Determination of Residual Strains of Cured Adhesives and Their Effects on Warpages in Electronic Packaging

Polymeric adhesives are popular in the application to electronic or optoelectronic packaging. Residual strains (stresses) of the adhesives have to be determined and analyzed for better reliability design of the joints or bump joints. The purposes of this study are to quantify residual strains of two adhesives: A (paste type) and B (film type) due to chemical shrinkage, stress relaxation, and thermal- and moisture-loadings, and to investigate their effects on the warpage of die attachment assembly. First of all, the mechanical properties of the two adhesives are measured in terms of temperatures by dynamic mechanical analyzer (DMA) and thermal mechanical analyzer (TMA). The residual strains are documented by testing fully-cured adhesive/silicon bi-material plates under thermal and moisture loading using Twyman-Green (T/G) interferometry system associated with Timoshenko´s bi-material theory and finite element method (FEM). Finally, the warpage of silicon/adhesive/Cu tri-material plates with both adhesives under thermal and moisture loading are also investigated by T/G experiments and confirmed by Suhir´s solution and FEM results. For adhesive-A, the results of the bi-material plate suggest that the residual strains are only induced by CTE mismatch during thermal loading, rather than other factors, right after the fully cured adhesive cooling down to room temperature. On the other hand, for adhesive-B, it was found that the additional residual strain of this film adhesive caused by chemical shrinkage plus stress relaxation, is about 2.26times10^-3, which accounts for 85% of thermal-residual strains, after the cured adhesive-B in the bi-material plate cooled down to room temperature. Experimental results also indicated that moisture-absorption expansion of the both adhesives is the major cause of decreasing warpage for the bi- material plates at 29degC/55% RH, other than stress relaxation. In contrast to the results of the bi-material plate, the warpages of- the tri-material plates are found to be insensitive to the moisture-absorption, chemical, and thermal strains of adhesives, but not to their elastic moduli, resulting from their relatively thin-layer and compliant nature. This finding is based on the present consistent results of T/G experiment, Suhir´s solution, and FEM analysis.