Abstract
In this paper the authors have investigated the thermal fatigue reliability of lead-free solder joints. They have focused their attention to the formation of the intermetallic compound and its effect on the initiation and propagation behaviors of fatigue cracks. An isothermal fatigue test method was used in this study to improve the efficiency of fatigue study, and several different lead-free solder alloys, Sn-Ag-Cu, Sn-Ag-Cu-Bi, Sn-Cu and Sn-Zn-Bi were investigated. There are two kinds of fracture mode in lead-free solder joints, one is solder fatigue mode, and the other is an interface fatigue mode. Based upon the experimental results, it was found that not only is the mode transition of the fatigue crack affected by the properties of the intermetallic layer but also is affected by the tension strength of the solder material. If the tension strength is lower than a critical value, the fatigue cracks in the solder joints appear within the solder domain, that is the solder fatigue mode, and their fatigue life can be assessed by Manson-Coffin's law of the bulk solder material. On the other hand, if the strength stress is higher than that value, the interface between solder and Cu pad breaks much earlier than the solder fatigue life, that is the interface mode.
