The Influences of Solder Composition and Pad Finish on the Reliability of Fine Pitch BGA Solder Joints

In this paper, fine pitch Plastic-ball-grid-array (PBGA) packages were assembled onto FR-4 printed-circuit-boards (PCBs) using various lead-free SAC solders including Sn3.0Ag0.5Cu, Sn3.8Ag0.7Cu, Sn1.0Ag0.1Cu. The solder ball diameter is 0.3 mm and pitch is 0.5 mm. The pad finishes on PCBs are OSP/Cu or electroless Ni/Au. Characterization of the solder joint microstructure, intermetallic compound (IMC) morphology was conducted. It is found that the composition of solder had no obvious effect on the IMC evolution of solder/OSP pad, but could significantly influence the IMC growth of solder and Ni/Au interface. Particle-like Ag3Sn formed on CuNiSn ternary IMC in the case of Sn3.0Ag0.5Cu, Sn3.8Ag0.7Cu. The assembled microelectronic devices were subjected to drop and bending tests. The results showed that while the same solder alloy was used, OSP pad shows better drop and bending reliability than Ni/Au pad. If same pad finishes are used, Sn3.8Ag0.7Cu showed bad drop and bending test performance compared with other solder alloys. The work can give some directions on the solder alloy design and choice of pad finish in electronic packaging and assembly.