For lidless with stiffener ring packages, the contact surface area is indicated in the top view aspect of the mechanical drawing. Many of these package types use multiple die in the package, and the thermal solution should be constructed to fully contact all of them. Also, the stiffener ring is higher than the die surface, and thus an island or pedestal needs to be constructed to reach into the stiffener ring to allow proper contact to the die. The thermal solution should not touch the stiffener ring in any direction. Thus, the minimum island size should be the overall area of all die in the package, and the maximum area should be sized so that it does not interfere with the stiffener ring. Some stiffener rings have a non-rectangular shape at the inner corners, so the island must be sized appropriately. The following figure is a top-view example of a lidless device with stiffener ring for reference. Use the appropriate packaging and pinouts guide to find the values for specific devices.
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)) × (45 − (2 × 6.00)) or 35.5 × 33 mm. 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 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.
