Narrowing the gap in flip chips

Due to the high I/O density, low-cost assembly, and low package profile, DCA is the technology of choice in many current applications. However, DCA configurations are vulnerable to thermally induced strains and the resulting solder-joint fatigue, which, in turn, leads to reduced reliability. In recent years it has been found possible to obtain dramatically higher (5-10X) reliability and to operate successfully with much larger chip sizes, under comparable operating conditions and with identical solders, by underfilling a chip. Underfilling is done by filling the gap between the chip and substrate with epoxy, which also surrounds the solder joints. Underfilling between the chip and substrate reduces solder-joint deformation and shear strain.