Effects of Oven and Laser Sintering Parameters on the Electrical Resistance of IJP Nano-Silver Traces on Mesoporous PET Before and During Fatigue Cycling

Abstract

Inkjet printing of conducting traces offers well established advantages and disadvantages as an alternative to electroplating of interconnects in flexible electronics. Assessment and optimization of their reliability is, however, often more complicated than commonly recognized. This is the case for an approach based on the deposition of silver nano-particle inks onto mesoporous PET substrates. In this case heating leads the trace resistance to drop not only because of the shrinkage and cure of the organic matrix holding the particles together, but also because some of that matrix 'disappears' into the substrate pores. The substrates can however only sustain relatively brief excursions above their glass transition, nominally 75°C, so it is not always practical to sinter the traces completely by conventional means. That has consequences such as ongoing reductions in resistance over time or under cyclic loading. Laser sintering does however offer the opportunity for much better fusing of the particles without excessive heating of the PET. The present work addresses effects of sintering parameters such as time/temperature and power/speed in oven and laser sintering, respectively, on the initial resistance and its evolution in subsequent low cycle fatigue testing. Interconnects of an average width of 80 µm and thickness of 550 nm were printed and post processed by one of two different sintering techniques: a) Convection oven sintering, and (b) Laser sintering. The resulting resistances were quantified, and samples finally subjected to tensile cycling with amplitudes of 1-2% and in-situ monitoring of the resulting resistance changes using a four-point probe. As expected, the resistance increased in each cycle as the substrate was stretched and it decreased again during unloading. However unlike for other kinds of traces, even though a remaining viscoelastic strain on the substrate prevented the complete elimination of the strain on the trace, the resistance of oven sintered traces usually ended up slightly lower after each cycle than before it. This effect was stronger for higher strain amplitudes, but it could be reduced or eliminated by longer preceding sintering of the traces. While a reduction in resistance may seem preferable to an increase, an even better solution would be a lower initial resistance that remained insensitive to subsequent fatigue cycling. This could be achieved by laser sintering, but careful optimization was required as too low a power did not prevent further resistance drops in cycling while too high ones led to significant degradations in fatigue resistance.