EARLY INTERFACIAL REACTION AND UNDER-COOLING SOLIDIFICATION BEHAVIOR OF Sn--3.5Ag/Cu SYSTEM

For electronic packaging technologies using lead-free solders, one of the major prob- lems related to reliability for the solder interconnects is the existence of the interfacial intermetallic compound (IMC). The interfacial failures between IMC and solder alloy often lead to loss of function in interconnects and result in product failure. Therefore, considerable attention has been focused on study of formation, growth and control of IMC during solder process. In this paper, the early interfa- cial reaction in Sn-3.5Ag/Cu (UBM) system and the system's melting and solidification characteristics were investigated using differential scanning calorimeter incorporating with reflow process. The results show that during heating the diffusion of Cu atom into Sn-3.5Ag solder results in the formation of Sn-Ag-Cu ternary alloy at the interface before melting of Sn-3.5Ag solder and the ternary eutectic system melts at a temperature nearly 4 lower than Sn-3.5Ag solder's melting temperature. The early interfacial reaction also leads to earlier wetting of the liquid solder alloy at the interface, and consequently brings about formation of scallop-type Cu-Sn intermetallics layer with a certain thickness as well as makes the initial Sn-3.5Ag/Cu system changed into Sn-Ag-Cu/Cu system, which makes the undercooling of the solder alloy decrease obviously.