Mechanical and Electrical Properties of Cu/Sn-3.5Ag/Cu Ball Grid Array (BGA) Solder Joints after Multiple Reflows

The microstructural evolution, die shear strength, and electrical resistivity of Cu/Sn-3.5Ag (wt.%)/Cu ball grid array (BGA) solder joints were investigated after 1 to 10 reflows using scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron probe microanalysis (EPMA), bonding testing, and a four-point probe station. A Cu₆Sn₅ intermetallic compound (IMC) was formed at both the upper and lower interfaces after one reflow. The IMC thickness increased at the lower interface with increasing reflow number, whereas the IMC morphology and thickness remained virtually unchanged at the upper interface, irrespective of the reflow number. The amount of Cu₆Sn₅ IMC contained in the solder ball increased with increasing reflow number. These microstructural evolutions with increasing reflow number strongly affected the mechanical and electrical properties of the solder joint.