Solder size effect on interfacial reaction and growth behavior of Cu–Sn intermetallic compounds in cross-scale Sn3.0Ag0.5Cu/Cu joints between stacking TSV chips during step-reflow processes

In this study, Sn3.0Ag0.5Cu (SAC305) solder balls with various diameters were used to fabricate cross-scale SAC305/Cu joints (from micro-bump joints to flip chip-BGA and then to board level BGA joints). Four different reflow peak temperatures of 300, 250, 200 and 150 °C were adopted for step-reflow processes. The interfacial reactions, evolution of microstructures, interfacial intermetallic compound (IMC) growth and Cu6Sn5 grain morphology change in cross-scale SAC305/Cu joints were investigated systematically. The results show that the proportion of IMC in cross-scale SAC305/Cu joints increases with decrease of solder diameter. The IMC layer in small joints is thicker than that in large joints during step-reflow processes because small solder joints have sufficient supply of Cu atoms for growth of Cu6Sn5. During step-reflow processes, the thickness of interfacial IMC layer increases with increasing the number of reflow cycles in crossscale SAC305/Cu joints regardless of diameters of solder balls.