The microstructural investigation and thermo-mechanical reliability evaluation of the Sn-3.0Ag-0.5Cu solder bumped flip chip package were carried out during the thermal shock test of the package. In the initial reaction, the reaction products between the solder and electroless Ni-P bump of the chip side were composed of two layers, i.e., a continuous layer of (Ni,Cu)(3)Sn-4 and discontinuous particles of (Cu,Ni)(6)Sn-5, while a continuous (Ni,Cu)(3)Sn-4 IMC layer was formed between the solder and electroless Ni-P layer of the package side. It is found that the primary failure mechanism of the solder joints in this type of test was thermally activated solder fatigue failure. For the interpretation of the failure mechanism of the package, finite element analyses were also conducted. The finite element analyses revealed that the cracks were induced by the accumulation of the plastic work and creep strain. Inelastic strain accumulations are primarily affecting the crack initiation and growth time, so called the life of the solder joint in this thermal shock test condition. (c) 2005 Elsevier B.V. All rights reserved.