Void free processing of flip chip on board assemblies using no-flow underfills

A consistent problem that has plagued the introduction of innovative no-flow underfill materials into volume flip chip onboard/in-package production is voiding of the underfill during processing. In addition to outgassing voids typically due to moisture and solvents in the substrate or outgassing of the underfill, voids are produced as a result of the squeeze flow experienced by the underfill during chip placement. This paper presents a systematic development of optimal flip chip on board assembly process for no flow underfill materials presenting process parameters compatible with four commercially available no flow fluxing underfills. A novel hybrid process is developed that combines a capillary flow dynamic with no-flow fluxing underfills resulting in virtually a void free assembly process. The results of the initial studies are used to choose an optimal process for the materials. The newly developed edge patterned hybrid no-flow process has resulted in near void-free assemblies capable of passing 2000 cycles without an electrical failure for the -40 to 125°C AATC reliability test.