Wafer level CSP with ultra-high thermal reliability lead-free alloys

Wafer-Level Chip-Scale Packaging (WLCSP) has become increasingly popular in portable electronics. One of its main characteristics is its reduced scale and that the solder balls are attached directly to the device. One of the main challenges in WLCSP is how to overcome the effects of thermal mismatch between the silicon die and the printed circuit board that arise from these characteristics. Use of new solder alloys is one of the ways to mitigate thermal fatigue stresses resulting from coefficients of thermal expansion mismatch. Tensile tests and high temperature creep tests were used for initial screening of the alloys and understanding the potential impact of each addition on the reliability of the solder in the final application. Here improvements in thermal, mechanical and metallurgical properties of the new alloy Maxrel Plus are discussed and compared to SAC405. Based on drop shock test, single ball shear test (high temperature storage, PCT and MSL1), thermal cycling test and intermetallics measurement results, we conclude that Maxrel Plus is specially recommended for use in WLCSP.