THE FORMATION OF CU3SN INTERMETALLIC ON THE REACTION OF CU WITH 95PB-5SN SOLDER

This study characterizes the interfacial reactions that occur when Cu is soldered with 95Pb-5Sn solder. A continuous layer of Cu//3Sn epsilon phase forms during the soldering process. Previous studies suggest that the intermetallic layer spalls off during soldering. However, the present work shows that the intermetallic layer is intact after soldering and that any spalling observed is due to improper polishing. A new polishing technique was developed to preserve the intermetallic layer. The Cu//3Sn has a fine columnar grain structure that is very brittle. Both intergranular and transgranular fracture modes are observed. The size of the intermetallic layer is dependent upon the length of time the solder is molten. The rate of formation of epsilon phase was measured and used to determine an activation energy for diffusion of Sn in 95Pb-5Sn of 13 kcal/mol. This study is relevant to the electronic packaging of integrated circuit chips.