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The effect of concurrent vibration and electrical power loads on the solder interconnections of a surface-mount power transistor package has been investigated in this work. Both cyclic and constant power loadings were separately combined with vibration over a wide amplitude range. Single load vibration and power cycling tests were conducted for comparison. In addition to lifetime analysis, the failure modes occurring under each test case were carefully studied from cross-sectional samples, and the failure mechanisms were rationalized with the help of finite-element calculations and microstructural analysis. A substantial reduction in interconnection lifetimes was observed in the combined load tests as compared with the lifetime under single load tests. Three different failure modes were found within all the different test cases: 1) ductile crack propagation through bulk solder, 2) recrystallization assisted crack propagation, and 3) mixed mode propagation with both mechanisms. The failure mode changes were dependent mainly on the magnitude of plastic strain induced by the mechanical vibration. The results of this study provide insight in designing more comprehensive reliability tests as well as achieving higher levels of test acceleration without compromising the validity of results.