R. Tao, S.P. Phansalkar, C. Kim, A.M. Forster, B. Han
National Institute of Standards and Technology,
United States
Keywords: advanced packaging, molding, epoxy molding compounds, cure kinetics, modeling, manufacturing process
Summary:
Cure kinetics of epoxy molding compounds (EMCs) is a fundamental material property that affects the molding process of semiconductor chips and final package performance. Accurate kinetics information is essential to improve the numerical Design of Experiments and simulation of manufacturing processes; therefore, understanding the cure kinetics is important to accelerate R&D in optimizing current manufacturing productivity and developing future advanced packaging technologies. EMCs are highly formulated, highly-filled thermosetting epoxy polymers contain many proprietary additives. Today’s EMCs often consist of two or more resin systems to meet ever-increasing stringent specifications as packaging technologies evolve towards miniaturization and 3D heterogeneous integration. In the current industry practice for EMC cure kinetics evaluation, due to measurement challenges related to the small polymer fraction, only the main reaction is considered. However, in multi-resin EMC systems, a second reaction is clearly identified but has been long, conveniently, overlooked by the semiconductor community. This information is critical for post-mold curing (PMC) to achieve optimal properties, for accurate prediction of stresses during the manufacturing process, and for thermo-mechanical reliability of the final package. In this presentation, we report a comprehensive cure kinetics study on industrial EMCs from leading vendors for different applications. We present experimental results from non-isothermal tests by differential scanning calorimetry (DSC). We show DSC data at different heating rates, which are judiciously chosen, to reveal the second reaction exotherm for multi-resin EMCs. For the fully-cured materials, two glass transition temperatures (Tg) are observed, attributed to the two network structures formed during the two-step curing of the EMC. The second reaction completes the overall reaction from about 0.8 to 1.0 of conversion, which is critical for the design of cure schedules, especially the PMC procedure during manufacturing. We present model-fitting and model-free methods to obtain the apparent activation energy and optimized constants for the complicated EMC kinetics. In addition, the relationship between the Tg and the conversion is reported by measuring partially-cured samples. The informed cure kinetics and cure-dependent Tg are critically required for accurate prediction of the thermo-mechanical behavior of advanced packages during various steps of molding processes, most critically, the evolution of warpage and residual stresses.