The contact area depends on the geometry of the lid. For forged lid and wire-bond chip-scale devices, the lid area generally mimics the package footprint area. Because of this, it is easy to understand and offers the most thermal contact possible for the device. For stamped lid flip chip and many standard wire bond packages, the lid contact area can be different from the package dimensions and can be calculated using the top view perspective of the package. The following figure is a top-view of an example stamped lid. The values indicated in red should be used for calculating surface area.
The height or Z direction is generally indicated by the A parameter, which measures from the seating plane to the height of the device. The associated table in the appropriate user guide specifies the MIN, NOM, and MAX specification to allow for proper tolerance calculation for the neighboring component clearance, as well as when the heatsink is shared between multiple devices and tolerances needs to be calculated between devices. The following figure shows an example of a cross section from Xilinx mechanical drawings. The A parameter is used to determine package height.
