Two subjects are included in this thesis; one is to construct the Coffin-Manson equation of the unleaded SnAgCu solder according to the experimental results provided by the Metal Research Laboratory (MRL) of Industrial Technologies Research Institute (ITRI). The results of CSP thermal cycle fatigue and SOJ pull tests and the corresponding stress and strain distributions solved from FEM analyses have been used to derive the Coffin-Manson equation for the SnAgCu solder. The other subject is to investigate the effect of CTE mismatch on the fatigue life of solder balls in the opto-electronic packaging. The solidified shapes of the different solder balls after undergoing the re-flow process are predicted by employing the Surface Evolver package program. The FEA meshes of the solidified solder balls in opto-electronic packaging are built according to the output results of the Surface Evolver program. The maximum equivalent plastic shear strain range of the solder after under one thermal cycle process is calculated by employing the MARC finite element package. The fatigue lives of solder balls under different arrangements are estimated according to the proposed Coffin-Manson equation. The effect of solder ball parameters, i.e. solder volume, solder offset distance, solder DNP and solder material on the reliability of different solder balls has also been explored in this thesis.