidic in nature.

• Change pumice more frequently.

• Check pumice grade being used.

• If equipment is suitable, consider use of aluminium oxide asreplacement of pumice.

• If chemical pretreatment is used, increase the thickness of copperetched to 1,0 micron.

Heavy surface oxidation

Heavy surface oxidation should be confined to those panels that have beenwet processed prior to resist lamination. If the copper oxide is not removed itmay cause a "lock in" of some resists.

Corrective Action

• Check to ensure that the rinse water after electro-plating is clean andthat the panels are rinsed sufficiently.

• Increase the temperature and airflow volume during the dryingoperation.

• Ensure that any through holes are dry before stacking the panels

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Anti-tarnish

The anti-tarnish coating on base laminate as received from the laminatesupplier is normally of the chromate type. Excessive chromate conversioncoating may not be removed during cleaning and could either lead to a "lockin" of some resists or may etch slower than surrounding copper.

Chromate layers are normally removed using an acidic medium. Alkalinepumice spray or pumice brush operations may lead to incomplete removal ofthe chromate layer.

Corrective Action

• Ensure that the pumice suspension is acidic in nature.• Change pumice more frequently as a build-up of chromate may occur

in the pumice suspension. If electrolytic methods are used to removethe chromate layer, ensure that the correct anodic voltage is applied forthe correct time

Dry Film Photoresist Exposure and Development

It is not generally recognised that the combination of exposure anddevelopment may provide problems during the etching operation.

As part of the resist matrix, a dry film resist comprises oligomers, sensitisersand photoinitiators that during exposure to ultra violet radiation combinetogether to form polymers and stable reaction bi-products. Insufficientexposure will lead to an excessive quantity of these chemicals remaining onand in the resist matrix. These unused chemicals react in the acidic media ofthe etchant to produce an oily product which, if not removed, will float on thesurface of the etchant.

Underdevelopment of the resist leaves a scum residue on the copper surfacesthat will not be completely removed by rinsing and will prevent etching of thecopper surfaces. Underdevelopment also leaves a larger than normal "foot" atthe base of the resist track profile. This "foot" will be undercut during etchingand fall into the etchant. This again may lead to organic contamination in theetchant.

Over development provides an unstable sidewall to the resist that is attackedby the cupric chloride etchant. This will leach out photoinitiators, oligomersand imaging agents from the resist, which unless removed by carbontreatment, will provide an oily residue in the etchant.

It is recommended that the dry film resist content of the developing solution ismaintained below 0,2 m2 / litre. If the resist content is higher there is a dangerthat the resist is re-deposited in a particulate form which is very adherent tothe copper surfaces and difficult to remove by water rinsing and preventsetching. Any solid particulate material, especially sodium carbonate, willpenetrate into the unexposed resist and prevent development of theunderlying resist.

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Rinsing is an integral part of development. Rinse stations should be at least50% of the development stations.

Development should be maintained at 1,3 - 1,7 times that of the time forminimum development breakpoint.

Corrective Action

• Check that the lamination temperature is not too high and that the hotrollers have a good thermal profile. Hot spots on the hot roller may leadto heat polymerisation of some resists. Panels should be cooled asquickly as possible to room temperature and not stacked until thiscooling process has taken place.

• Check for correct exposure level. In the case of PHOTEC resists,ensure that the Step Tablet exposure is maintained between Step 7-9.

• Ensure that the phototools are of good quality with no scratches orpinholes in the opaque areas. The phototool should have opaque areasof sufficient density to prevent partial exposure of the resist in areasthat should not be exposed. Scratches or "touch-up" marks in the clearareas of the phototool will lead to partially polymerised resist that maybe removed during development and re-deposit back on to the panel ata later stage of development.

• The acuity (the change between the clear areas and the opaque areasof the phototool) should be a sharp boundary. If there is a gradualchange between clear and opaque areas this will lead to partialexposure along the edges of these tracks and development problems.In severe cases this may increase organic levels in the etchant.

• Check for correct developer concentration. Follow the resistmanufacturer recommendations for concentration and developersolution temperature.

• Ensure that there is no particulate matter in the development solutionby using a filter of 10-30 micron filter size

• Check for the resist loading in the developing solution. (See Appendix1 for the analytical method.)

• Ensure that the breakpoint of the development is correct in order toobtain a stable sidewall of the resist

• Rinse water temperature should be between 12-27 0C and rinse watertime should be at least 50% of the development time

• If necessary, increase the hardness of the rinse water to 8 - 12 dH.

• Reduce scum formation by using water containing less than 10mgm/litre of divalent cautions for make up and replenishment.

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• Reduce the amount of antifoam and ensure that it is a compatible typewith the resist being developed. The higher the concentration ofantifoam, the higher the formation of scum and the chance of residuesremaining on the panel after development.

• Clean all transport rollers to prevent any particulate matter pressinginto the undeveloped resist.

• Ensure that all spray nozzles in the development section are notblocked or worn. Worn, blocked or partially blocked nozzles may causepoor or underdevelopment.

• Ensure that as the polyester foil is removed no flakes of this materialare formed that will be attracted to the resist surface by electrostaticelectricity.

• Check the resist after development and drying to see if the resist iscorrectly developed and rinsed. Over development or rinsing with toosoft water can leave a soft resist that can be attacked by the etchant.After development and drying, place the panel in a tank of water forone minute. Take out the panel and hold it vertically. The water shouldimmediately run from the resist (i.e., it should immediately de- wet).The resist should have a gloss appearance without any matte areas

• Ensure that the resist is thoroughly dried after development andrinsing. The resist swells slightly during development and will shrink assoon as it comes in contact with an acidic medium. Heating the resistat this stage stabilises the sidewall and shrinks back the resist to itscorrect size and prevents leaching into the etching solution.

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ETCHING

The primary etchant that is used to produce innerlayers and boards by thetent and etch process is a non-proprietary solution of cupric chloride. Thisetching solution contains cupric chloride and hydrochloric acid. The etch rateand the etch factor is determined by the concentrations of these twochemicals in solution.

The basic chemistry of the etching mechanism is:

CuCl2 + Cu0 Cu2Cl2

Cu2Cl2 + 4 HCl 2CuCl32- + 4H+

During this reaction there is a build-up of the insoluble Cu2Cl2 (cuprouschloride). Insoluble Cu2Cl2 precipitates onto the surfaces preventing etching atthese points. Further reactions take place with the oxidant to oxidise thecuprous to cupric chloride.

With hydrogen peroxide each gram of copper etched requires 75 ml of 100volume hydrogen peroxide and 1 ml of 35% hydrochloric acid.

Cu2Cl2 + 2HCl + H2O2 2 CuCl2 + H2O

With chlorine:

2 CuCl32- + Cl2 2CuCl2 + 4 Cl-

It is normal for these reactions to be controlled by the use of an Oxidation-Reduction Potential control system (ORP). However, the oxidant added tocontrol the copper chloride etching solution is not only consumed by oxidisingthe copper from the cuprous to the cupric state.

Organic material dissolved from the dry film resist is also oxidised and thetotal reaction consumes a higher amount of oxidant. It has been shown by x-ray mass analysis that the oily residues present on the surface of the cupricchloride etchant is a mixture of oligomers, photoinitiators and imaging agent. Ithas been determined that the oxidising agent in the etchant affects allaqueous developable photoresists. It has also been determined byexperiment that it is essential to control the Oxidation Reduction Potentialclosely as the range of eliminating or obtaining copper spots on an etchedpanel is within a range of 20mV.

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Control of Etching Solution

• Maintain the oxidation-reduction potential at 540 +/- 40 mV. Below500mV the incidence of copper spots increases considerably.

• Continuously carbon treat the cupric chloride etchant to removeorganic material.

• Remove any sludge formed on the surface of the etching solution atleast once per week.

• Clean once per week the conveyor and squeegee rollers, especiallyrollers at the entrance of the etching machine.

• The "oily" layer on the surface of the etching solution may be reducedby heating the developed boards to a temperature of 40-60 0C prior toetching.

• The oil-adsorbing mat (polypropylene-type) in the etching machineshould be maintained regularly.

• If pumped through the spray nozzles, the oily layer forms an emulsionthat is strongly adherent to copper surfaces.

• Increase the addition rate of the oxidant (hydrogen peroxide or chlorinegas) for organic material.

• Ensure that the filters on the etching machine are maintained regularly.Filters capable of removing 10-30 micron particles are recommended.

• Ensure that the total etch time is at least 1,3 times the minimumetching time.

• Cuprous chloride concentration increases as the oxidation reductionpotential decreases. At 540 mV the concentration is about 1 gm /l andat 450 mV it rises to about 20 gm /l.

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Resist Stripping

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RESIST STRIPPING

The objective of resist stripping is to remove the resist from the copper panel(including fine lines and spaces), while ensuring a non-oxidised surface.

Most alkaline type dry film resists do not dissolve in the stripping solution butare detached from the copper surface in small flakes. This extends theworking life of the stripping solution.

Stripping solutions are normally alkaline metal hydroxides, such as sodium orpotassium hydroxide, or based on amines such as mono or tri ethanolamineand tetra methyl ammonium hydroxide.

The stripping solution should break the polymer chain at the cross-linkingpoint of the three dimensional structure, which is formed during thepolymerisation of the resist and before the bond between the resist and thecopper surface is broken. The stripping mechanism depends not only on thecross-link density of the resist but also the number of carboxylic acid groupson the polymer chain. Therefore, the type and concentration of strippingsolution should be optimised for each resist and these must be set to allowthe stripping solution to have time to penetrate the resist and break thepolymer chain before the resist-to-copper bond is broken.

If stripping trials are conducted in the laboratory prior to production it must benoted that the resist characteristics do change during the electrolyticdeposition of copper and tin-lead or tin. This means that any stripping trialsshould be conducted on panels that have been through the deposition cycleused on production.

The trials are performed not only to optimise the stripping time and flake size,but also to set the concentration of stripping solution that minimises theswelling of the resist in the solution. To obtain complete stripping of the resistfrom within fine lines and spaces the resist must be removed before it swellsand is trapped by mechanical forces within these fine traces.

The stripped flake size for any resist depends on four major factors:

• The type of stripping solution• The concentration of the stripping solution• The temperature of the stripping solution• Design of the stripping equipment

When using any stripping solution the stripping time and stripped flake size isa balance of concentration and temperature. The higher the concentrationresults in a faster stripping time but with a larger flake size. Conversely, alower concentration will give smaller flake size but the stripping time is muchlonger. Increasing the temperature will reduce both the stripped flake size andthe stripping time.

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Stripping is a diffusion-controlled mechanism. A high volume and spraypressure through each spray nozzle is required. Typically, a minimum flow of4 litres per minute at a pressure in excess of 1,5 Bar is recommended. Whenalkaline metal hydroxide solutions are used for stripping electroplated boards,a pressure as high as 6-10 Bar is often used.

An ideal equipment configuration is for the spray jets in each stripping moduleto be angled at 30 degrees in the four axes of the board. This will ensure thatthe resist is stripped between fine tracks. Where there is a possibility ofoverhang of plated metals onto the resist surface, particularly in high currentdensity areas, it is beneficial to have a flooded stripping cell as the finalstripping section. With PHOTEC resists it is helpful if the plating overhang isless than 7-8 microns onto the surface of the resist.

Antifoam may be necessary to prevent foaming of the solution. Any antifoamused must be in a minimum concentration and compatible with the strippingsolution being used. The antifoam used in the development solution willprobably not be suitable for the stripping solution. The total alkalinity of thestripping solution determines which type of antifoam is suitable.

To prevent the dry film from being dissolved in the stripping solution, it isnecessary to filter the solution to remove the stripped flake of resist. Thisfiltration will extend the working lifetime of the stripping solution. There aremany different types of filtration methods, including external or internal beltfilters or angled screen types. PHOTEC resists have been demonstrated towork with all of these. If an angled screen is used as the filtration method, theactual angle of this mesh may have to be adjusted to obtain the best results.

For optimum stripping it is necessary to replenish the stripping solution. Thismay be achieved by either analytical or by “bleed and feed“ methods. In allcases, the area of PHOTEC resist stripped should not exceed 0,5 squaremetres per litre of stripping solution. The following example demonstrates theeffect of stripping solution concentration on stripping time and stripped flakesize.

Dry film resists thickness: 50 micronStripping solution: Sodium Hydroxide

Temperature: 550CAgitation: Zero

Concentration (%) Complete removal of resistTime (sec.)

StrippedFlake Size

0,5 360 ~2 x 10 mm*1,0 150 ~10 x 20 mm*1,5 100 Large sheet**2,0 70 Large sheet**2,5 60 Large sheet**

*Self releasing from copper **Requires agitation to release from copper

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Questions and Answers

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COMMONLY ASKED QUESTIONS

Where is PHOTEC manufactured? Where is it slit?

PHOTEC is currently manufactured in Japan and Malaysia. It is producedwith Japanese products and slit on machines that are specified and run byHitachi Chemical engineers.

Enthone-OMI has been the exclusive distributor and marketer of PHOTECdry film resists throughout Europe. Backed by unmatched technicalsupport, PHOTEC dry film rolls are custom slit at Enthone’s ISO 14001certified facility in The Netherlands.

How is consistent quality and supply assured?

All manufacturing is carried out under the same exacting conditions. Theproducts used during manufacture are identical and quality control is thesame.

There is always 6-8 weeks of “Jumbo rolls “ in stock at Enthone Benelux.This stock is based on our forecast for sales over a period of 5-6 weeksand the delivery time from the manufacturing site.

What is a Jumbo Roll?

A jumbo roll is a master roll that is produced during manufacture. This rollis typically 1,5 - 1,6 metres wide and with a length of up to 2 kilometres.

How do we know which type of PHOTEC to stock?

The type of PHOTEC inventoried is based on customer current andforecasted buying patterns.

What cutting widths are available?

Enthone slitting facility is capable of cutting widths according to customerrequirements within the range 145mm to 610mm with an accuracy of +/-1mm.

What are the standard core sizes?

The core size relates to the internal diameter of the core. 3 inch and 5 inchcore sizes are available.

What is meant by a 6-inch core size?

The 6-inch core was originally quoted when large diameter paper coreswere first introduced. It was a standard core for electrical coils that utilisedthe outside diameter of the roll. The paper cores had a wall thickness of0,5 inches; the inner diameter was the normal 5-inch.

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What class of clean room is recommended?

Class 10,000 clean room is required for today’s PWB lines and spaces(i.e. 75 micron). The air quality should be checked regularly and all cleanroom procedures must observed.

How should rolls be stored?

To prevent deterioration of the dry film resist, reduction in photosensitivity,edge fusion, etc. the rolls should be stored between 5 - 20 OC with arelative humidity of 40 - 60 %.

If the temperature is lower than 5 0C water may condense on the rollsleading to softening of the resist and the potential for the defects,including:

• Increased adhesion of the film at the lamination stage resulting inresist lock-in, as well as plating or stripping problems.

• Thinning of the resist (especially around holes) leading to tentfailures and other defects.

If the temperature is higher than 200C for any length of time edge fusionand deterioration of the photosensitive properties may result.

Note: When the temperature is reduced, the relative humidity in the air willINCREASE.

What is the thickness of the polyethylene and polyester (Mylar) films?

The polyethylene has a thickness of 25 microns.The polyester thickness for the standard resists is 20 microns.

What is the optimum cleaning cycle prior to lamination?

The optimum cleaning cycle imparts the correct surface topography andcleanliness to achieve the best resist adhesion values. This may beachieved by chemical or a combination of chemicals and abrasivetechniques.

What is the maximum unevenness of the copper surface?

PHOTEC resists have good conformance properties. However, for 40 or50 micron thick resists, the macro roughness of the glass weave within thesubstrate should not exceed 9 microns.

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What conformance (conformability) may be achieved with PHOTECresists?

PHOTEC dry film resists provides excellent conformance propertiesversus competitive resists. When laminated properly, PHOTEC resists willconform to substrate defects of up to 9 microns when using a 40-micronthick resist. In the case of a scratch defect, the width of the scratch shouldbe less than 100 microns.

Note: A resist is a viscous liquid when laminated. Fluid dynamics will notallow it to flow into deep narrow cuts.

What surface roughness is recommended for optimum adhesion of theresist to the substrate?

The main purpose of a micro-roughened surface is to provide key lockingpoints for mechanical adhesion of the resist. Surface roughnessrecommendations include:

Ra 0,2 – 0,4 micronsRmax 2,5 – 3,0 microns

Ra is the average distance of the roughness profile from the centre line ofthe surface.Rmax is the deepest individual roughness depth over the total measuredlength.

To ensure sufficient anchor points for the resist, a minimum of 25 peaksper 100 microns is recommended.

What is the resist adhesion mechanism to the copper substrate?

Immediately after lamination the adhesion mechanism is purelymechanical. After a period of ~5 minutes a chemical bond forms betweenthe resist and the copper substrate. This chemical bond is a key factor indetermining the resistance to under plating or undercut during etching.

Is there any adhesion promoters in the PHOTEC chemical constituents?

Unlike other resists, PHOTEC does not contain any sulphur-containingcompounds within the chemical formulation. PHOTEC does contain asmall amount of nitrogen, which is in the imaging agent formulation.

How does PHOTEC achieve good adhesion values?

The main adhesion forces are achieved by the covalent chemical bondformed between the hydrogen atom in the acrylate molecule and thecopper substrate.

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How long may panels be held between lamination – exposure –development?

Typically, panels may be held 3-4 days between lamination and exposure.However PHOTEC may be held up to 7 days if they are covered with nontransparent black polyethylene and kept stored at a temperature of lessthan 20 0C and a relative humidity of 40-60%.

Between exposure and development ideally they should be developedwithin 24 hours but can be held up to 3 – 4 days if kept covered with blackpolyethylene and stored in controlled conditions.

NOTEPHOTEC in common with all dry film photoresists are unable to bedeveloped if exposed to a light intensity of 3 – 5 mJ/cm2. The yellow lightsused in the process areas do have an element of light in the visible lightwavelength and all dry films are sensitive to visible light.

What happens if panels are stored longer than recommended or outsidethe recommended storage conditions?

Lock-in of the resist to the substrate and incomplete development mayoccur leading to plating or etching defects.

What wavelength of light is most suitable to expose PHOTEC resists?

PHOTEC has a peak spectral absorption at 360 nanometres. Each type ofPHOTEC has a slightly different spectral response.

Checks of light output using a radiometer are not entirely accurate sinceeach radiometer is calibrated for a slightly different light wavelength. Tocontrol exposure it is ideal to set the exposure level using a graduatedgrey scale.

What is the relationship between exposure energy and time ofexposure?

Exposure Energy (E) = Light Intensity (I) x Time (T)

E = I x T

mJ/cm2 = mW/ cm2 x t

What happens to the exposure time when the exposure lamps get older?

As the lamps age with use, two effects can be observed.

• The lamp experiences a reduction in power. The electrical currentsetting on the lamp must be increased to maintain the same lightoutput. Unless this is carried out, the exposure time becomes longer.Frequent checks must be made to ensure that the exposure energyapplied to the resist is constant.

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• There is a shift in the peak spectral output from the lamp. This meansthat there is a spectral mismatch between the peak spectralabsorbance of the resist and the peak spectral output of the lamp.Once again, a longer exposure time or a reduction in the quality of theside wall profile of the resist is observed. This results in lowerproductivity or reduction in quality.

What is the minimum hold time between exposure and development?

The resist is exposed by free radical polymerisation and the molecules inthe resist are disturbed by the action of the ultra violet light radiation.Unless the exposure machine is well ventilated to remove the heat emittedby the lamps (not all the electrical energy supplied to the lamp is emittedas light), a part of the energy is transformed into heat. This heat will againdisturb the molecules within the resist.

The time between exposure and development must allow for themolecules in the resist to normalise. The photoinitiators may still migratewhen the resist is above room temperature and for a short period of timeafter the exposure energy is switched off.

Time between the exposure and development is the time taken for theresist to reach room temperature. If development is conducted before theresist attains room temperature, the side wall profile of the resist will not beoptimum.

What happens if the polyester support film is removed too long afterexposure and before development?

If the polyester is removed prior to or during exposure the oxygen in theatmosphere may react with free radicals and form non-reactive products.The polymerisation of the resist is accomplished by free radicals and ifthey are converted to a non-reactive form the polymerisation will bestopped.

What is the minimum exposure level that will prevent development of theresist?

Most dry film resists only require an exposure level of 3-5 mJ/cm2 toprevent development. This amount of energy can arise on the resist if theresist is held too long under yellow light conditions within the exposureroom. Yellow lights have a small element of white light within the outputspectrum that many resists, including PHOTEC resists, are sensitive.

What is the adhesion value of the polyester support foil to the resist?

A nominal value for all types of PHOTEC resists is 20 grams per twocentimetre wide strip.

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Is a clean room required for lamination and exposure?

The objective of lamination and exposure is to reproduce a reliable imageon the copper surface that is on the phototool. Dirt and dust will cause aproblem in giving either short or open circuits. For normal 125 micron linesand spaces, an air conditioned room of Class 10,000 is suitable. As thelines and spaces become closer together, the standard of the clean roommust improve. For 50 or 75 micron lines and spaces, the clean roomshould be Class 1000.

Will I get the same exposure energy on the resist surface as shown onthe top of the exposure frame?

No. There are interposed films between the light source and the dry filmresist surface.

• Glass or polyester exposure frame covers sheet• Phototool• Phototool protective coating• Polyester cover sheet of the resist

All of these films reduce the light passing through. In some cases theactual amount of light reaching the resist surface is only 50% of thatmeasured on top of the exposure frame.

How do you determine the actual light loss and obtain the correctexposure?

With a glass exposure frame it is not possible to place a U.V radiometerbetween the two glass plates.

By placing the radiometer on top of the exposure frame, check therelationship between the applied number of milijoules per squarecentimetre and the step obtained on the graduated grey scale. Once thishas been established, set the number of milijoules applied by measuringthe milijoules every working shift.

How should the phototool and the exposure frame be cleaned?

Dependent on the class of circuit being produced the number of cyclesbetween cleaning is variable.

Generally, for lines and spaces of 150 microns and above the phototool iscleaned with either solvent or sticky roller after every 25 exposures. Forlines and spaces of <150 microns it is good practice to clean after every 5or 10 exposures. After each cleaning the phototool should be inspectedwith a magnifying glass, to check for fibre or particulate matter.

It is good working practice to install an ionization blower above each printmachine.

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How often should the phototool be changed?

It is good practice to use new phototools for each batch of circuits.However, it is normal to change the phototool after every 250 exposures.The action of placing the substrate on the phototool will lead to scratches ifit not covered with a protective coating.

You should also change the phototool if the light intensity passing thoughthe tool drops by a value of 10% versus the light intensity on a new tool.

What is a latent print out image?

The latent print out image is the formation of an image of the phototoolonto the PHOTEC surface. A print out image is formed by a change in thestructure of the imaging agent incorporated within the resist.

In the case of PHOTEC resists, the image is photochromic. This meansthat on exposure to light the resist changes to a darker colour. The actualcolour change depends on the type of imaging agent and the degree ofexposure. A higher level of exposure will give more contrast between theexposed and unexposed resist.

What is the optimum exposure level for PHOTEC?

Refer to the technical data sheet for the type of PHOTEC resist beingused. In general, it is normal to expose the resist to a grey scale ofbetween Step 7 and 9. After development, the step is determined whenthe PHOTEC resists appears on the panel.

What is the Minimum Development Time (MDT)?

The minimum development time is the time taken to remove theunexposed resist on the copper surface. The minimum development timeis also called the "“breakpoint “ or “ wash off point“. This time is reportedas a percent of the usable development machine or a percent of thenumber of spray bars in the machine.

The Total Development Time is the minimum development time multipliedby a factor of 1,3 to 1,7.

Total Development Time = Minimum Development Time x 1,3 to 1,7.

Is there an easy way to check the break point prior to lamination?

Using a water-soluble marking pen, draw several lines on the cleanedpanel in the direction that the panel will be developed. Laminate overthese markings.

During development, observe at which point the marked lines are removedfrom the panel. The actual resist will be removed one spray bar before themarkings are removed.

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What is the shape and size of the “foot“ on the PHOTEC resist profile?

At the interface of the resist to the copper surface the resist will protrudeonto the copper surface. This protrusion is what is known as the “foot“.The foot should not be greater than 2-3 microns long.

Is there an easy way to determine if post development rinsing issatisfactory?

After the panel has been developed, rinsed and dried, take the panel andimmerse it in the rinse water for one minute. Remove the panel and drainvertically. If the panel completely de-wets the rinsing is effective. If thepanel remains wet, development residues are present and rinsing hasbeen ineffective.

Another way to determine satisfactory rinsing is to check the hardness ofthe resist with a fingernail. After soaking the dried panel in the rinse waterfor one minute, scrape the surface with the fingernail. If the resistscratches or is removed slightly it is possible that rinsing is not effectiveand the rinse water should be changed.

Why is drying necessary after post development rinsing and prior tocupric chloride etching?

During post development rinsing, the resist swells slightly. Drying shrinksthe resist back to a normal state and reduces the organic contamination inthe etching solution.

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Appendices

Appendix Title

1 Analytical method to determine the concentrationof dry film resist in developing solutions

2 Exposure

3 Asian/European Specification

4 Water Hardness

5 Hitachi/Strouffer/Riston Step Tablet

6 Line width variation between artwork andfinal etched panel

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APPENDIX 1

Analytical Method to Determine the Concentration of Dry FilmResists in Developing Solutions

General

An aqueous dry film resist contains within its chemical structure a number ofcarboxylic acid groups (R – COOH). The number of these groups is specificfor each type of resist. In a development solution these carboxylic acid groupshave reacted with the alkali metal carbonate (Na2CO3 or K2CO3) to form acarboxylate group [i.e. R – COONa)].

The analytical method used determines the alkali metal carbonateconcentration and continues further to determine the dry film resistconcentration.

Three reactions must be taken into consideration:

Na2CO3 + HCl NaHCO3 + NaCl

NaHCO3 + HCl NaCl + H2O +CO2

R – COONa + HCl NaCl + R – COOH

Method

1. Mix the developing solution thoroughly.

2. With a safety pipette take a 5ml sample of the developing solution andplace in an Erlenmeyer flask.

3. Add 30 ml of deionised / demineralised water.

4. Add a few drops of 1% phenolphthalein indicator solution to theErlenmeyer flask. ( Colour of the solution will change to a red violet )

5. Using a magnetic stirrer and rotator agitate the solution evenly.

6. Titrate the solution with a 0,1 N Hydrochloric acid solution slowly until thecolour of the solution changes from red violet to the colour of the originalsolution.

7. Note the volume of 0,1N Hydrochloric acid used = A ml

8. Continue with the same solution.

9. Add a few drops of 1% Methyl Orange indicator solution. The colour of thesolution will change to an orange colour. (continued on next page)

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10. Titrate slowly with 0,1N Hydrochloric acid until the colour changes to pink.

11. Note the volume of 0,1N Hydrochloric acid used. = B ml

Note: The first titration determines the concentration of Sodium Carbonate

= A ml 0,1N Hydrochloric acid

Na2CO3 + HCl NaHCO3 + HCl

The second titration determines the Sodium Bicarbonate concentration plusthe Carboxylate concentration

= B ml. 0,1N Hydrochloric acid

NaHCO3 + HCl NaCl + H2O + CO2 = A’ ml 0,1 N HCLR- COONa + HCl R- COONa = NaCl = B’ ml 0,1 N HCl

Calculations

Mls 0,1 N HCl for volume A is equivalent to mls 0,1 N HCl for volume A’

Therefore we can calculate the mls 0,1N HCl equivalent to the carboxylate Concentration B’

B’ = B – A

1. Na2CO3 % = 0,1 x 106 x Aml x100 Sample volume 5ml x 1000

2. Dry film resist = Film calculation factor x 0,1 x B’ ml concentration 5

Note:

0,1 = 0,1 N HCl If a different Normality is used a conversion factor must be applied

106 = Molecular weight of Na2CO3 . If Potassium Carbonate is used for developing then the factor 138,2 is used (138,2 are Molecular weight of Potassium Carbonate)

FILM CALCULATION FACTOR : 7,25 (mean value)

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APPENDIX 2

EXPOSURE

To calculate the correct exposure time from a known exposure time and theresulting step obtained the following equation or table may be used.

This calculation only applies to a 21 Step Density Tablet with a density rangeof 0,05 – 3,05 and a ∆ D = 0,15.

Exposure time for the desired step (T): (Desired Step – Reference Step) = Reference time (TR) √ 2 ( S ) (SR )

( S – SR) T = TR √ 2

Step Obtained by Exposure Multiple to Obtain Step 8

1 1,1132 8,003 5,664 4,005 2,836 2,007 1,418 1,009 0,7110 0,5011 0,3512 0,2513 0,1814 0,13

Example:

To find the correct exposure time when Step 5 is obtained in X seconds toreach a desired Step 8.

Multiply X by 2,83

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APPENDIX 3

Asian/European Specification

Based on customer requirements both in Asia and in Europe, the followingsurface specification is applied for production of 100-micron lines and spaces.(The specification applies to each surface of a 500 mm x 500-mm panel.)

Item Specification

Macro roughness (glass weave roughness) < 4 micron

Nodules Zero

Scratches < 1 ( depth < 5 micron) ( length < 100 micron)

Pin Holes < 1 < 50 micron Diameter 0>50 micron Diameter

Indentations all indentations must be < 4 microns Deep

Diameter ( mm ) Number < 0,25 No Spec. 0,25 – 0,5 < 50 0,5 – 0,75 < 25 0,75 – 1,00 < 4 > 1,00 0

Panel Edge Smooth.

Periphery of tented hole No burrs or dishdowns

Surface roughness R max 2,5 – 3,0 microns R a 0,24 – 0,4 microns Peaks and valleys 20 – 26 per 100 microns

Moisture in through hole None

Water stains None

Water Break test Water should remain on the surface for minimum 30 seconds

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APPENDIX 4

Water Hardness

Water hardness is measured in milligram equivalent per litre of calcium ormagnesium ions.

1 milligram equivalent is equal to 20,04 mg/l Calcium ion 12,16 mg/l Magnesium ion

1 degree on the DIN scale is equal to 0,357-milligram equivalent per litre.This is equal to 7,15 mg/l Calcium ion 4,34 mg/l Magnesium ion

1 degree on the DIN scale is equivalent to: 1,786 degrees French Hardness 1,237 degrees English Hardness

If the water hardness of the immediate rinse after development is too low, acalculated addition of a 200 gm per litre solution of anhydrous magnesiumsulphate is added.

An addition of 1,075 ml for every 10 litres of rinse water will raise thehardness by 1 degree DIN.

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APPENDIX 5

Hitachi/Stouffer/Riston Step Tablet

HitachiStouffer

Step

41 tablet41 tabletDensity

HitachiStouffer

Step

21 tablet21 tabletDensity

RistonRistonStep

1725

TabletTablet

Density

123456789

1011121314151617181920212223242526272829303132333435363738394041

0.000.050.100.150.200.250.300.350.400.450.500.550.600.650.700.750.800.850.900.951.001.051.101.151.201.251.301.351.401.451.501.551.601.651.701.751.801.851.901.952.00

1

2

3

4

5

6

7

8

9

10

11

12

13

1415161718192021

0.05

0.20

0.35

0.50

0.65

0.80

0.95

1.10

1.25

1.40

1.55

1.70

1.85

2.002.152.302.452.602.752.903.05

1234567891011121314151617

12345678910111213141516171819202122232425

0.050.550.600.650.700.750.800.850.900.951.001.051.101.151.201.251.301.351.401.451.501.551.601.651.70

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APPENDIX 6

Line Width Variation Between Artwork and Final Etched Panel

Pretreatment Prior to Lamination

The copper surface prior to lamination has chemicals that will cause a strong“lock-in” adhesion of the resist.

Chemicals include:

• Copper oxides Cu2O, CuO• Sulphur compounds from anti-tarnish formulation• Nitrogen compounds

Corrective Action: Ensure surfaces are thoroughly clean and dry prior tolamination.

Lamination

Too high a temperature causes some heat polymerisation of the PHOTECpolymer.

Corrective Action

Ensure that the surface temperature of the panels immediately afterlamination is between 40-60 oC. Adjust hot roller temperature and/or speed oflamination in order to achieve this.

Cooling After Lamination

Boards remain at elevated temperature for a prolonged time after lamination.Stacking the boards may create a large thermal mass, causing the boards tocool slowly.

Corrective Action

To achieve a rapid cooling rate after lamination, rack panels one centimetre apart. Use refrigerated air equipment to cool panel.

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Extended Time Between Lamination and Exposure

The chemical bond between the resist and copper will increase with time. Thiswill interfere with the development mechanism.

Panels are held too long without cover.

Corrective Action

Ensure panels are exposed as quickly as possible after lamination. If panelsare held more than 24 hours between lamination and exposure cover panelswith black polyethylene film.

Dry film photoresists are sensitive to a white light wavelength (“yellow” lightswill emit this light as well). Dry film photoresists will not develop if exposure tolight energy is greater than 2,8 mJ/cm2 and becomes more difficult if exposureis 0-2,8 mJ/cm2.

Exposure

If the exposure temperature is too high, migration of the photoinitiators alongthe path of the exposed areas will result. This may lead to partial exposure ofthe resist. Due to refractive index, off contact exposure will cause increases inline width.

Corrective Action

Ensure that the exposure equipment interior cooling system is switched onand is effective. Panel temperature during exposure should not be higher than30 oC.

Ensure that phototools are in direct contact with the photoresist.

Ensure that any protective film on the phototool does not have a refractiveindex that will cause light scattering.

Cooling After Exposure

If panels are warm after exposure the migration of photoinitiators will continuealong the path of the exposed areas. This will lead to exposure of the resistoutside the opaque areas of the phototool.

Corrective Action

Rapid cooling of the panels after exposure is required. Ensure that thepolyester protective foil is not removed for a prolonged period prior todevelopment.

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Development

The development mechanism is a diffusion controlled process. The pressurewithin each spray bar of the nozzle should be a minimum of 1,5 Bar. Thepressure applied to the surface of the panel will depend upon:

• Equipment design• Type and condition of pumps• Height of nozzle above panel surface etc.

Overall correct development conditions are essential in order to reproduceetched tracks equal to artwork.

Corrective Action

Check:

• Actual development solution temperature against indicatedtemperature on the control module

• Resist loading in the development solution to ensure it is below 0,2m2/litre

• Nozzle condition. Replace worn or blocked nozzles

• Speed and rotation of pump

• Solution volume output from pump. Replace worn impeller and/orimpeller housing

Water Rinsing After Development

Rinsing after development is an integral part of the development mechanism.Inadequate rinsing may leave resist alongside the exposed resist, which inturn will result in under etching.

The volume of water, spray pressure, temperature and water hardness are all-important factors. Recommended water rinsing time should be at least 50% ofthe development time. Too “soft” water will not stop the development actionand may lead to poor rinsing of development resist.

Corrective Action

Check:

• Water spray pressure is greater than 1,2 Bar

• Water temperature is between 12 and 25 oC

• Hardness of water is 8-12 DIN

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Drying After Development

If panels are stacked after development and prior to etching, ensure thatpanels are completely dry and cooled to room temperature prior to stacking.

Etching

The etching stage may produce incorrect line width dimensions if theparameters are not controlled correctly.

Corrective Action

Ensure that the correct chemical composition of the etchant is attained.

Ensure that the Oxidation-Reduction Potential is 540-600mV.

General

Ensure that the photoresist is used on the ‘First-in – First out’ basis and that itis stored under the correct environmental conditions, i.e. temperature 5-20 oC,relative humidity 40-60%.

Documents

PHOTEC - O2web.telecom.cz/interconti/pdf/enthone/Photec Technical Process... · marketer of PHOTEC dry film resists ... the nomenclature “aqueous type resist ... The monomers may