Mechanical Reliability and Bump Degradation of ACF Flip Chip Packages Using BCB (Cyclotene™) Bumping Dielectrics Under Temperature Cycling

Interface degradation between dissimilar materials in the flip chip packages with anisotropic conductive adhesive joint is susceptible to mechanical and electrical failure upon temperature cycling. Particularly the mechanical reliability of flip chip using anisotropic conductive films (ACFs) often depends upon the interface characteristics between the bumping/passivation dielectrics and adjacent materials. This paper investigates the delamination and cracking in polymeric bumping/passivation dielectrics (Cyclotene™ 4024) and the damage mechanism of interconnect bump with BCB bumping dielectrics upon temperature cycling. Adhesion and fracture surface morphology after die shear testing show that the weakening and delamination of BCB passivation layer at the die corner is the main cause of the reduction of die adhesion strength after the thermal cycling. The sliding trace on fracture surface after three-point bending fracture reveals that cyclic shear displacement in a polymeric BCB layer fatigue the interface bonding or interconnect bump upon thermal cycling, leading to mechanical delamination and functional bump failure. Passivation cracks also develop around the circumference of BCB passivation over aluminum pad. A model is presented for the degradation of interconnect bump with surrounding BCB passivation.