Thermomechanical behavior of underfill/solder mask/substrate interface under thermal cycling

With the increasing application of flip-chip technology in the microelectronics industry, the adhesion strength of interfaces in flip-chip microelectronic structures has become an important issue for manufacturing and operation. In this paper we present an experimental investigation of the adhesive strength of underfill material to solder mask coated FR-4 substrate under thermal cycling. The effects of the number of thermal cycles on interfacial strength were investigated by using the button shear test. The relationship between interfacial strength and the thickness of solder mask was also examined. Furthermore, the morphologies of fracture surfaces of the test specimens were analyzed by scanning electron microscopy. The results of this study show that the interfacial strength of the underfill/solder mask/substrate joint was significantly reduced by thermal fatigue. Finally, the degradation behavior and possible mechanisms were then determined on the basis of these observations.