Metallurgical behaviors of lead-free solder alloys with respect to Ag content in a ball grid array package around melting temperatures

We report a new failure phenomenon in the ball grid array (BGA) package mounted on a board. The composition of BGA solder was Sn-1.0 wt%Ag-0.5 wt%Cu (SAC105) alloy. In-situ bending of the mounted board at 223 °C led to a BGA joint crack between Cu 6 Sn 5 intermetallic compound (IMC) and solder. The composition of SAC105 solder is considerably off-eutectic due to the low amount of Ag, which leads to about 8 °C gap between liquidus and solidus temperatures. In this temperature range, solid and liquid coexist, called semi-solid, which has a unique microstructure of solid Sn grain in liquid films. Such a structure is very weak for the tensile stress because of easy moving of grain inside the liquid film. On the other hand, no such failures were observed in the BGA package using Sn-3.0 wt%Ag-0.5 wt%Cu alloy. The pull test of individual SAC105 solder balls not only confirmed the same failure mode but also quantified the weak adhesion of SAC105 solder ball at 223 °C. This result suggests the careful choose of solder alloy for board level assembly when a mechanical fluctuation is expected.