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Mechanism of Moisture Diffusion, Hygroscopic Swelling and Adhesion Degradation in Epoxy Molding Compounds
Keywords: moisture, adhesion degradation, mechanical behavior
Epoxy Molding Compounds (EMC’s) are widely used as the encapsulants for protecting the packages against the unprotected environments. One disadvantage of these materials is that the epoxy molding compound absorbs moisture from environment. This will cause many reliability problems, including interface delamination and popcorn effect during the solder reflow process. We have investigated the effect of moisture on many encapsulated packages and bulk EMC’s. Firstly, the moisture absorption of EMC’s shows a dual-stage non-Fickian behavior, which was modeled by a finite element analysis in our previous works. Secondly, the desorption experiments show an irreversible residual moisture content, which remains in materials at temperatures lower than glass transition temperature. Although, the elastic modulus shows not to be affected by moisture, the glass transition temperature shifts slightly toward the lower values. The influence of moisture on the poisson’ ratio was investigated using the image correlation technique. A high increase of Poisson ’ration from 0.22 to 0.48 was observed as a result of moisture absorption. The core of this work is the effect of moisture on the adhesion of EMC’s. A sandwich specimen was developed in order to measure the interface fracture toughness of the Leadframe/EMC interface. The adhesion was measured periodically as a function of exposed time to humid environment and the interfacial fracture toughness was gained by the four-point bending test. In order to consider the effects of the residual stresses from the manufacturing process on the measured fracture toughness, a finite element analysis of the whole manufacturing process was applied, which considers the influence of cure shrinkage, CTE-mismatch and hagroscopic swelling of the EMC. A degradation model of the adhesion as a function of moisture concentration is suggested in this work, which allows for the measurement of the interfacial moisture diffusion.
M. Hossein Shirangi,
Fraunhofer Institute IZM, Micromaterial Center Berlin
Berlin BW 13355,

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