Reliability Impacts on Flip Chip Packages: Moisture Resistance, Mechanical Integrity and Photo-Sensitive Polyimide (PSPI) Passivation

In this paper, the effects of moisture sensitivity during preconditioning (30 degrees C/60% RH, 192 hours) tests and material property changes after reliability stressing on flip chip chip-scale package (FCCSP) were comprehensively investigated by various experimental data as well as theoretical explanation. Since the integrity of FCCSP is dependent on the mechanical integrity of underfill and PCB used in package assembly process, deep insight was given on the material properties in order to understand degradation mechanism induced by the combined stresses of preconditioning test and environmental stresses in a sequence. As a result of DOE (Design of Experiment) for 2 different PCB, failures were found only in J PCB because of a large CTE change before and after moisture absorption. After moisture absorption, large CTE change from 27.78 to 32.04 of J PCB could aggravate the thermal mismatch between a PCB and an underfill, it caused shear displacement by more than 2,309 ppm in the interface. According to the DOE for underfill, we verified that higher modulus underfill could improve the reliability of flip chip packages. Based on our works, we recommend the optimized value for underfill modulus is from 8 GPa to 12 GPa. We explained logically two different failure mechanisms of delamination. One is induced by CTE mismatch of PCB, and the other is by underfill modulus by means of electron microscope. Finally, as to reliability concerns of moisture resistance arisen from the absence of the photo-sensitive polyimide (PSPI) passivation layer, we demonstrated that potential risk is minimal if FCCSP is assembled with an appropriate underfill as well as PCB.