Packaging materials are important factors in cost, process, and performance of packaged devices. To minimize costs associated with redesign or rework, it is critical to select the right materials for a package at the design phase. Finite element analysis and other simulation tools have been widely used to optimize the target material properties and design parameters in microelectronic packaging. However, many materials issues faced by assembly engineers cannot be predicted by simulation engineers alone. Partnerships between package design engineers with materials suppliers can be used to help identify possible design issues and provide optimized materials systems during the design phase. In this paper, two examples are given to show the complex interactions of molding compounds with other package components in assembly processes, wherein simple design FEA could not adequately predict the resultant package performance. The results of this work illustrate the need for packaging design engineers to work closely with materials suppliers during the package design stage to develop design rules for the complex material s/component interactions in addition to those predictable by simple FEA simulation.