Growth behavior of intermetallic compounds and early formation of cracks in Sn-3Ag-0.5Cu solder joints under extreme temperature thermal shock

The microstructure evolution and growth mechanism of interfacial intermetallic compounds (IMCs) as well as the mechanism for early formation of cracks in Sn-3Ag-0.5Cu solder joints of quad flat packages (QFPs) during extreme temperature thermal shock between 77 K and 423 K were investigated. The Cu-Sn IMC layer at the Cu lead/solder interface and the Ni-Cu-Sn IMC layer at the solder/ENIG pad interface gradually thickened as well as the IMCs morphologies changed during extreme temperature thermal shock. Scallop-like Cu-Sn IMC layer and needle-like Ni-Cu-Sn IMC layer both transformed to plane-like IMCs. New Cu3Sn phase was formed at the interface between Cu lead and Cu6Sn5 IMC layer after 250 cycles. The (Ni, Cu)3Sn4 IMC layer was completely converted into (Cu, Ni)6Sn5 IMC layer after 150 cycles resulting from the diffusion of Cu atoms from Cu lead and Sn-3Ag-0.5Cu solder to the solder/pad interface. The time exponent (n) values of Cu-Sn and Ni-Cu-Sn IMC layers were 0.66 and 0.34, respectively, indicating that the controlling mechanisms for Cu-Sn and Ni-Cu-Sn IMC growth were bulk diffusion and grain-boundary diffusion, respectively. Cracks were formed both at the solder/Cu-Sn layer interface and at the solder/Ni-Cu-Sn layer interface after 250 cycles, due to the CTE difference between the solder and IMC, and to the thickened and flattened IMC layer. The great stress concentration resulting from the large temperature variation (∆T = 346 K) led to the early formation of cracks. With the increase of thermal shock cycles, the pull strength of Sn-3Ag-0.5Cu solder joints decreased and the fracture location changed from within the solder to partly in Cu-Sn IMC layer and partly along the solder/Ni-Cu-Sn layer interface, which indicated that the fracture mechanism transformed from ductile fracture mode to brittle fracture mode.