Pcb Manufacture

8/8/2019 Pcb Manufacture

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Printed Circuit Boards


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PRINTED CIRCUIT BOARDS

Format of Course

A series of 10 one hour long lectures / tutorials

Delivered by

Dr Gary J Callon

Dr Graham Berry


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Course Content

History of PCB Development

The rapid advancements during the 50s and 60s

Developments up to the modern day

Materials choices, properties of substrates, solders, metal

interconnects, chip and component packages etc.


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Course Contenturface Mount Technology

Multi layers associated technology, interconnecting layers,

vias, electroless & electroplating etc.

Future Trends in PCB Manufacture


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The PCB

Basically consists of a planar (flat) substrate which has

electronic components mounted on it that are interconnected

by conductive tracks.


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The PCB

Obvious Advantages

Large number of components can be fitted and connected

together on a small, flat substrate aided by advancements

in component reduction, thin line widths of interconnects.

Multi layer possibilities allows more complex circuitry

without taking up more room.

Mass production high volume throughput, reduces cost to

customer.


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HISTORYLate 1890s Great Advancements in Telephony

Early 1900s Great Advancements in Wireless Radio

Wiring becoming more and more complex as more components

being used.

High error rate as more and more wires needed to connectcomponents improvement needed


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HISTORYAlternative concept of using a planar substrate with mounted

components and patterned interconnects dates back to early 1900s

In 1903, Albert Hanson, from Berlin, filed a printed wire patent

based on stamped or cut out brass or copper foil adhesively bonded

to paraffin paper.

Included the concept of a double sided board with interconnections

between top and bottom layers. Interconnects basically crimped!

Also suggested the use of conductors deposited from conductive inks

and electrodeposition


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HISTORY

Edison had also been asked by the Sprague Electrical Co. how to

draw conductive traces on paper.

He suggested [1] applying glue or polymer and dusting with conductive

powder. [2] Patterning a dielectric with silver nitrate solution and

decomposing the salt to the metal. [3] applying gold foil to the

adhesive.

[1] and [2] can basically be applied as printing techniques Edisonrealised the importance of mass production!


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HISTORY

Edison could have pondered further and included vacuum deposition

technology and copper plating.

Americas most prolific inventor had patented these!

NOTE Edisons suggestions were ADDITIVE techniques

Putting material only where it is needed.


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HISTORY

Early circuitry generally ignored Subtractive techniques & favoured

Additive.

Generally conductive interconnects today are made via subtractivetechnology i.e. cover whole substrate with copper and etch or mill

away unwanted material.

There is considerable interest at the moment in being able to return to

additive technologies. Less waste, effluent, chemicals etc.


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HISTORY

In 1913, Arthur Berry filed for a patent which described

a method of manufacturing circuits for electric heaters,

in which metal was etched away. His patent described

the process of coating metal with a resist prior toetching, an improvement over die-cutting, which left

stress-concentrating sharp corners. Later, Littlefield

described a similar methodology.

Bassist also described a photoengraving process verysimilar to print plate production.


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HISTORY

The next inventor to rise to the occasion was

Charles Ducas, whose patent described both

etching and plated-up conductors. One version

involved electroplating a copper, silver, or gold

pattern onto a low-temperature metal alloy

through a contact mask.


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HISTORY

Another Ducas process involved forming

grooves in dielectrics such as wax and filling

them with conductive paste, which was thenelectroplated. Both sides of the dielectric could

be circuitised, and Ducas went on to describe

multilayer circuits and a means of

interconnecting the layers.


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HISTORY

Frenchman Cesar Parolini disclosed

improvements in additive processing when he

patented the printing of patterns with adhesiveonto dielectric, followed by applying copper

powder to the wet ink. This was Edison's basic

concept and one of Ducas's methods, but

Parolini implemented it fully and added theconcept of jumper wires


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HISTORY

1920s and 30s many other inventors exploit

print and plate technologies.

E.G. In 1933, Franz added conductive carbon

particles to polymer ink for printing on

cellophane or similar lamina and, perhaps aware

of Parolini's earlier work, added a copper platingstep.


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Introducing the Father of the PCBPaul Eisler Born in Austria in 1907

In early 1930s worked for HMV improving radio reception on

trains.

Life was difficult being Jewish in Austria/Germany at the time

Escaped to England, eventually interned as an enemy alien butReleased in 1941

Used his creativity to aid the defeat of the Nazis


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Introducing the Father of the PCBIn late 30s in England, Eisler took a radio he had made to bits and

took it apart and replaced all the wire-to-wire connections with

flat circuitry he made from strips of copper foil varnished on

Bakelite-backed paper.

Nearly sold the idea to Plessey in 1936 but production staff turned

it down women labour was cheaper and more flexible!

By 1943, The US Army were applying some of the techniques

Developed by Eisler in the production of rugged &

robust radio sets.


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Introducing the Father of the PCBAt the end of the war, Eisler and his wife were

making electrodeposited copper foil on planar

substrates and etching it with ferric chloride.

Founded Technograph Printed Circuits Ltd and

filed many patents for printed heating and

electrical interconnect circuitry.

His most important patents dealt with etching


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Introducing the Father of the PCB

Etching had been perfected by the printing industry

hundreds of years before in plate making.

An etch resist was used that was mechanically scraped

away

During the 1800s, photosensitive coatings were perfected

that enable the widespread use of photoengraving.


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Introducing the Father of the PCB

Eislers technique however was different

Printers generally etched into a layer of thick copper

Eisler used thin, electrodeposited copper foil and therefore

Etched completely through the metal to isolate conductors.

He became embroiled in legal patent arguments in US

because British patents had constantly referenced as used

in the printing industry


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The Rise of the PCB in the 1950s

PCB becomes commonplace in consumer products

Aided strongly by US releasing invention for

common use in 1948

Eisler therefore unsuccessful with US patentattempts


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In the 1950s, every electronic component generally had wire

leads, and the PCB had holes drilled for each wire of each

component.

The components were then soldered to the PCB. This

method is called through-hole construction.

This could be done automatically by passing the board over

a ripple, or wave, of molten solder in a wave-soldering

machine.


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Through-hole mounting is still useful in attaching

physically-large and heavy components to the board.

However, the wires and holes are wasteful. It costs money to

drill the holes, and the wires are merely cut off.


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1960s and Onwards

Surface Mount Technology Developed

Components made with small contact pads that are

physically held by solder to the conductors.

Solder paste generally applied by screen printing process

and components mounted on. Solder paste also acts astemporary adhesive.

PCB passed through IR oven to cure solder


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1960s and Onwards

Surface Mount Technology Developed


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Advantages of Surface Mount

smaller components

no need to drill holes through abrasive boards

simpler automated assembly

small errors in component placement are corrected

automatically as the molten solder pulls the component

into place by surface tension

components can be fitted to both sides of the circuit

board


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1960s and Onwards Copper Foil Bonding

Subtractive technique of copper foil bonding to wholesubstrate and removing unwanted metal becomes

generally adopted. Copper foil is typically 10s of

microns in thickness.

Substrates developed that are compatible with thistechnology. Generally matted glass fibres with epoxy

based e.g. FR4, FR5 etc.

Patterned Conductive Interconnects


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1960s and Onwards Copper Foil Bonding

Copper foil is generally pressed while heated onto the

substrate with adhesive

Substrates developed which are made of epoxy and glassfibres to which this process can be done Bonding aided by

Tg of the epoxy. Substrates such as FR4 become prominent.


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1960s and Onwards Copper Patterning

Unwanted copper etched away with ferric chloride remember Eisler!Photolithography exploited and developed further to produce

patterns

Light sensitive photoresists used as patternable barriers for etch

Design technology developed to produce circuitry as exposable

medium for transfer into photoresist e.g. photomask clear

acetate with dark emulsion as opaque regions


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Copper clad substrate covered with photoresist

Photomask placed over photoresist during exposure

Photolithography with a Photomask


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After exposure, photoresist developed

Copper left exposed in regions to be removed


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Exposed copper etched away with ferric chloride

Photoresist then removed


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A basic photomask for a simple PCB can be made today with

an inkjet transparency and a printer. Circuit design made on pc.

Photoresist covered, copper clad FR4 can be bought from RS etc

A light box can be easily made for exposing

Of course in industry much more sophisticated equipment is used!- much better resolution and quality.

Photomask acetates designed on CAD packages and printed


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Techniques also developed since the 1960s to screen print resist

onto copper layer no exposure undertaken.

Resolution not as good as photolithography in terms of resolved

feature sizes but a good method for mass production of low cost

boards.

Alternatives for Patterned Resist


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A milling tool can be used to cut away the unwanted copper to

leave a desired pattern.

This requires sophisticated plotting equipment in either X,Y or

X, Y, Z axis control.

This is another Subtractive Method

Alternative for Patterning Copper


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Simple Schematic for Producing a 1 layer PCB


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Some of the Areas of Interest for the Rest of the Course

Development of Substrate Materials properties etc.

Development of Solders

Development of Components e.g. reduction in size etc.

Multilayers PCBs with up to 24 layers interconnecting layers

& electroplating, chemistry etc.

Problems with PCBs and measures to overcome them, etc.

Documents

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