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 [email protected]
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2
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 layerMetal 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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CuCu
Cu oxide
coatingCu
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
Resi
stiv
ity /
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
0 10 20 30 40 50 600123456789
Resi
stiv
ity /
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 0 10 20 30 40 50 60
0123456789
Resi
stiv
ity /
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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0 5 10 15 20 250
5
7075808590
Resi
stiv
ity /
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
pasteCu 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