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A systematic experimental work has been carried out to understand the mechanism of Au diffusion to the solder interface as well as to investigate the effect of Au and Ni layer thickness on the reliability of BGA solder joint. BGA solder balls of Sn-37wt%Pb has been bonded on BGA solder bond pads of Au/electrolytic Ni/Cu by reflowing at 225°C for 0.5 minutes. The thickness of the Ni layer was varied from 0.35 mm to 2.8 mm and the thickness of Au was varied from 0.1 mm to 1.3 mm. Solid state aging up to 1000 h at 150°C have been carried out to simulate the ultimate interfacial reactions during an operation life of electronics devices. Cross-sectional studies of interfaces have been conducted by scanning electron microscopy (SEM) equipped with an energy dispersive X-ray (EDX) analysis to investigate the interfacial reaction phenomena. Ball shear tests have been carried out to obtain the interfacial strength and to correlate with the interfacial reaction products. After the shear tests, fracture surfaces have also been investigated to understand the fracture modes.