Reliability of power cycling for IGBT power semiconductor modules

Power cycling capability is one of the most important reliability items in application semiconductor modules. This paper describes the mechanism of power cycling by analysis of the structure of lead-based solder and joint failure due to fatigue was studied to ascertain improvement in the power cycling lifetime. Lead-based solder has a relatively weaker yield-strength which means that plastic deformation leads to deterioration. Therefore, tin-silver based solder was selected as a higher yield-strength material, and a new composition which has both excellent mechanical properties and wettability was developed. Concerning the failure mechanism of power cycling for this newly developed solder material, solder joint failure and aluminum wire bond failure were clearly distinguished, and the lifetime prediction curve was clarified by FEM analysis and detailed failure analysis. Furthermore, it was proved that the deterioration of new tin-silver based solder is caused by thermal fatigue with the grain growth of tin. Thus, longer power cycling capability is achieved by using this newly developed solder instead of conventional lead-based solder.