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Numerical and experimental techniques were employed to assess the thermomechanical behavior of ceramic and organic flip chip packages under power and accelerated thermal cycling (ATC). In power cycling (PC), the non-uniform temperature distribution and different coefficients of thermal expansion (CTE) of each component make the package deform differently than in the case of ATC. Conventionally, reliability assessment is conducted by ATC that assumes uniform temperature throughout the assembly. This is because ATC is believed to be a worse case condition compared to PC, which is similar to actual field conditions. For ceramic and organic flip chip ball grid array (FC-BGA) packages, numerical simulations of ATC and PC were performed by a combination of computational fluid dynamics (CFD) and finite element analyses (FEA). For PC, CFD analysis was used to extract transient heat transfer coefficients while subsequent thermal and structural FEA was performed with heat generation and heat transfer coefficient from CFD as thermal boundary condition. The numerical simulations were compared with an in-situ, real-time moire´ interferometry experiment. It was found that, for certain organic packages, power cycling was the more severe condition that caused solder interconnects to fail earlier than ATC, while ceramic packages fail earlier in ATC than PC. Accordingly, qualification based on ATC testing may overestimate the life of the package.