Additives effects on growth pattern modification of Cu6Sn5-based intermetallic compounds 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 seconds 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-0.1 Co 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 (Ni or Co) at the outer region of (Cu, Ni)₆Sn₅ or (Cu, Co)₆Sn₅ IMCs was responsible for the morphology transit, 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.