Study on FO-WLP Warpage Behavior - Influence of Process Temperature and Geometry
Controlling warpage effects in fan-out wafer level packaging (FO-WLP) is of key importance for realizing reliable and cost-efficient system in packages (SiPs). During the manufacturing process however, warpage effects occur caused by a combination of mismatch in thermal expansion coefficient of different materials and cure-shrinkage effects of polymeric materials. An approach of controlling the warpage could be realized by assessing a numerical simulation workflow of the FO-WLP process chain in which the relevant material properties and geometry are needed as input. Since there are many different steps included in the FO-WLP process, accompanied by complex material behavior (e.g. curing of polymers), this workflow is not straight forward. In this study the first couple of FO-WLP processing steps are investigated in detail by performing extensive thermomechanical material characterization, temperature dependent warpage measurements and numerical simulations. The warpage measurements indicate an irreversible effect after exceeding certain processing temperatures, which are significant for the final warpage at room temperature. A new approach for measuring the coefficient of thermal expansion is discussed, where a temperature profile is used based on occurring temperatures in the process instead of the typical three identical temperature ramps according to the standards. A positive side effect of this approach is that it allows to determine possible shrinkage effects. Within the simulation model, the shrinkage values are used, as well as different CTE values in order to represent the hysteresis effect observed in the experiments. Very good agreement between experiment and simulation is achieved which is shown for three demonstrators having different epoxy mold compound thicknesses.