η-η transformation of interfacial Cu6Sn5 in solder joints

The η-η´ transformation of interfacial Cu₆Sn₅ in solder joints was studied. Two cooling rates were selected to obtain different grain sizes of intermetallic compounds (IMCs). In the case of air cooling, the compounds were tiny and no η-η transformation was detected in the as-soldered state. Under furnace cooling, however, thick and coarse interfacial IMCs were obtained and η was still stable. After aging 2d at 150°C, η-η´ transformation was detected no matter what cooling rate. This indicates that effects of IMC thickness and grain size are negligible in some way. It is also shown that aging at 150°C promoted the interfacial η-η´ transformation and the time needed was comparatively short to that of long-term aging study of solid/solid growth kinetics. These provide us a significant reference to understand the obtained reliability data and evaluate the possible threats of transformation to the interconnects. For Ni-bearing solder joints, interfacial (Cu,Ni)₆Sn₅ was detected. Only η-phase was discovered after soldering and aging.