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 stresses 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 transient temperature effects during thermal cycling. To include temperature effects a fracture toughness induced by applying thermal 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 mechanical toughness method overestimates the underfill/package reliability that becomes subject to temperature change. The difference is explained using fracture energy concept. © 2010 Elsevier Ltd. All rights reserved.