Abstract: Thermal fatigue life of ceramic ball grid array(CBGA)devices under thermal cycling conditions was presented in -55℃~125℃.Failure mechanism of the soldered joints including the germinating position and expanding direction of the cracks were observed and analyzed by optical microscopy.Results show that the crack in soldered joints germinats in the borderline all around the out most solder balls.With the increasing of thermal cycling times,the cracks expand from the borderline of out most solder balls to the ball center along the interfaces.It is found that the germination and expanding of the micro-cracks are caused by highly concentrated stress and strain as well as interaction between thermal cycling and creep.