Effects of fine size lead-free solder ball on the interfacial reactions and joint reliability

As the trend of smaller mobile electronic products, smaller solder balls are needed for electronic packages such as chip scale package (CSP). The purpose of this study was to investigate the effect of solder ball sizes on the interfacial reaction and their joint reliability. In this study, 400, 300, and, 200 μm diameter solder balls of 98.5 wt.% Sn, 1 wt.% Ag, and 0.5 wt.% Cu(SAC105) were reflowed on the Cu/OSP and electroplated Ni/Au pads at 250°C. In the case of the reaction with Cu/OSP, as solder ball sizes decreased, the thickness of Cu6Sn5 intermetallic compounds (IMCs) increased and Cu concentrations of bulk solder balls increased. The Cu dissolution rate of finer solder balls into molten solder was faster than larger solder balls due to its high pad area/solder volume ratio (A/V ratio). Fast Cu dissolution into molten solder promoted the thicker IMC growth. In the case of the reaction with electroplated Ni/Au, (Cu, Ni)6Sn5 IMCs were only formed at the interface of 400 μm solder ball. Both (Cu, Ni)6Sn5 and (Ni, Cu)3Sn4 IMCs were formed at the interface of 300 μm solder ball. The absolute amount of Cu in 200 μm solder ball was smaller than 300 and 400 μm solder ball due to its smaller volume, therefore (Ni, Cu)3Sn4 IMCs were formed by the insufficient amount of Cu at the interface of 200 μm solder ball. The substrate design with smaller A/V ratio and the SAC alloy with higher Cu contents (>0.5 wt.%) were recommended to reduce the IMC growth.