Wafer-applied underfill film laminating

Abstract

Wafer-applied underfill systems for flip-chip bonding have been developed to overcome throughput limitations imposed by capillary underfill systems. The wafer-applied underfill system consists of a highly filled film laminated to a bumped IC wafer and a liquid flux adhesive dispensed on the board. The wafer is diced after film lamination and the individual chips carry the underfill through the assembly process. The flux adhesive is dispensed at the bond site just prior to chip placement. These underfill materials undergo most or all of the required cure through a single reflow oven pass. A critical concern for a successful wafer-applied underfill bond is the prevention of discontinuities or voids in the underfill layer. Voids in the underfill can weaken the solder reinforcement otherwise provided by the underfill or permit the formation of between-bump shorts caused by solder extruding into the void. The wafer-applied underfill film laminating process presents one possible source for discontinuities. Since the film is laminated to a bumped wafer, the complete encasement around each solder bump by the wafer-applied underfill film is a necessity. A lamination process used to transfer the wafer-applied underfill film to the bumped wafer without such discontinuities will be described. This lamination process has been shown to provide uniform lamination over a full wafer with complete encasement of the solder bumps. Wafer-applied underfill and flux material characteristics and reliability results also will be reported.