Water-assisted sub-critical crack growth along an interface between polyimide passivation and epoxy underfill

Direct chip attach (DCA) microelectronic packaging technology is gaining pr ominence due to its numerous advantages. Delamination (debonding) of the un derfill epoxy/ polyimide passivation interface of a DCA during hydro-therma l reliability testing has always been one of the salient problems. We have studied the water-assisted sub-critical crack growth along this interface a nd our measurement offers important clues as to the origins of the poor hyd ro-thermal testing results for these interfaces. A modified asymmetric doub le cantilever beam (ADCB) testing technique has been used to measure the su b-critical crack growth velocity v at various relative humidities and tempe ratures as a function of the crack driving force (strain energy release rat e) G*. The presence of a significant partial pressure of water p(H2O) produ ces a marked decrease (by up to a factor of 12) in the threshold G* for cra ck growth at measurable velocities. Above the threshold log v rises linearl y with rootG* but then enters a regime where the crack velocity (v = v*) is almost independent of rootG*. Finally, at the values of G* corresponding t o rapid crack propagation in the absence of water, log v increases very rap idly with G*. By analogy to the classic work on water-assisted sub-critical crack growth in silica-based glasses, where very similar features are obse rved, we believe that the sub-critical crack growth along the polyimide-epo xy interface results from stress-assisted hydrolysis of primary covalent bo nds, in our case ester bonds across the interface. The regime of rootG* jus t above the threshold corresponds to a physicochemical situation where the water activity (p(H2O)) at the crack tip is the same as that of the gaseous environment. In the regime where v = v* approximate to constant, the water activity at the crack tip is below that in the environment and the crack g rowth velocity is limited by the transport of water vapor to the bonds ahea d of the crack tip. We develop a model of this crack growth following Wiede rhorn 1967 that allows us to predict the sub-critical crack growth as a fun ction of G* for arbitrary relative humidity and temperature conditions.