Novel insights in growth of intermetallic compounds between Sn–3.0Ag–0.5Cu solder and flexible PCB substrates under strain

The study investigated the effect of tensile and compressive strain on the interfacial reaction of SAC305/Cu system. Experimental results indicated that the thickness of Cu₆Sn₅ layer on non-strained samples slightly decreased after aging for 120 h and increased with prolonged aging time. The applied tensile strain had an enhanced effect on the growth of Cu₆Sn₅ phase, while the applied compressive strain inhibited the growth of Cu₆Sn₅ phase. However, the thickness of Cu₃Sn layer at SAC305/Cu solder joints increased gradually with increasing aging time, regardless of whether strain was or not applied on the solder joints. Besides, the mean diameter of Cu₆Sn₅ grain increased in three systems as the aging time was prolonged to 480 h. The morphology of Cu₆Sn₅ grain on non-strained samples gradually changed from round-shape into polyhedron-shape with increased aging time. The morphology of Cu₆Sn₅ grain under tensile or compressive strain was polyhedron-shape. The Kirkendall voids appeared within Cu₃Sn layer in three systems after aging for 120 h and the amount of voids increased gradually with prolonged aging time. The applied tensile or compressive strain had an enhanced effect on the formation of voids within Cu₃Sn layer and the enhanced effect of applied tensile strain was much remarkable than that of other systems.