Effects of underfill material properties on the reliability ofsolder bumped flip chip on board with imperfect underfill encapsulants

摘要

The effects of underfill material properties on the reliability ofsolder bumped flip chip on printed circuit board (PCB) with imperfectunderfill encapsulants were studied. Three different types of underfillimperfections were considered; i.e., (1) interfacial delaminationbetween the underfill encapsulant and the solder mask on the PCB (crackinitiated at the tip of underfill fillet); (2) interfacial delaminationbetween the chip and the underfill encapsulant (crack initiated at thechip corner); and (3) the same as (2) but without the underfill fillet.Five different combinations of coefficient of thermal expansion (CTE)and Young's modulus with the aforementioned delaminations wereinvestigated. A fracture mechanics approach was employed forcomputational analysis. The strain energy release rate at the crack tipand the maximum accumulated equivalent plastic strain in the solderbumps of all cases were evaluated as indices of reliability. Besides,mechanical shear tests were performed to characterize the shear strengthat the underfill-solder mask interface and the underfill-chippassivation interface. The main objective of the present study is toachieve a better understanding of the thermo-mechanical behavior of flipchip on board (FCOB) assemblies with imperfect underfill encapsulants