Dispersion and the Worst Case of Thermal Fatigue Life of Solder Joints in Vehicle Electronic Devices

This paper presents mining the worst case of thermal fatigue life of solder joints on chip components used in vehicle electronics. Typical 36 cases with various solder shapes were examined. Thermal fatigue life including not only the crack initiation but also crack propagation was evaluated by using the finite element method. Based upon the case study´s results, it was found that when a crack propagates along the side of the component, the fatigue life of the components will not be affected by the dispersion of the solder joints. However, when the crack propagates into a fillet of solder, the fatigue life will drop greatly, and it will be affected remarkably by the dispersions of the shape of solder joints. Then, based upon the analytical results, it was found that there is a strong interactive relation between the solder joints on two sides of the component, the structural asymmetry was included in the sensitivity analysis processes. It was shown that compared with a standard model (which has a symmetrical structure where fatigue cracks propagate along the side of the component) the worst case has a big unbalance between the values of the two side solder joints. The fatigue cracks change their route from the side of the component to fillet of solder in the smaller side, and the fatigue life of the component shows an 80% decrease. It means that the shape of solder joints should be designed to control the failure mode.