Electromigration Reliability of SnAgCu Solder Joints in WLCSP

This study focuses on the electromigration behavior and the mean-time-to-failure (MTTF) analysis of SnAgCu solder joints in the temperature range of 130 to 160°C with current density of 0.96-1.59×l04 A/cm
²
. The Sn4.0Ag0.5Cu (SAC405) solder bumps are in a WLCSP assembly with Ti/Cu UBM on the chip side and Cu pad on the substrate side. The effective activation energy and exponent associated with current density of SAC405 solder joint are obtained, which is 0.89 eV and 2.23. The electromigration simulation of a three-dimension dual bumps model was employed to investigate atom density and void formation in the solder joints. The goal was to establish a relationship with the experimental electromigration results and to devise a technique for predicting the lifespan of electromigration. Threshold failure atom density 0.966 was gained. The test structures may experience both electromigration and thermomigration under severe condition. The most vulnerable area is the junction between RDL and UBM on the solder ball where the electron flows downwards. The direction of thermomigration in relation to electromigration plays a vital role in determining the failure rate and final morphology of the solder joints.