Using the previous figure as an example, the two numbers in red indicate the minimum island size. The values in blue can be used to calculate the maximum area. For this example, the minimum island size is 29.16 × 29.23 mm, and the maximum island size assuming a purely rectangular island is (47.5 - (2 × (6.00 + 0.20))) × (45 - (2 × (6.00 + 0.20))) or 35.1 × 32.6 mm. In the equation, 6 mm denotes the max width of the stiffener ring, and 0.2 mm denotes the distance from the edge of the stiffener ring to the edge of the package. It is suggested to size this somewhere in between these boundaries. These values can be different for your selected package, as this is only an example of how to calculate island size. The following figure shows the pedestal extruding from the heatsink base calculated from the device package mechanical drawing to allow proper contact to die surface.
There are two vertical (height) parameters of interest, one from the seating plane to the top of the stiffener ring and one from the stiffener ring to the die surface. The first parameter is denoted by the letter A in the mechanical drawing. That is to be used to understand the clearance under the heatsink for nearby components like capacitors as well as to properly size the attachment for the heatsink. The height difference between the die surface area and stiffener ring is denoted by the A3 parameter and is needed to properly size the minimum height of the pedestal or island for the heatsink. The following figure is a diagram denoting the A parameter used for height for clearance and attachment considerations.
The following figure is a diagram denoting the A3 parameter. It indicates the minimum height requirements for the pedestal or island constructed into the heatsink.