The use of underfills in electronic applications is becoming more prevelant with the decrease in package pitch to 0.5 mm and the increase in I/Os. Underfill encapsulation is typically used in flip chip on laminate assemblies to more evenly distribute and minimize the solder joint strains, thus improving thermal cycling fatigue life. The material constitutive and damage behavior of underfills is however poorly understood. Typical underfill material data sheets often do not provide the parameters required for development of accurate predictive models. In this paper a new methodology for preparation of thin uniaxial test samples for mechanical testing of underfills has been used to better understand the non-linear constitutive behavior of underfills. Bulk underfill samples exhibit different behavior because of non-uniform curing and the effect of sample thickness on the response of underfill layers. A microscale tension-torsion testing machine has been used to measure stress-strain, creep, and stress relaxation behavior if several underfills as a function of temperature. Thermal-fatigue reliability response of various permutation of underfill materials have been analyzed using statistical models. The effect of thermal aging, thermal cycling, and moisture preconditioning on the constitutive behavior of materials have been analyzed. Models have been developed to represent the underfill behavior in an operating range of −40 to 125C.

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