SAM interpretation of interfacial anomaly in flip-chip BGA package with 65 nm Cu/low-κ integrated circuits device

In the course of qualifying a flip chip BGA package with a 65 nm Cu/low-κ device, an unusual interfacial phenomenon was detected at the interface between die and underfill using a scanning acoustic microscope. Inhomogeneous acoustic intensity with oriented "scallop-like" pockets was found around the solder bumps at the die and underfill interface. Close inspection of the underfill material after cross-sectioning and planar lapping revealed non-uniform filler distribution, resulting in the formation of resin-rich "scallop-like" pockets around the bumps. A correlation between inhomogeneous acoustic intensity and filler distribution is discussed and the physical methodology of analysis is described in this paper.