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Mechanical reliability issues such as cracking of LSI chips and deterioration of electronic performance of them caused by mechanical stress and strain in multi devices sub-assembly (MDS) structures were discussed analytically and experimentally. Local thermal deformation due to thinning of the LSI chips for mobile application causes large distribution of residual stress from -300 MPa to +150 MPa in the chips. The values of the maximum and the minimum stresses are strong function of the thickness of the LSI chips. In flip chip assembly structures, high tensile stress occurs near the edge of the back surface of the chips and at the edge of small bumps. High compressive stress remains in the center area of the thinner chips because of the reduction of their cross section. Such a wide range of the residual stress causes wide distribution of the shift of electronic performances of devices. Therefore, it is very important to optimize the MDS structures to improve the reliability of products.