The Micro-Electro-Mechanical Semiconductor (MEMS) sensor packages are an advanced multimaterial composite system. These packages comprise polymeric materials like prepregs, solder-mask, insulation, and conductive adhesives. Prepregs are glass fiber reinforced epoxy laminates. Only a low material sensitivity to environmental influences will ensure the sensors' reliable performance during their application lifetime. To this end, the potentially applied materials undergo defined thermal and moisture-dependent material characterization. In this contribution, the influence of moisture and temperature has been studied for five different prepreg materials, which are commonly applied as a substrate material in a MEM'S sensor. The measured thermal and moisture dependent material properties are the basis for a numerical diffusion analysis and a virtual hygro-thermomechanical reliability assessment.