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

1

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

3

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

4

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

5

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

6

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

7

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

8

~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

9

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

10

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

11

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

12

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

13

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

14

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

15

Glass substrate

16

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

17

Acknowledgements

EPSRC for funding through the Innovative Manufacturing and Construction Research Centre

(IMCRC) Materials Research School, Loughborough

University for PhD studentship funding

18

Thank you for your attention