Effect of solder bump geometry on the microstructure of Sn-3.5 wt% Ag on electroless nickel immersion gold during solder dipping

Continuous miniaturization of solder joints in high-density packaging makes it important to study how the joint size could affect the solder microstructure and thereby the subsequent in-service reliability. In this study, a printed circuit board with electroless nickel immersion gold (i.e., Au/Ni-P) over Cu bond pads of size approximately ~80 µm and ~1500 µ~m in diameter was dipped into a Sn-3.5Ag solder bath. The study shows that the smaller bumps, which cool more quickly, include much finer Ag3Sn particles. In addition, substantial differences in the thickness of the interfacial intermetallics and the microstructure for different dipping times are observed for different bump sizes. The results from a combined thermodynamic-kinetic model also suggest that the solder bump geometry can influence the dissolution kinetics of the pad metal into the molten solder and therefore the microstructure at the solder-pad interface and within the bulk solder.