A Closed Form Method Combined with the Steinberg’s Solution to Estimate Number of Fatigue Cycles for Lead-Free Solders

Abstract

Solder joints of components are normally the most susceptible to the vibration loading conditions. The Steinberg’s closed form solution has been widely used in industries to identify high risk components to be candidates for detailed finite element (FE) durability analysis under vibration loads. Unfortunately, the Steinberg’s closed form solution is only applicable to the SnPb, not the lead-free materials (SAC); therefore, it would be troublesome to identify high risk SAC components, especially if there are numerous SAC components in the BOM. This paper is to present a method that would be able to combine with the Steinberg’s closed form solution to identify high risk SAC components. This method is developed from comparison between SnPb and SAC fatigue vs. strain range relationship using the high and low cycles of fatigue Coffin Manson closed form equation. In addition, this method can also predict solder joint fatigue cycles of one material using the already-derived fatigue cycles from another material without re-running detailed FE analysis. This additional feature would help, for example, to make prediction if re-ball from SAC to SnPb or vice versa would pose any risks. It is strongly recommended to utilize this method when assessing lead-free components under vibration as currently it is only available method to serve this purpose.