Mechanism of Moisture Diffusion and Adhesion Degradation in Epoxy Molding Compounds

Interfacial adhesion is of important concern for multilayered structures such as microelectronic packages . Moisture can influence the interfacial adhesion through three mechanisms. The first mechanism is the intrinsic aggregating effect of water molecules upon direct presence at the interfaces and degrading the interfacial adhesion by bonding to the polymer chains. The second mechanism is that the absorbed moisture changes the mechanical properties of polymeric materials. For example moisture can change the elastic modulus and shift the glass transition temperature of polymers to lower values. This mechanism leads normally to a slight difference in the mode mixity of the measured fracture toughness of moist sample when compared to that of dry sample. The third mechanism is the swelling of polymeric materials upon exposure to moist environments and causing an additional mismatch between volumetric expansions of substrate and adhesives. This is even more pronounced when the joint between a polymer and metal is investigated. Since the metallic substrate is impermeable to moisture, only the polymeric adhesive absorbs moisture and causes mismatch in hygroscopic strains. In order to measure the intrinsic fracture toughness of a moisture preconditioned sample, the influence of hygroscopic swelling which induces an apparent change in the measured fracture toughness should be isolated.

 In this study a systematic investigation of absorption and desorption of moisture in epoxy molding compounds was conducted. Absorption of moisture was found to show a dual-stage non-Fickian behavior. The exposure of sample upon a virtual saturation (end of the first absorption phase) to dry environment was found to lead to an almost dry state with slight residual moisture content at the end. However, a dry state was not achieved when the samples with higher initial moisture content (which were kept in humid environment for a longer time) were exposed to dry environments. A residual moisture content was present which was a complex function of time exposed to moisture, sample geometry and temperature at which the desorption process takes place.

Adhesion between copper and EMC was also investigated under moisture. It was found, that some of adhesion loss upon moisture absorption up to virtual saturation may be recovered, when a proper annealing condition is used. However, if the samples were exposed for a longer time to humid environment, none of the adhesion loss can be recovered, because the second stage of moisture absorption has a permanent degrading effect.

Shirangi M.H., Fan X.J., Michel B., “Mechanism of moisture diffusion, hygroscopic swelling and adhesion degradation in epoxy molding compounds”, Proc. 41st International Symposium on Microelectronics (IMAPS), 2008, Providence, RI, USA, pp.917-923.  

regards.

Hossein

Contact: hossein.shirangi(at)izm.fraunhofer.de