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

The effects of underfill material properties on the reliability of solder b umped flip chip on printed circuit board (PCB) with imperfect underfill enc apsulants were studied in this paper, Three different types of underfill im perfections were considered; i.e., (1) interfacial delamination between the underfill encapsulant and the solder mask on the PCB (crack initiated at t he tip of underfill fillet), (2) interfacial delamination between the chip and the underfill encapsulant (crack initiated at the chip corner), and (3) the same as (2) but without the underfill fillet. Five different combinati ons of coefficient of thermal expansion (CTE) and Young's modulus with the aforementioned delaminations were investigated. A fracture mechanics approa ch was employed for computational analysis. The strain energy release rate at the crack tip and the maximum accumulated equivalent plastic strain in t he solder bumps of all cases were evaluated as indices of reliability. Besi des, mechanical shear tests were performed to characterize the shear streng th at the underfill-soldier mask interface and the underfill-chip passivati on interface. The main objective of the present study is to achieve a bette r understanding in the thermo-mechanical behavior of flip chip on board (FC OB) assemblies with imperfect underfill encapsulants.