A Lightweight Integrated Electronics Module (iem) Packaging Design for the Messenger Spacecraft

Introduction 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. MESSENGER is the first spacecraft that will orbit the planet Mercury [1]. The mission design has provided two launch windows, with a primary window in March 2004 and a secondary window in May 2002 [2]. The spacecraft will be commanded to perform two deep space maneuvers, two Venus flybys, and two Mercury flybys during its long cruise phase, and will be inserted into Mercury orbit in April 2009. The spacecraft will then orbit Mercury for the one-year mission with an 80 inclination in a 12-hour period and periapsis latitude and elevation of 60 N and 200 km, respectively. Science data will be collected by a suite of onboard instruments that include the Mercury Dual Imaging System (MDIS), the Mercury Atmospheric and Surface Composition Spectrometer (MASCS), the Gamma-Ray and Neutron Spectrometer (GRNS), the X-Ray Spectrometer (XRS), the Magnetometer (MAG), the Mercury Laser Altimeter (MLA), and the Energetic Particle and Plasma Spectrometer (EPPS) [3]. The Integrated Electronics Module (IEM) as shown in Figure 1 is the central control element for the entire spacecraft. There are two IEMs onboard the spacecraft for redundancy. Each IEM accommodates five Printed Wiring Boards (PWBs). These as shown in Figure 1, in order from left to right, are the DC-DC power converter board, the Solid State Recorder (SSR) board, the Main Processor (MP) board, the interface board, and the Fault Protection Processor (FPP) board. The five PWBs are plugged to a motherboard that is not shown in the figure.