Study of Polyimide in Chip Package Interaction for Flip-Chip Cu Pillar Packages

Chip-package interaction (CPI) is a key area for achieving robust copper bump interconnection in flip-chip packages. Polyimide (PI) has been widely used in electronic package products to provide structural support to protect electronic devices from excessive stress. Passivation crack and LK/ELK delamination are two polyimide related failures in flip chip packages and pose significant challenges to achieving robust CPI design. A 3D finite element model was developed to evaluate the role of polyimide in CPI stress mitigation. For the passivation crack, a failure metric, in-plane radial passivation stress has been proposed to evaluate the passivation crack risk. The simulation shows that the in-plane radial stress of passivation layer is well correlated with the test results. The location of passivation crack matches the peak radial stress location within the passivation layer. The simulation compared the in-plane radial stress between package with and without PI and found the in-plane radial passivation stress of packages with PI layer was significantly reduced compared with packages without PI layer. The simulation demonstrates PI can provide structural protection against in-plane radial stress caused by thermal cooling after chip mass reflow. For ELK delamination, the simulation shows PI opening (PIO) has a big impact on the ELK peel stress. It has been found PIO plays a different role in ELK peel stress due to in-plane shear deformation and out-of-plane thermal expansion. When stress is induced due to in-plane shear deformation, smaller PIO and stiffer PI can provide better stress buffer on ELK layers. Larger PIO and stiffer PI can help to reduce the ELK peel stress induced by out-of-plane thermal expansion. This study presents two failure stress metrics from the perspective of fracture mechanics and further investigates the role of PI in flip chip Cu pillar packages.