Influence of chemistry and stresses on reliability of polymer and substrate interfaces

Abstract

Gradual failure of the interfaces in corrosive environment was observe when the interfaces were subjected to non-catastrophic mechanical loadings. Coupling agents are used as additives in epoxy based adhesives to improve the adhesion by introducing chemical bonding across the interface. The chemistry of coupling agent, acidity, temperature, and stress have been observed to affect the rate of adhesion strength degradation and jeopardize the reliability of electronic packages. The degradation of the interface between coupling agent enhanced epoxy base underfill and glass substrate were investigated. The gradual degradation of adhesion strength is a mechanochemical process, whch involve stress assisted chemical reaction between attacking chemicals and the stressed bonds at the crack tip. The debondling process under the combined influence of chemistry and stress is dependent on the activation energy of the reaction (ΔG₀^*), as well as the activated area coefficient per density (λ/ρ). In thls study, a combination of mechanical testing methods is developed to quantify the interfacial debonding process under controlled chemical environment and mechanical loading. Shear fracture test, an adaptation of the die shear test, is developed to characterize the degradation of the interface under controlled chemistry. Immersed tapered double cantilever beams test is used to characterize the combined influence of chemistry and stress on the interfacial degradation. Debonding rate of underfill adhesives with silane coupling agent, titanate coupling agent, and zirconate coupling agent were characterized at different temperatures, acid concentrations, and applied loadings. The results indicate that acid is a catalyst in the degradation of interfacial bonding. The interface enhanced with silane coupling agent has the highest activation energy ΔG₀^* for hydrolysis, which means that it is the least susceptible to acid attack compared to adhesive with titanate coupling agent and zirconate coupling agent. The rate of debonding under elevated temperature is determined by both the exponential term and the pre-exponential product term in the chemical kinetic model. The interfacial degradation is strongly dependent on temperature because of the double exponential functional form of the chemical kinetic model for degradation. A small change in the activation energy would result in a large change in the degradation rate of the interface. The effect is particularly significant as temperature elevated. The influence of applied stress on the interfacial degradation rate is characterized by the activated area coefficient per density λ/ρ. Data in this study indicated that silane coupling agent, with the highest λ/ρ, is most susceptible to stress assisted debonding compare to titanate and zirconate system. On this basis, package delamination can be minimized by selecting material systems with high activation energy and activated area coefficient per density.

Date of Award	2001
Original language	English
Awarding Institution	The Hong Kong University of Science and Technology