Effects of Initial Morphology on Growth Kinetics of Cu₆Sn₅ at SAC305/Cu Interface during Isothermal Aging

Solder/Cu joints are important components responsible for interconnection in microelectronics. Construction of the solder/Cu joints through liquid/solid (L/S) reactions accompanies the formation of the Cu-Sn intermetallic compounds (IMCs) at the joint interface. The Cu₆Sn₅ IMC exhibits remarkable distinctions in thickness and morphology upon increasing the L/S reaction time. Effects of the initial characteristics of thickness and morphology on the growth kinetics of Cu₆Sn₅ during subsequent isothermal aging were investigated. SAC305 solder was reflowed on a Cu electroplated layer at 265 °C for 1 to 60 min to produce the Cu₆Sn₅ IMC with different thickness and morphology at the SAC305/Cu interface. The as-fabricated SAC305/Cu joint samples were aged at 200 °C for 72 to 360 h to investigate the growth kinetics of Cu₆Sn₅. The results show that the initial characteristics of thickness and morphology significantly influenced the growth kinetics of Cu₆Sn₅ during the subsequent solid/solid (S/S) reaction. A prolonged L/S reaction time of 60 min (L/S-60) produced a scallop-type Cu₆Sn₅ IMC with a larger grain size and a thicker thickness, which reduced the quantity of fast diffusion path (grain boundary) and the magnitude of concentration gradient, thus slowing down the growth rate of Cu₆Sn₅. According to the growth kinetics analysis, the growth rate constant of Cu₆Sn₅ could be remarkably reduced to 0.151 µm/h^0.5 for the L/S-60 sample, representing a significant reduction of 70 % compared to that of the L/S-1 sample (0.508 µm/h^0.5 for L/S reaction time of 1 min).