Copper Filled Conductive Adhesives for Printed Circuit Fabrication - - PowerPoint PPT Presentation

Copper Filled Conductive Adhesives for Printed Circuit Fabrication

D.A. Hutt1, S. Qi2, R. Litchfield1, B. Vaidhyanathan2, D.P. Webb, S. Ebbens, C. Liu1

1Wolfson School of Mechanical and Manufacturing Engineering 2Department of Materials

Loughborough University D.A.Hutt@lboro.ac.uk

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Outline

• Introduction
• Copper as a substitute for Silver
• Copper Conductive Adhesive Preparation
• Characterisation of Printed Tracks
• Alternative Curing Methods
• Reliability
• Functional Circuit Fabrication
• Conclusion

Introduction

• Printing now applied to most areas of electronics
• Conductors, components, displays
• Aim to achieve
• Low cost
• High volume, e.g. reel to reel processing
• Agility, e.g. low volume prototypes
• Substrate variety, e.g. polymers, FR4
• Reliability
• Many conductor inks based on silver
• Lower cost / more abundant alternatives needed

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Printed Conductors

• Nanoparticle based inks
• Inkjet print nanoparticles

and sinter

• Conductive inks and adhesives based on micron

sized particles or flakes

• Screen / stencil

print and cure

• Printing and component assembly steps – often

separated

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Curing e.g. 80 OC – 150 OC

adhesive Conductive particle

sinter

Copper as a Substitute for Silver

• Conductive adhesives / inks rely on low

resistance metal to metal contact

• Silver widely used:
• Ag oxide has good conductivity
• Copper offers lower cost
• Direct substitution

problematic

• Copper surface oxidation

results in poor conductivity

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Oxide layer Metal particle

Copper Powder Preservation

• Copper powder treatment method developed
• Remove oxide and apply protective coating
• Self-assembled monolayer (SAM)
• Enables powder to be stored in

air (in freezer) for several weeks

• Coating breaks down during

thermal cure – metal to metal contact

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Cu Cu

Cu oxide

coating Cu

SAM

etch

Adhesive Cu SAM coating Bare Cu Cured Adhesive

Copper Conductive Adhesive

• Etched and applied protective SAM coating to

spheroidal copper powder

• Average 10 µm particle size
• Powder stored in the freezer for several weeks
• Conductive adhesives prepared
• Two formulations of epoxy adhesives tested
• One part adhesive – cure for 60 min @ 150oC
• Two part adhesive – cure for 15 min @150oC
• Both 85.7 wt% Cu loading

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Thermal Curing Procedure

• Stencil printed stripes of Cu adhesive on glass
• Curing (150oC) under an inert (Ar) atmosphere
• Poor conductivity if cured in air
• Resistivity measured using four point probe

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~2 cm

Ar inlet Ar outlet Glove box Hot plate Samples Rubber tube Oxygen analyzer

Resistivity of Fully Cured Samples

• Cured copper samples show low resistivity
• One part resin – best results
• Comparable to commercial Ag filled adhesive

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0.0 0.3 0.6 0.9 1.2 1.5 1.8 2.1

Resistivity / 10-4 Ohm.cm

Cu paste A (Cu mixed with resin A) Cu paste B (Cu mixed with resin B) Commercial Ag paste

One-part resin Two-part resin Commercial Ag resin

Microstructure

• Shrinkage of the adhesive

leads to reduced resistivity

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5 min cure 60 min cure

10 20 30 40 50 60 1 2 3 4 5 6 7 8 9

Resistivity / 10-4 Ohm.cm Curing time / min

Microwave Curing

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• Microwave curing has also been investigated
• Using an inert atmosphere
• Aim to improve the thermal profile

Sample holder Thermal imaging camera Ar inlet Ar outlet Microwave cavity Quartz tube

Microwave Curing

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• Comparable resistivity and microstructure

achieved, but in shorter curing time

20 min cure

10 20 30 40 50 60 1 2 3 4 5 6 7 8 9

Resistivity / 10-4 Ohm.cm Curing time / min

Conventional curing Microwave curing

Reliability

• Storage in ambient environment leads to small increase

in resistivity

• Approx. 6 to 7% increase in 6 months
• 85oC / 85% relative

humidity testing

• Exposed tracks

show large change in resistivity

• Tracks protected

with a conformal coating show much greater reliability

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5 10 15 20 25 5 70 75 80 85 90

Resistivity / 10-4 Ohm.cm Aging time / h

Commerical Ag paste

Cu paste A (No coating) Cu paste A (Conformal coating)

Circuit Printing and Assembly

• Combining circuit formation and component

interconnection into a single process

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Thermal Cure Stencil print copper paste on substrate Place components into uncured paste Functional circuit

stencil blade paste Cu paste deposit

Functional Copper Circuits

• Single layer test circuits prepared using

stencil printing and component placement

• Low cure

temperature enables polymer substrates to be used

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Glass substrate

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Conclusion

• Copper as an alternative to Ag in printed

electronics is challenging

• Using organic coatings Cu can be preserved

for use in conductive adhesives

• Reliability of Cu adhesives requires further

investigation

• Microwave heating can speed up the curing

process

• Functional circuits demonstrated combining

printing and assembly

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Acknowledgements

• EPSRC for funding through the Innovative

Manufacturing and Construction Research Centre (IMCRC)

• Materials Research School, Loughborough

University for PhD studentship funding

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Thank you for your attention