Cooling rate dependent underfill fracture toughness

Abstract

The failure of organic packages during thermal cycling is often associated with failure of the underfill by fracture. The fracture toughness of underfills measured by applying a mechanical load to the material at a constant temperature is used as a measure of the propensity of underfill fracture. However, this fracture toughness does not take into account temperature effects including cooling rates. To include temperature effects a fracture toughness based on thermally applied stresses is defined and a method to measure this thermally induced fracture toughness is described. Results on two commercial underfills are presented. Comparison of the conventional, mechanically induced fracture toughness and the new, thermally induced fracture toughness shows that underfill fracture toughness including thermal effects is significantly smaller than the conventional values. This indicates that the conventional method overestimates the stress required to introduce fracture in these materials.