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MESSENGER (MErcury Surface, Space ENvironment, GEochemistry, and Ranging) is a mission to orbit and explore the planet Mercury. MESSENGER will carry out comprehensive measurements for one Earth-year. The subsystem that controls the MESSENGER spacecraft is called the Integrated Electronics Module (IEM). The IEM will operate the MESSENGER spacecraft, store data, and autonomously detect and mitigate onboard faults. In addition to meeting challenging requirements, the IEM must be available for integration on the spacecraft only 18 months after the start of full engineering development. The adoption of the 6U compact PCI (cPCI) standard has simplified the IEM development effort, reducing the time and cost that would otherwise be required. The 6U cPCI standard dictates board dimensions and the backplane electrical interface. The use of the standard has allowed some of the IEM boards to be specified and procured with a competitive selection process in a minimal amount of time. Prototype IEM systems have been assembled using commercially available backplanes, card racks, and Ethernet cards. Low-cost off-the-shelf cPCI tools such as board extenders and bus analyzers have been used to aid development. These components and tools would have to be developed at a significant cost if a proprietary or non-commercial board format had been adopted. The 6U cPCI specification itself does not address board level mechanical and thermal requirements, but features were added to the boards to meet these requirements while retaining compatibility with the cPCI standard. This paper details the benefits and lessons learned that have resulted from the use of the 6U cPCI standard in the development of spacecraft avionics.