Heatsink Coldplates Not Flat

[Patrick_Bateman]

I was wondering why heatsink cold plates are not perfectly flat? Wouldn't that improve thermal performance and distribute pressure evenly? I am assuming it's because of some cost reason.

 

[CountMike]

They are mostly just so slightly convex in the middle because real CPU chip is in the meddle of IHS and that's where most heat is. so most contact is required.