Solder ball reliability assessment of WLCSP — Power cycling versus thermal cycling

Abstract

Failure analysis and its effects are major reliability concerns in electronic packaging. More accurate fatigue life prediction can be obtained after the consideration of all affecting loads on the electronic devices. When an electronic device is turned off and then turned on multiple times, it creates a loading condition called power cycling. WLCSP use a package technology at the wafer-level, which is an extension of the wafer fab process where the final device is a die with an array pattern of the solder interconnects. The die is the heat source causing non-uniform temperature distribution. The solder ball reliability assessment of wafer level chip scale package (WLCSP) is done through Finite element analysis (FEA) under two different loads. In this paper, the power cycling is done to check its solder ball reliability by estimating stresses and failure life cycles. The mismatch in coefficient of thermal expansion (CTE) between components used in WLCSP and the non-uniform temperature distribution makes the package deform resulting in thermal stresses. Cross-sectioning of the PCB was also performed and it has been observed that the life of any assembly also depends on the copper content in the PCB board. This study shows the contradictory results from the literature, i.e. the reliability of thinner board will be more compared to thicker boards. The thermal cycling test is also done on the same package to compare the effect of power cycle and thermal cycle. For more comparative analysis, accelerated thermal cycling (ATC) is performed on WLCSP. The internal heat generation from power cycling shows more failure and more accurate results compared to thermal cycling.