Stress variation analysis during curing process of epoxy underfill

In cases when epoxy resin is used as underfill, its volumetric shrinkage and elastic modulus increase during the curing process generate stress, which often leads to inferiorities such as deformations or cracks in the packaging. Therefore it is quite important to grasp the variation of resin properties during the curing process and to properly analyze the resin curing stress based on reliable data. However, it is almost impossible to directly measure the resin curing stress itself. Therefore, in this report we tried to predict the curing stress variation from the volumetric shrinkage and the elastic modulus during the curing process. To measure the elastic modulus during cure, we made use of ARES-G2, a rheometer by TA Instruments Inc., which can measure both strain and stress. We conducted in-situ measurements of the resin elastic modulus during cure by oscillation, varying the temperature and the frequency at the same time. As for the volumetric shrinkage, the surface level of the resin in a pycnometer was measured during cure, which represented the resin volume. Second, to combine these two data, resin curing rate is introduced. For each measurement datum, the resin curing rate was calculated with the Arrhenius equation according to each temperature profile. Then the curing stress for each curing rate was calculated using the data of the elastic modulus and the volumetric shrinkage gained above. Finally, we conducted an experiment to measure the curing stress. The resin was stuck onto a thin slide glass and the amount of the glass warpage was measured while being cured. The warpage amount was converted into the stress value considering the glass material property. The experimental result and the analyzed result of the resin curing stress showed good agreement, which demonstrated the effectiveness of these analysis methods.