The Determination Of The Residual Stress State And Its Effect On Peel Tests In Polyimide Coatings.

Abstract

ABSTRACTThe determination of residual stresses in thin polymer coatings is crucial in the prediction of failure due to delamination and cracking. This is especially important in the electronics industry where thin polymer dielectric layers are used. The dielectric layers are often under three-dimensional constraints and have holes through which metal interconnects run. These act as locations of high stress concentration. Three complementary methods have been developed which will provide a better base for understanding the delamination and performance of thin coatings.These techniques include, a holographic interferometry method for determining the residual stresses in coatings (˜10 μm thick); a deformation calorimetry technique for establishing an energy balance for peel tests; a set of experiments for measuring the nine elasticity coefficients for orthotropic films. The combination of these methods allows a comparison between the residual stresses present in coatings and the contribution of those residual stresses to the work needed for peeling. The knowledge of the stress state and elasticity coefficients is essential for determining the elastic energy in a coating. The total elastic energy which can be stored in a stressed coating increases with the coating thickness. Stressed coatings exceeding a critical thickness will spontaneously delaminate if the elastic energy in the coating is greater than the energy required for debonding. The elastic energy in the coating per unit area at the critical thickness is a measure of the interfacial strength.