Large Die Size Lead Free Flip Chip Ball Grid Array Packaging Considerations for 40nm Fab Technology

To reduce the RC latency, leading edge silicon nodes employ porous SiO2 dielectrics in the interconnect stack. Introduction of porosity lowers the dielectric constant, k, but also significantly decreases both the elastic modulus and fracture toughness of the dielectric. As such, devices manufactured in silicon processes that use low K (90nm, 65nm, and 55nm) and even more so extremely low K ( 45nm, 40nm, and 28nm) interlayer dielectrics are substantially more prone to fracture as a result of package induced stresses than non porous higher K dielectrics. Since the package stresses scale with die size and package body size and inversely with bump pitch, manufacture of large die and package size flip chip devices made with extremely low K dielectrics has proven to be challenging. The stress challenge is further exacerbated by the RoHS requirements for lead free packaging which requires higher process temperatures and somewhat higher yield point solders. The combination of increased stress and reduced mechanic...