Flip chip processing using wafer-applied underfills

Wafer-level underfill has the potential to substantially increase the implementation and usage of flip chip technology in the electronics industry. The development of wafer-level underfills can bring the financial benefits of wafer-level processing to flip chip assembly and packaging. In order to realize these benefits, the wafer-level flip chip process should be transparent to standard assembly lines. Experimental studies have identified a number of process-related defects, including underfill voids, underfill outgassing, and die misalignment resulting from the solid to liquid transition of the wafer applied underfills and their associated surface tension, in the current study, aspects of assembly processing that relate to wafer-level flip chip assembly quality are examined. A parametric study of the effect of underfill coating uniformity on assembly quality and underfill voiding is presented. To address the surface tension driven die misalignment, a method is explored for reducing die misalignment through the use of fillet-constraining solder mask patterns. In addition, a theoretical description of the forces acting on a wafer-level flip chip die is developed to better understand the influence of process and design parameters on assembly yield.