System-in-Package is an appealing alternative compared to the integration on a PCB or in a chip. A big variety of different packaging solutions (including 2.5/3D integration) makes it difficult to choose the most appropriate solution for a given specification. Simulation-based design flows gain importance, but lack the straightforward access to actual design data. We propose a new description format as well as a corresponding methodology to manage and process assembly and packaging design data. Based on established software concepts (XML/XSLT), our Assembly Description Format (ADF) integrates well into existing design environments and features a high flexibility to consider distinct design aspects.