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

Copper Filled Conductive Adhesives for Printed Circuit Fabrication D.A. Hutt 1 , S. Qi 2 , R. Litchfield 1 , B. Vaidhyanathan 2 , D.P. Webb, S. Ebbens, C. Liu 1 1 Wolfson School of Mechanical and Manufacturing Engineering 2 Department of Materials Loughborough University D.A.Hutt@lboro.ac.uk 1

Outline  Introduction  Copper as a substitute for Silver  Copper Conductive Adhesive Preparation  Characterisation of Printed Tracks  Alternative Curing Methods  Reliability  Functional Circuit Fabrication  Conclusion 2

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 sinter  Inkjet print nanoparticles and sinter  Conductive inks and adhesives based on micron sized particles or flakes  Screen / stencil Conductive Curing particle print and cure adhesive e.g. 80 O C – 150 O C  Printing and component assembly steps – often separated 4

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 Oxide layer Metal  Direct substitution particle problematic  Copper surface oxidation results in poor conductivity 5

Copper Powder Preservation  Copper powder treatment method developed  Remove oxide and apply protective coating  Self-assembled monolayer (SAM) SAM coating Cu oxide SAM Cu etch Cu Cu Cu coating Adhesive  Enables powder to be stored in air (in freezer) for several weeks Bare Cu  Coating breaks down during Cured Adhesive thermal cure – metal to metal contact 6

Copper Conductive Adhesive  Etched and applied protective SAM coating to spheroidal copper powder  Average 10 µ m 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 @ 150 o C  Two part adhesive – cure for 15 min @150 o C  Both 85.7 wt% Cu loading 7

Thermal Curing Procedure  Stencil printed stripes of Cu adhesive on glass  Curing (150 o C) under an inert (Ar) atmosphere  Poor conductivity if cured in air  Resistivity measured using four point probe Ar outlet Rubber tube Glove box Samples ~2 cm Ar inlet Oxygen Hot plate analyzer 8

Resistivity of Fully Cured Samples 2.1 Cu paste A (Cu mixed with resin A) Cu paste B (Cu mixed with resin B) Commercial Ag paste 1.8 Resistivity / 10-4 Ohm.cm 1.5 1.2 0.9 0.6 0.3 0.0 One-part Commercial Two-part resin Ag resin resin  Cured copper samples show low resistivity  One part resin – best results  Comparable to commercial Ag filled adhesive 9

Microstructure  Shrinkage of the adhesive leads to reduced resistivity Resistivity / 10 -4 Ohm.cm 9 8 7 5 min cure 6 5 4 3 2 1 0 0 10 20 30 40 50 60 Curing time / min 60 min cure 10

Microwave Curing  Microwave curing has also been investigated  Using an inert atmosphere  Aim to improve the thermal profile Microwave cavity Thermal imaging camera Ar outlet Quartz tube Sample holder Ar inlet 11

Microwave Curing  Comparable resistivity and microstructure achieved, but in shorter curing time 9 Conventional curing Resistivity / 10-4 Ohm.cm Microwave curing 8 7 6 5 4 3 2 1 20 min cure 0 0 10 20 30 40 50 60 Curing time / min 12

Reliability  Storage in ambient environment leads to small increase in resistivity  Approx. 6 to 7% increase in 6 months  85 o C / 85% relative 90 Commerical Ag paste Resistivity / 10 -4 Ohm.cm 85 humidity testing Cu paste A (No coating) Cu paste A (Conformal coating) 80  Exposed tracks 75 show large 70 change in resistivity  Tracks protected 5 with a conformal coating show much 0 greater reliability 0 5 10 15 20 25 Aging time / h 13

Circuit Printing and Assembly blade Cu paste deposit stencil paste Stencil print copper paste on substrate Thermal Cure Functional circuit Place components into uncured paste  Combining circuit formation and component interconnection into a single process 14

Functional Copper Circuits  Single layer test circuits prepared using stencil printing and component placement  Low cure temperature enables polymer substrates to be used Glass substrate 15

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 16

Acknowledgements  EPSRC for funding through the Innovative Manufacturing and Construction Research Centre (IMCRC)  Materials Research School, Loughborough University for PhD studentship funding 17

Thank you for your attention 18