Reliability Improvement Research of 2.5D CoWoS package

Abstract

The 2.5D CoWoS(chip on wafer on substrate) package contains many materials such as ASIC(Application Specific Integrated Circuit), HBM(High Bandwidth Memory), Si interposer and substrate. The CTE (coefficient of thermal expansion) of various materials are different and the structure is complicated. Temperature changes cause structure deformation because of thermal mismatch. This thermal mismatch causes great thermal stress, which results in reliability failure. Reliability failure mainly occurs in the thermal cycling test, and may cause the underfill material and EMC (epoxy molding compound) delamination and crack. Therefore, this document focuses on CoWoS thermal stress failure, analyzes and verifies different substrates and assembly materials, confirms the optimal failure improvement solution through the finite element simulation model and the actual verification data. According to the test data, the underfill material and EMC delamination and crack are mainly caused by the large materials CTE. During the thermal cycle, the stress is concentrated and the EMC adhesion is insnderfillficient. Selecting Low CTE substrate core and underfill materials, increase substrate core and Ring thickness and optimizing the assembly parameter can effectively reduce product CTE, Improve reliability failure problems. Therefore, through the above analysis and improvement of thermal mismatch stress failure, the CoWoS package reliability pass rate can be provided.