Abstract: In this parper, initiation and propagation of the crack in flip chip solder joints under thermal shock, were studid and growth mechanism is analyzed by crack growth path and distribution of accumulated plastic work density and plastic strain at solder joints. Based on this study, it is seen that the crack formed outside solder joint, located in the interface between interfacial IMC and solder matrix, as cycles increasing, the crack propagated into solder matrix, and grew along the pad, the distribution of accumulated plastic work and density is consistent with crack growth direction. The discussion from crack growth mechanism shows that the interface locate in double-stress-concentration, and the plastic work is bigger, therefore the crack is inclined to form in the interface between the IMC and solder joint, as cycles increasing, the plastic strain of solder matrix is increased, which causes the region of higher solder matrix providing condition for crack propagation.