Abstract
Delamination of the underfill/passivation interface due to thermal fatigue is a major cause for failure of DCA assemblies. Hence, we have developed a new technique based on a non-contact fiber optic displacement sensor to investigate the crack growth rate per cycle da/dN of an interfacial crack under thermal fatigue conditions. 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 rate depends on the maximum difference in the strain energy release rate of the crack in each cycle. This paper outlines the theoretical basis of the technique and provides initial results obtained for a commercial underfill 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.
