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 µm 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. [ABSTRACT FROM AUTHOR]