Brittle fracture induced by phase transformation of Ni-Cu-Sn intermetallic compounds in Sn-3Ag-0.5Cu/Ni solder joints under extreme temperature environment

Electronic assemblies without thermal protection have to be subjected to extreme temperature environments during deep space exploration. In this study, extreme temperature thermal shock test from 77 K to 423 K was conducted to investigate the phase transformation and growth behavior of interfacial Ni-Cu-Sn intermetallic compounds (IMCs) in Sn-3Ag-0.5Cu (SAC305)/Ni solder joints, as well as their effect on solder joint reliability of Plastic Ball Grid Array (PBGA) assemblies under extreme temperature environment. Double layers of (Ni, Cu)3Sn2 and (Cu, Ni)6Sn5 IMCs were formed at the SAC305 solder/Ni interface after reflowing. (Cu, Ni)6Sn5 IMCs were found to gradually transform into (Ni, Cu)3Sn2 IMCs during extreme temperature thermal shock by transmission electron microscopy (TEM). After 300 cycles, (Cu, Ni)6Sn5 IMCs at the (Ni, Cu)3Sn2/Ni interface completely disappeared, while a new (Cu, Ni)6Sn5 IMC layer was formed on the top surface of SAC305 solder ball. The growth of interfacial Ni-Cu-Sn IMCs was significantly accelerated by the high thermal stress induced by the thermal expansion mismatch between SAC305 solder and Ni, and the large temperature variation (ΔT = 346 K). The transformation of (Cu, Ni)6Sn5 to (Ni, Cu)3Sn2 and the fast growth of brittle (Ni, Cu)3Sn2 IMCs led to the initiation and propagation of cracks within the (Ni, Cu)3Sn2 IMC layer under high thermal stress after 300 cycles, which finally caused the brittle fracture of SAC305 PBGA solder joints after 350 cycles.