Investigation of Mirror Image Package Performance Under Thermal Cycling and Thermal Shock Tests

摘要

Reliability is one of the most critical issues encountered in mirror image assemblies. In order to improve the reliability, typical structures of chip scale packages (CSPs) were analyzed that included flex interpose and molded flex types. A comparison of those CSPs between the mirror image and single sided was performed using reliability testing under thermal cycling (0 to 100°C) and liquid-to-liquid thermal shock (-55 to 125°C). The experimental results were then analyzed further using finite element modeling (FEM). From this study, the mechanism of the degradation of reliability of mirror image CSPs was understood. It was found that the reliability degradation was dependent not only to the out-plane deflection of board and assembly sequence but also to what types of CSPs were used. Flex interpose CSPs are more compliant and therefore less dependent on each other in relation to the mirror sides. However, on the other hand, mirror image CSPs with less compliant structure may have lower reliability. A novel mirror image design was therefore proposed after analyzing several various approaches to improve the reliability. A preliminary result shows it is possible to improve the reliability of mirror image CSP to the same level as that of single-sided CSP. Moreover, it is possible to get a higher reliability design of mirror image assembly if the CSPs with lower reliability are mirrored with certain types of CSPs.