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Underfill fracture toughness as a function of cooling rate

Organic package failure during thermal cycling to low temperatures is often associated with underfill failure such as fracture. To categorize the propensity of underfills to fracture the fracture toughness measured by applying a mechanical load to the material at a constant temperature is used. However, this fracture toughness is insensitive to cooling rate. In order to account for cooling rate a fracture toughness based on a thermally applied stress is defined and a method to measure this thermally induced fracture toughness and results on a commercial underfill are presented. Thermal shifting factors used for the construction of a relaxation modulus master curve were obtained and used to calculate the rate-dependent thermal stress and thermal stress intensity factor. In separate experiments the mechanically induced fracture toughness of the underfill is determined at various isothermal temperatures. The two fracture toughness values are compared. The thermally induced underfill fracture toughness was found to be nearly 50% smaller than the one induced mechanically. Slower cooling rates resulted in greater thermal fracture toughness. The use of thermally induced fracture toughness values allows more realistic modeling of package lifetimes.