We investigated the mechanism of crack propagation in micro-solder joints with a semiconductor structure and developed a new crack propagation model. In micro-solder joints, solder crack might propagate not only at the interface itself but also near the solder and land interfaces. Conventional crack propagation models based on fracture mechanics, however, cannot explain this phenomenon. In our model, the fatigue life of solder was evaluated based on accumulative damage during crack propagation, and a crack path was automatically calculated. Using this model, we analyzed the crack path of a ball grid array (BGA) structure and determined that this model can reproduce the above phenomenon.