SAC305 solder fatigue crack propagation under 3-point bending cycle test condition

SAC305 solder is the most common lead-free solder used in electronic industry to connect micro-electronic packages to the printed circuit board. Although the material is quite robust against cycling loading, solder joint reliability is still one of the major factors limiting the lifetime of electronic assemblies and accelerated stress tests are needed to speed up solder joint reliability assessment. In this study, the authors propose an isothermal 3-point bending cycling test (3-PB) applied to a simple test vehicle (TV), which consists of a silicon die with backside metal pads soldered to a metal substrate. The dimension of the die and the solder pads are similar to characteristic size of lead frame-based power packages used in microelectronics. A non-destructive method (Scanning Acoustic Microscopy, SAM) is used to study the solder fatigue crack propagation during cycling test. It is found, that for these TV assemblies, the majority of the solder lifetime is governed by the crack growth regime, while crack initiation accounts only for about 10%~20% of the total lifetime. Crack investigation is done for different loading conditions and finite-element analysis is performed to assess accumulated equivalent creep strain of solder joints for the different test conditions. Experimental and simulation results are then combined into a solder joint fatigue model for lifetime prediction for crack initiation, crack growth rate and end-of-life.