What does the hole in the P4 heatspreader do?
 

The hole in the P4's heat spreader is a result of the manufacturing process. The IHS is glued to the processor substrate with epoxy at the Intel plant. The hole allows gases and pressure to escape while the epoxy cures. Without the hole the pressure would deform the epoxy bond and make the connection between substrate and IHS uneven.

It is perfectly safe to cover and/or fill it with thermal paste.

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