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Control of Void Formation in Adhesively Bonded Joints
Keywords: adhesive bonding, voids, modeling
Adhesively bonded joints are ubiquitous in electronic assemblies that are used in a wide range of applications, which include automotive, medical, military, space and communications. The steady drive to reduce the size of assemblies in all of these applications, while providing increased functionality, generates a need for adhesive joints of higher strength, improved thermal and electrical conductivity and better dielectric isolation. All of these attributes of adhesive joints are degraded by the presence of voids in them. The quest to minimize voids in bonded structures motivated this study of their formation in a solvent cast, die bond epoxy film, which undergoes a liquid phase transition during cure. Void formation is influenced by a number factors, which include wettability of the bonded surfaces, adsorbed water, amount of solvent retained in the film, volume of entrapped air, thermal profile of the cure schedule, and clamping pressure during cure. We have been exploring the interaction of these parameters and their impact on void retention in bonded joints with the aid of a moving boundary, finite difference model that tracks the movement of volatile species into and out of the void under diffusion control. The position of the void interface is updated at each discrete time step by integrating the product of the time step and flux of diffusing species over the area of the interface. The internal pressure of the void is determined by application of the Young-Laplace equation, while Henrys law is used to estimate the concentration of diffusing species adjacent to the void interface. The calculation proceeds for a time equivalent to the integral of the time temperature product required to achieve a 70% cure state of the adhesive, at which point the void interface is immobile. The adhesive we have chosen for this study is transparent in both the pre and post-cured states, so by sandwiching the material between glass slides we are able to observe void formation and evolution during cure. This allows us to directly observe the effects of various parameters for comparison with our predicted behavior.
Nina S. Dytiuk, Member of Technical Staff
Cambridge, MA

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