Interface Embrittlement Between 63Sn-37Pb Solder and Au Layer. Part 2: Quantitative Compositional Analysis of Intermetallic Compound Reaction Layers

The interface gold (Au) embrittlement failure mode resulting when a Au protective finish is not completely removed by the soldering process. has been investigated This failure mode places high-reliability electronics, which use the eutectic 63Sn-37Pb (wt%, abbreviated Sn-Pb) solder and Au protective finishes, at risk for latent service failures. Part 1 documented the Au/Sn-Pb) interface microstructures and especially, Kirkendall void formation, as a function of solid-state aging. Part 2 determines the compositions of the Au-Sn IMC layers and particles using electron probe microanalysis (EPMA). Two interface IMC layers were identified: a high-Au, secondary Au-Sn IMC layer adjacent to the Au layer, and a low-Au, primary Au-Sn IMC layer next to the Sn-Pb solder. The secondary IMC layer had compositions between the δ (AuSn) and ε (AuSn₂) phases. The primary IMC layer exhibited compositions between the ε (AuSn₂) and η (AuSn₄) phases. The compositions varied as a function of the solid-state aging parameters as well as solder layer thickness. The controlling mechanism was Kirkendall voids and their effect on Au diffusion along the Au/secondary Au-Sn IMC interface. Gold-Sn IMC particles in the solder exhibited compositions like those of the primary Au-Sn IMC layer, but were a function of Au scavenging from the Sn-Pb microstructure. Nickel diffusion, which occurred into the remaining Au and IMC layers, had a driving force that was sensitive to either solder layer thickness or the remaining Au layer thickness, the latter being correlated to the former. Gold diffusion was also measured, which took place in the Ni layer.