Plugging Vias WConductive Ink

8/10/2019 Plugging Vias WConductive Ink

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Process for Plugging Lowto High Aspect Ratio Vias

with Conductive Ink inProduction Volumes

Michael OHanlon, DEK Flemington NJ

Lynne Dellis, DupontResearch Triangle Park, NC

2004 IPC Printed Circuits Expo, APEX and Designers Summit


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Why Via Fill?

Performance Enhancement

Via-in-PadThermal Conductivity

Surface Planarization (Sequential build-up)

Electrical ConductivityBuried Vias

Traditional Uses Vacuum in-circuit testingNon-solder vias

Prevention of contaminant penetration


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Why Conductive Via Fill?Higher reliability through electrical conductivity

Higher thermal conductivity than traditionalepoxies

Increased board density Via in pad technology Surface planarization for sequential build-up technology

Reduced cost Plated through holereplacement for buried vias

Reduced plating thickness for through holes


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ApplicationsDemand for higher component and PCB densities ispushing aspect ratios up to 13:1 and beyond for

multilayer applications

Via in pads

Stacked vias

Standard Via & Land(dog bone pattern)

Conductive ViaFill BGA Land


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Applications

Inner Layer productionLow aspect ratios usually a max of 3:1No through hole plating required


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Via fill process requirements

Fill quality after printing

Minimum material left to simplify/assist theplanarization process

Surface After Printing


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Via fill process requirementsFill quality after planarization

Mulitlayer

100% +/- .002 inchesno dish downno divots

Inner Layerspecifications more lenient

Minimum air voidsno large voids expandingacross the via diameter


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Current process

Figure 4 -

Mask

Typical: Hand Held Squeegee

Vacuum assist table

Mylar mask

Multiple print strokes

Squeegee application:


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Current process

Critical Process Parameters:

Figure 4 -

Print SpeedPressure

Squeegee Angle

Material Roll Volume

Vacuum Pressure

Mask

Number of Print Stokes, 1... 10 more?


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Current process IssuesSetup variables: New setup every board


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Current process IssuesSetup variables: New setup every boardInsufficient fill: How many print strokes? 1... 10 More?


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Voiding: Air tapped per print stroke


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Voiding: Air tapped per print stroke

Poor process control: No control over critical parameters


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Voiding: Air tapped per print strokePoor process control: No control over critical parametersPoor throughput: 5mins per board, more?


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Voiding: Air tapped per print strokePoor process control: No control over critical parametersPoor throughput: 5mins per board, more?Excessive handling of material: Creates air voids in material


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Current process IssuesInsufficient fill Air voids

Limited throughput

Operator dependent


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Desirables

Increase throughputPrototype to production levels

Reduce wasteRepeatable process

Improved fill


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Alternative Process Solution Automatic Stencil Printer with Enclosed Print Head:

Computer controlled process parameters

No operator influenceSave and recall product files in seconds

Repeatable process Automatic stencil alignment

Less top side materialPaste management

No air induced into the paste Automatic board transfer

50 - 60 Units Per HourTrue Process Control


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The Enabling Technology An Enclosed Print Head holds the paste material within

a chamber which is in contact with the stencil at all time.

The chamber is filled andmounted to the Printer.

Once filled the operatordoes not handle thematerial again.

The entire chamber isstored when not in use.

A silicon bladderholds the material

Material is added through ports when needed.

By eliminating the need to handle the material air voidsare not introduced in the paste.


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The Enabling TechnologyThe Enclosed Print Head provides a Direct downwardpressure to fill the via, rather then using Vacuum to suckthe paste into the via hole.


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Direct ImaginingCritical Process Parameters, all computer controlled:

Paste pressurePrint speedSqueegee Angle

Material Roll Volume Vacuum pressure


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Direct ImaginingNo Vacuum needed to suck material (and air) into the via.


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No paste roll to maintain


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No Squeegees


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No Squeegees (angle and pressure eliminated)Typically one print per cycle for


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No Squeegees (angle and pressure eliminated)Typically one print per cycle for


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Print TrialsPrint Trials were run at two customers sites

Printer: DEK model ELA Proflow 300mm enclosed headPaper stencil cleanerTooling, magnetic pillars

Material: Dupont CB-100 Viscosity = 115 145 PaS

Particle size = up to 75 micron3-5% solventTest boards:

Dupont test board,Double sided FR412 X 18 X 0.062 0.011 plated via (5.6:1)

Customer product,Multiple layer FR412 X 18 X 0.153 0.020 plated via (7.6:1)

12 X 18 X 0.180 0.018 plated via (10:1)

Printer: DEK model VISTA Proflow 500mm enclosed head

Vortex stencil cleanerTooling, magnetic pillars

Material: Dupont CB-100 Viscosity = 115 145 PaS

Partial size = up to 75 micron3-5% solvent

Test boards:Customer product

Multiple layer FR424 X 17.5 X 0.063 0.015 plated via (4:1)12 X 18 X 0.040 0.011 plated via (3.6:1)


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Print Trails

Inspection Method

X-ray (wet print)Percent of fill & air voids

Cross sections (after curing) Air voids

Visual Inspect(after curing and sanding)

Pad surface quality


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Critical Process ParametersPrint speed:speed of the enclosed head during the print cycle.

Paste pressure:Pressure within the head during the print cycle.

Multiple Strokes:Number of print strokes per board

Separation speed:How fast the board separates from the stencil after printing.

Stencil design: Aperture to Via diameter ratio

Board support:Dependent on fill criteria


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Print Speed vs. Paste Pressure At higher paste pressures the via holes fill faster allowing afaster print speed.

For mid range aspect ratios (5:1)Paste pressure = 2.5 - 3bar

Print speed = 20-30mm/secPrint stroke = 1

For high aspect ratios (10:1)Paste pressure = 3.5 4 barPrint speed = 20mm/sec

Print stroke = 2


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Print Speed vs. Paste PressureHowever , high paste pressure may cause bleeding underthe stencil to occur sooner, requiring more frequent stencil

wipes.

Stencil

Via Via

board


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Multiple strokesBy utilizing a positive pressurewithin the enclosed Head to forcethe material into the via hole, thelarge air voids are eliminated.

1

board

Via

stencil

2If required


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Separation speed A fast separation speed could draw in air which wouldnot be seen until after sanding.

Too fast 3mm/secStencilBoard

Via 1) At print height

2) During separation

3) After release


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Quality of FillRation = 9:112 X 18 board

20mil via

176mil board

Paste press = 3.5 barPrint speed = 25mm/secPrint strokes = 1Sep speed = 3mmsec

Via hole fill = 90 100% 0-10


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Quality of FillLarge air voids eliminated

SqueegeesEnclosed Head

l f ll


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Quality of FillRation = 9:112 X 18 board

20mil via (after plating)

176mil board

Paste press = 3 barPrint speed = 25mm/secPrint strokes = 2Sep speed = 3mmsec

Via hole fill = +115% 0+15

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Carrier A carrier was used to act as a dam for theover print, with blotter paper between the

board and carrier.

Carrier

Blotter paper

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Carrier

The carrier effectively dams the exitpreventing over print.

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Carrier However , a small ditching issue was seen onthe exit side.

Carrier

Ditching due to thematerial pulling out ofthe via as it is separated

from the carrier.

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CarrierThe solution is to allow a small over print.This can be done with a spacer between theboard and carrier

The spacer allows a nailhead on the exit side toresolve the issue.

Carrier

Carrier

Spacer added


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Thank you for your time

Michael OHanlon, DEK Flemington NJ

Lynne Dellis, DupontResearch Triangle Park, NC

2004 IPC Printed Circuits Expo, APEX and Designers Summit

Documents

Plugging Vias WConductive Ink