Influence of thermal ageing on void content and shear strength for selected lead free solder die-attach

In flip-chip packaging technology, thermal performance can be improved by attaching heat spreader to the backside of the heat generating silicon die via thermal interface material. Solder thermal interface materials (STIMs) are preferred to their polymer-based counterparts because they relatively offer higher thermal conductivities. Unlike lead-based solders which have had a longer application history, lead-free solders are relatively new; thus, the reliability of these lead-free solders are subject of research interest. In this study, lead-free solder preforms with compositions of 96.5Sn3Ag0.5Cu, 99.99In, 97In3Ag and 58Bi42Sn were used for silicon die attachment on copper heat spreader. The void content and shear strength of the soldered joints after thermal ageing at 125°C for 50 hours, 100 hours, 200 hours and 300 hours were evaluated. Generally, the four studied solder joints show decrease in voids with thermal ageing though 96.5Sn3Ag0.5Cu and 58Bi42Sn comparatively manifested more reduction in voids. The shear strengths of 96.5Sn3Ag0.5Cu alloy and 99.99In are shown to be relatively more stable throughout the ageing time. The results also indicate that samples with lower void content comparatively result in higher shear strength and vice versa. These results are especially important for high temperature application environments of Pb-free STIMs like in automotive systems.