Understanding the height and contact area of the heatsink to the device is important for any thermal solution. For lidless with stiffener ring, it is critical in determining proper heat sink fit. In all cases, this information is needed to understand the vertical tolerance and for calculations to ensure the proper amount of pressure is applied to the device so that no inference to neighboring components can exist. Using the Mechanical Drawings chapter from the appropriate packaging and pinouts document, locate the package outline drawing for the selected device/package.
The heatsink should be sized so that it has full contact to the top surface plane of the device to avoid potential hot spots or mechanical failure on the device. In many cases, it is oversized beyond the surface area without issues, however, in some cases there are limits to this sizing. The contact area and height specifications depend on the package and lid type.