Dissolution kinetics of BGA Sn–Pb and Sn–Ag solders with Cu substrates during reflow

An investigation has been carried out to compare the dissolution kinetics of the Cu pad of the ball grid array (BGA) substrate into the molten conventional Sn–Pb solder and Sn–3.5Ag solder. A fixed volume of the BGA solder ball (760 μm diameter) was used on the 15–18 μm thick Cu pad having a circular area with a diameter of 650 μm. The dissolution measurement was carried out by measuring the change of Cu pad thickness as a function of time and temperature. A scanning electron microscope (SEM) was used to observe the microstructure of the solder joint and to measure the consumed thickness of Cu. A fast dissolution of the substrate occurred in the beginning for the molten solder/solid Cu reaction couple. But the dissolution in Sn–Ag was much more higher than that in Sn–Pb solder. The rates of Cu dissolution were measured for different soldering temperatures ranging from 225 to 240 °C and activation energies of 54 and 116 kJ/mol were found for the dissolution reaction in Sn–Ag and Sn–Pb solder, respectively.