In this thesis, the relation between shear load and displacement for the lead-free solder (Sn3.0Ag0.5Cu) and the tin-lead solder (63Sn37Pb) are investigated. Except that, a new shear strength of the solder balls is suggested with considering the plastic strain energy of the solder balls. Three diameters of the Sn/Ag/Cu and Sn/Pb solder balls are studied. The variation of the plastic strain energies for the balls undergone different number of thermal cycles is compared. The effect of high temperature aging on the shear strength is also discussed. The difference between the failure fractures of the Sn/Ag/Cu and Sn/Pb solder ball are executed by using SEM. The experimental results show that the failure mechanism for the Sn/Ag/Cu is quite different from the Sn/Pb solder ball. Generally, the lead-free Sn/Ag/Cu solder is much ductile than the Sn/Pb solder ball in the shear test. Also the better fatigue performances are observed for the Sn/Ag/Cu solder balls.