Highly Reliable Pressure-Less Silver Sintering Joints

Abstract

Abstract Pressure-less silver sintering pastes with high reliability tested by temperature cycling test (TCT) under −40°C to 175°C were developed. The silver joints formed between Si die and Si3N4 active metal brazing (AMB) substrate can survive at least 1500 cycles without decrease in shear strength. In contrast, the strength of reference solders decreases to 62% to 86% of their initial values. The optimized silver joints can achieve average total void% of <1.3% and maximum single void% of <0.3%. Die tilt angle can be controlled within < 0.3°. Failure mode analysis of the joints reveals that during TCT in the bulk phase, silver sintering continues resulting in the formation of larger grain and pore size; while at the interface, the adhesion strength at the silver-substrate interface was gradually enhanced as compared to that at the silver-Si interface. Temperature cycling makes the adhesion strength at both interfaces more balanced accounting for the constant shear strength. SEM cross-section shows that for the as-sintered sample the porosity within the joint is not evenly distributed; the center area normally has a low porosity of < 4%, while at the edge there is a higher porosity of 18% to 24%. The average total porosity of the joint is ~ 13%. This value does not change after 800 cycles of TCT, but increases to 19% at 1500 cycles. The effect of TCT on the joints under conditions such as different surface finished substrate, varied bond line thickness and Si die size was also studied.