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MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is a mission to orbit the planet Mercury. The comprehensive scientific data collected through the one-Earth-year orbital mission phase will allow scientists to study and understand the environment and evolution of the innermost terrestrial planet. The five-year cruise phase and the harsh environment of Mercury orbit pose challenges to the spacecraft subsystem design in terms of balancing an extremely tight mass budget with robust thermal and mechanical designs. The packaging design for a low-cost, lightweight Integrated Electronics Module (IEM) is presented in this paper. The commercial 6U Compact Peripheral Component Interconnect (PCI) Printed Wiring Board (PWB) design has been selected to reduce development cost. Several unique features of the IEM packaging design include using the RAD6000 processor developed by BAE Systems in the Main Processor board, 64-Mb Hyundai TSOPs stacked two-high for 1 GB of SDRAM on the Solid-State Recorder Assembly, and a 32-mm Ceramic Column Grid Array as the PCI Bridge chip. The IEM chassis that accommodates five PWBs is designed with thin-wall aluminum for weight savings, and is fabricated by investment casting for cost savings. Extensive thermal and structural analyses have been performed to ensure that the IEM is capable of surviving and functioning during launch, cruise, and orbit. Environment tests have been conducted on the pre-engineering IEM to validate analytical results.