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

We have developed a new technique that uses a noncontact fiber optic displacement sensor to investigate the crack growth along polymer interfaces under thermal fatigue conditions. This technique has been used to test the underfill/passivation interface of a direct chip attach (DCA) assembly, the thermal fatigue driven delamination of which is a major cause for failure of DCA assemblies. The sample is prepared as a multilayered cantilever beam by capillary flow of the underfill over a polyimide coated metallic beam. During thermal cycling the crack growth along the interface from the free end changes the displacement of this end of the beam and we measure this displacement at the lowest temperature in each thermal cycle. The change in beam displacement is converted into crack growth knowing the geometry of the specimen. 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 angle. This paper outlines the theoretical basis of the technique and provides initial results obtained for a variety of underfills dispensed over a commercial (PMDA/ODA) polyimide. The technique was validated by comparing the crack growth measured by displacement changes with direct optical microscopy measurements of the crack length.