A system for underfilling in a chip package includes an underfill mixture that ameliorates the CTE mismatch that typically exists between a packaged die and a resin-impregnated fiberglass mounting substrate. In one embodiment, the system includes an underfill mixture that comprises a principal underfill composition of a rigid octaaminophenyl silsesquioxane (OAPS) that is used as a curing agent for a tetrafunctional, low viscosity, and relatively rigid TGMX epoxy resin. An embodiment is also directed to the assembly of a flip chip package that uses the underfill mixture.