Effects of addition participation in the interfacial reaction on the growth patterns of Cu6Sn5-based IMCs during reflow process

The growth pattern of intermetallic compounds generated between Cu substrate and the eutectic Sn-3.5Ag doped with small amount of additives (0.1 mass%), say, Ni or Co, was explored in this paper. During reflow experiments, the dwell time at peak temperature 250 °C was set up for 1, 2, 5, 10, 20, 30 and 60 s, respectively, to investigate the intermetallic morphology growth behavior. The intermetallics formed between Cu substrate and three kinds of solders, namely, Sn-3.5Ag, Sn-3.5Ag-0.1Co and Sn-3.5Ag-0.1Ni, were all identified as Cu6Sn5-based, although the additives participated in the interfacial reaction. However, the rounded shape of Cu6Sn5 was observed, while the (Cu, Ni)6Sn5 or (Cu, Co)6Sn5 intermetallics were polyhedral shaped (or faceted). The presence of additives, namely Ni or Co, at the outer region of (Cu, Ni)6Sn5 or (Cu, Co)6Sn5 IMCs was responsible for the morphology transition, which might lead to the increase of enthalpy change and thus result in the Jackson's parameter larger than 2. Another attractive result was the evolution of grain size distribution. For the grains of typical Cu6Sn5 intermetallic, the size distribution tended to be more narrow following with the extended reflow time, although the number of intermetallic was reduced. However, for the (Cu, Ni)6Sn5 or (Cu, Co)6Sn5 intermetallics, the grain size distribution became wider and spread out. Based on the basic theory of grain growth, the AGG (abnormal grain growth) pattern was suggested for the (Cu, Ni)6Sn5 or (Cu, Co)6Sn5 intermetallic growth, while Cu6Sn5 IMC exhibited the normal grain growth.