Effect of multiple reflow processes on the reliability of ball grid array (BGA) solder joints

This paper studies the mechanical behaviors and microstructures of the ball grid array (BGA) solder joints against OSP-coated Cu pads on FR4 substrates after multiple reflow process. A new Pb-free solder, Sn–3Ag–0.5Cu–8In (SACI), has been compared with the popular Pb-free solder Sn–3Ag–0.5Cu (SAC), and the traditional Sn–37Pb (SP) solder. It is found that SACI solder joints provide higher shearing strength over SAC and SP solder joints during multiple reflow. The shearing force of the three types of solder joints does not have significant change with the increase of the number of reflow cycles. The fracture of all SAC and SP solder joints mainly occurs in the bulk solder, whereas the SACI solder joints show a diverse manner, with various combinations of bulk solder facture, solder/IMC interface dissociation, and pad/resin interface failure. The differences in mechanical behaviors are interpreted in terms of diffusion and interaction, as well as chemical and thermal induced degradation. The microstructure evolutions of the solder joints during multiple reflow soldering are also examined. A continuous change of chemical compositions in the IMC layer/particles has been noticed. Large-shaped Ag–In–Sn phase near the IMC layer has been found within the SACI solder joints on the Cu pads.