PQFP and other Leadframe-based packages: For QFP packages, since the power consumption is usually low, heat sinks are not usually used. In cases where there is necessity, HQ packages are recommended. Xilinx has not tested any QFP packages with heatsinks on them.
Flip-Chip Packages: For the Flip-Chip BGAs that have been surface mounted, a sustained, direct compressive (non varying) force applied normal to the lid with a tool head that coincides with the lid or is slightly bigger will not cause any damage to the flip-chip solder bumps or external balls, provided the force does not exceedfive grams per external ball and the device and board are supported to prevent any flexing or bowing.
PBGA Packages: For all BGA packages, generally aboutfive grams per ballis acceptable. Provided the force is normal (perpendicular) to the surface and does not tend to provide a moment or prying type force, these estimates will be acceptable. When the component is mounted on a board and the board is supported on its side(s) so that the component and any heatsink have their central axes aligned horizontally, the mechanical forces require additional analysis.
NOTE applying to all of the above: Forces greater than the above recommendations might work; however, if the forces will exceed this limit, a careful Finite Element Analysis should be performed. The PC board needs to be properly supported to prevent any flexing resulting from such a force. Any bowing resulting from such a force can likely damage the package to board connections. It is recommended that any complicated mounting arrangements need to be modeled with respect to the thermal stresses that can result from the heating of the device.
Heat sink Information The heatsink vendors should be able to provide data for a specific heatsink. If the heatsink is excessively heavy, it may be mechanically mounted to the package using a constant force clip or, in some cases, to the board with pins for even greater strength. For heatsink vendor information, please refer to "Device Package User Guide" (UG112):
http://www.xilinx.com/bvdocs/userguides/ug112.pdf