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The interfacial reactions between Sn–50 wt% Bi (Sn50Bi), Sn–57 wt% Bi (Sn57Bi) and Sn–65 wt% Bi (Sn65Bi) alloys with the Au substrate aged at 80, 100 and 120 °C for 12–350 h are investigated in this study. The Au–Sn binary intermetallic compounds (IMCs) were observed at the interface, and the Au–Sn binary IMCs and ternary Au–Bi–Sn metastable (T) phase in the solder when the Sn–50Bi and Sn–65Bi couples. The Au–Sn binary IMCs and T phases were formed in the Sn–50Bi/Au couple aged at 100 °C and Sn–57Bi/Au couple aged at 120 °C for 100 h. However, this T phase was transferred into the AuSn4 phase when the reaction was extended over 72 h. The increase in the Bi content in the Sn–Bi alloys could make the large (Bi) phase segregate in the solder/Au interface and decrease IMC growth. The IMC growth mechanism in all reaction couples is diffusion-controlled. The lowest reaction activity energy value could be found in the Sn–57Bi/Au couple due to its lowest liquidus temperature.

A failure analysis of thermomechanical fatigued small outline J (SOJ) lead/Sn–X (X = AgCu and Pb) solder joints over different printed circuit board surface finishes underwent pull test has been conducted. Experimental results showed that the dimple-fracture feature was due to the occurrence of fracture in the solder matrix and the intergranular-fracture feature was due to the occurrence of fracture in the IMC layer. The growth rate of the IMC layer at the SOJ/SnAgCu interfaces is much faster than that of the IMC layer at the SOJ/SnPb interfaces. The increase in the thickness of IMC (Cu6Sn5) and the fatigue of the SnPb solder matrix upon thermal cycling resulted in the decline of the strength of the SOJ/SnAgCu/(Ni/Au) and SOJ/SnPb/(Ni/Au) solder joints, respectively. After a pull test, the existence of different phases and the coverage variation of these phases on the fracture surface disclosed not only the distribution of stress but also the thermal fatigue properties of bulk solder alloy and IMC between the SOJ lead and solder.

The effects of printed circuit board (PCB) surface finish and thermomechanical fatigue (TMF) on the formation and growth of intermetallic compounds (IMCs) between Small Outline J (SOJ) leads and Sn–37Pb solder were investigated. The thickness of the IMC layer formed initially at as-soldered SOJ/Sn–Pb interface over Ni/Au PCB surface finish was about 1.3 times of that over OSP PCB surface finish. The parabolic TMF cycle dependence clearly suggests that the growth processes are controlled primarily by solid-state diffusion. The diffusion coefficient for the growth of total IMC layer at SOJ/Sn–Pb interface over Ni/Au PCB surface finish is the same as that over OSP PCB surface finish and, thus, the total IMC layer at the SOJ/Sn–Pb interface over Ni/Au PCB surface finish is thicker than that over OSP PCB surface finish. Using the Cu–Ni–Sn ternary isotherm, the anomalous phenomenon that the presence of Ni retards the growth of the Cu3Sn layer while increasing the initial growth of the Cu6Sn5 layer can be addressed.