Skin effect and material degradation of lead-free solder joint under AC

Sine waveform AC skin effect of lead-free Sn95.5%Ag4.0%Cu0.5 (SAC405) solder joints was investigated experimentally. It was found that 1 MHz is critical frequency above which skin effect becomes essential for lead-free solder joints. Since our concerned frequency in this study is far below 1 MHz, skin effect is not ignored in computer simulations. Sine waveform alternating current (AC) was used to simulate the electromigration (EM) and thermomigration (TM) response. It was found that 1000 K/cm thermal gradient between top of solder joints and the bottom side induces much larger damage than pure AC electromigration. By using entropy based damage model, EM induced material damage was observed proportional to AC loading frequency f^0.40, current density j^0.28, and ambient temperature T^1.2. Frequency has an even larger impact on EM damage of solder joints than current density. Exponential relationship is observed between damage evolution and current loading time. Due to the material healing effect at load-off time, we can predict that lifetime of conductors subjected to AC current loadings should be much longer than those of DC under otherwise the same conditions.