Characterization of prepreg shrinkage and investigation of its influence on warpage simulation

Glass fibers pre-impregnated with a thermosetting resin matrix, so-called prepregs, are one of the base materials used in the state-of-the-art electronics industry. During printed circuit board (PCB) manufacturing, besides other influence factors, the chemical shrinkage of pure thermoset materials during the curing process was already identified in previous studies as one of the main causes of warpage of PCBs and electronic packages respectively.The aim of this study was to quantify the prepreg shrinkage caused by the curing of its resin matrix and investigate the impact on the results when considering these shrinkage properties in warpage simulation. The dimensional changes of the prepared test specimens when exposed to curing temperature were recorded using the method of digital image correlation. By providing the measurement data as objective, simulation based on an optimization algorithm was used to determine material specific values describing the observed shrinkage behavior. The deducted, orthotropic shrinkage behavior is implemented into advanced thermo-mechanical material models of dielectric board or package layers thus allowing an improved warpage simulation.Validation of the results by simulating further experimental trials proved that the presented approach of combining experimental measurements with virtual optimization provides accurate material characteristics describing the dimensional changes of prepregs during curing. The paper shows a panel level warpage calculation which has been performed to show the influence of the implemented enhanced material models of the characterized prepregs on the accuracy of the simulation results. It is shown that the improvement in the simulation quality versus a classical “CTE-only” warpage approach is significant. Thus the implementation of the resin and prepreg shrinkage behavior into dielectric material models finally can be considered as advantageous in order to carry out realistic and accurate warpage simulation of PCBs as well as semi-finished production panels during the manufacturing process.