A thermo-mechanical approach for fatigue testing of polymer bimaterial interfaces

We have developed a new technique that uses a noncontact fiber optic displa cement sensor to investigate the crack growth along polymer interfaces unde r thermal fatigue conditions. This technique has been used to test the unde rfill/passivation interface of a direct chip attach (DCA) assembly, the the rmal fatigue driven delamination of which is a major cause for failure of D CA assemblies. The sample is prepared as a multilayered cantilever beam by capillary flow of the underfill over a polyimide coated metallic beam. Duri ng thermal cycling the crack growth along the interface from the free end c hanges the displacement of this end of the beam and we measure this displac ement at the lowest temperature in each thermal cycle. The change in beam d isplacement is converted into crack growth knowing the geometry of the spec imen. The crack growth rare depends on the maximum difference in the strain energy release rate of the crack in each cycle and the mechanical phase an gle. This paper outlines the theoretical basis of the technique and provide s initial results obtained for a variety of underfills dispensed over a com mercial (PMDA/ODA) polyimide. The technique was validated by comparing the crack growth measured by displacement changes with direct optical microscop y measurements of the crack length.