Base flatness is critically important for minimizing thermal interface resistance. A warped or concave/convex base makes only partial contact with the heat source, leaving large air gaps. Even with TIM, these gaps degrade performance. For high-power components, a mirror-finish, lapped flat base is often specified. Typical flatness tolerances are measured in microns over the contact area. Some high-end coolers use a milled or convex base designed to deflect under mounting pressure to ensure perfect contact with the typically slightly concave integrated heat spreader (IHS) of a CPU.