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A Power Distribution Unit (PDU) is being developed by The Johns Hopkins University Applied Physics Laboratory for the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) spacecraft that will orbit the planet Mercury for one Earth year to complete the global mapping and the detailed characterization of the planet's exosphere, magnetosphere, surface, and interior. The PDU contains the circuitry for the spacecraft pyrotechnic firing control, power distribution switching, load current and voltage monitoring, fuses, external relay switching, reaction wheel relay selects, and power system relays. It also supports the Inertial Measurement Unit (IMU) reconfiguration, Integrated Electronic Module (IEM) select relays, solar array drives, propulsion thruster firing control, and propulsion latch valve control. To enable the mission to reach the distant planet, significant weight reduction for all spacecraft electronics must be achieved. This requirement has led to an advanced electronic packaging design that begins with component selection, printed wiring board design with very small feature sizes, and a compact interconnection scheme. The significant challenge in the packaging design of the PDU is how to implement state-of-the-art technologies to minimize system weight and meet the stringent reliability required by the MESSENGER power system. This paper will describe the detailed electronic packaging design of the PDU, including the use of Metal Oxide Semiconductor Field Effect Transistor (MOSFET) devices instead of conventional mechanical relays, high-density printed wiring board designs with blind and buried vias, and a modular packaging design to achieve significant weight reduction.