The last and arguably one of the more important parts of heatsink design is the heatsink attachment. The primary goal for most heatsink attachments is to apply an even 20 to 50 PSI of pressure for all lidded and lidless devices. InFO differs in that it requires a reduced 3 to 5 PSI due to the thin non-organic package substrate. SOM's target pressure to the heat spreader is 20 to 30 PSI. Pressure outside these ranges can cause the following problems.
- Degraded thermal performance
- Decreased overall reliability of the device
- Stressed corners or edges of the device, causing potential damage to the device
Because of those reasons, and as stated before, Xilinx does not recommend the use of adhesives for heatsink attachment to our devices. Furthermore, most clips that attach to our package substrate, are not recommended due to the following.
- Proper and even pressure to the device is difficult to control.
- Reliability to vibration, drops, and other G-forces is lessened.
- Amount of force is not enough with Z-clips to be considered a reliable option for heatsink attachment.
Therefore, Xilinx recommends using a dynamic mounting (pressure compensating) attachment that can apply enough force to at least four evenly spaced points around the device. This will allow proper design to these specifications. A common means to do this is with the use of spring screws at the four corners of the device. This generally requires through-holes in the board and thus should be considered prior to PCB layout so that the proper keep out and routing considerations are made for this attachment.