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

To satisfy the requirements of the newest mobile product, smaller solder ball is needed for electronic packages such as chip scale package (CSP). The purpose of this study was to investigate the effect of solder ball size on the interfacial reaction. 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 formed on the Cu/OSP and electroplated Ni/Au pads at 250°C. In the case of the reaction with Cu/OSP, as solder ball size decreased, the thickness of Cu6Sn5 intermetallic compounds (IMCs) increased and Cu concentrations of solder balls increased. In the case of fine solder ball, Cu dissolution rate into molten solder was faster than large solder ball due to its high pad area/solder volume ratio (A/V ratio). Fast Cu dissolution into molten solder promoted the IMC growth. In the case of the reaction with electroplated Ni/Au, (Cu,Ni)6Sn5 IMCs were only formed in 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. (Ni,Cu)3Sn4 IMCs were only formed at the interface of 200 μm solder ball. The absolute Cu contents of 200 μm solder ball was smaller than 300 and 400 μm solder ball due to its smaller volume. (Ni,Cu)3Sn4 IMCs were formed by the insufficient Cu contents in 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.