Stacked chip scale package (SCSP) is a new electronic packaging technology for non-CPU products, such as hand-held computing and communication devices. In this technology, one or more wire-bonded silicon chips are stacked on top of another flipped silicon chip, and an overmolded underfill encapsulant is used to both encapsulate and underfill the wire-bonded chip and the flip chip in a single process. In this paper, the cure behavior, thermal stability, filler content, and thermomechanical properties of five overmolded underfill materials have been studied using DSC, TGA, TMA, and DMA. Results showed that there is a strong correlation between thermomechanical properties and the filler content of the material. Based on measured thermomechanical properties, a "figure-of-merit" approach was used to estimate the thermal stress induced in the package upon cooling. Results showed that an OMUF material with a low T-g, low coefficient of thermal expansion (CTE), and low modulus can effectively reduce the package thermal stress. The reliability results are in good agreement with the predictions based on thermal stress estimation. (c) 2005 Elsevier B.V. All rights reserved.