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This paper focuses on the intermetallic compounds (IMCs) of Sn95.8-Ag3.5-Cu0.7/Au/Ni/Cu lap-shear solder joints subjected to thermal cycling and isothermal aging. The behaviour of interfacial Sn-Cu-Ni IMC layer, and Ag-Sn and Sn-Cu IMCs within the solder matrix over the testing timeframe is presented. The results of mechanical shear test on the solder joints at various time intervals are presented. It is found that the shear strength of a solder joint subjected to thermal cycling decreased with the number of cycles, corresponding to the increase in Sn-Cu-Ni IMC thickness and the change in dispersion pattern of Ag-Sn IMCs in the bulk solder matrix. Furthermore, brittle fracture of the solder joint along the interfacial Sn-Cu-Ni IMC layer is responsible for the deterioration of solder shear strength at 1000 and 2000 thermal cycles.