Our Lapped CPU Heatsink Test Platform

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Lapping is the popular term for sanding either the base of a heatsink or the IHS of a CPU to be flatter and smoother. The goal is to achieve closer mechanical mating between the CPU and the heatsink base for improved heat transfer, leading to better cooling. The recommended procedure is to lap both CPU and heatsink base for best results. Search the web and you'll find not only lots of detailed instructions and YouTube videos on how to lap a CPU, but also a wide range of cooling improvement claims.

I shopped locally for sandpaper of different grades, and ended up finding the really fine grades at an automotive supply shop. The superfine sandpaper is apparently used to create ultra-smooth paint finishes. I mostly followed these guidelines at Overclockers.net, but also referenced many other sources as well. An unused small double-pane framed window was used as the platform for the sandpaper.

How our Core i7-965 Extreme looked before lapping. You can see the nicks and scratches collected over the years. Note the greater wear in the corners and edges, which suggests they are raised above the rest of the area. Also note the bevelled and rounded edges around the perimeter: That is normal for an Intel IHS. An AMD CPU, in contrast , has an IHS that is much more square at the edges, usually a sharp corner where the top meets the sides.

This profile view of the CPU with a steel straight-edge (the back side with cork lining showing) against the IHS shows contact only at the edges; there is a gap through the middle. The IHS is not exactly concave, which impliers a curve. The edges are raised a bit higher than the center.

Curiously, running the straight edge 90 degrees perpendicular showed less of a gap, which suggests the concavity is not consistent.

One side of this spare window was used as the sanding platform. The numbers on each piece of sandpaper indicates its grit rating. The brownish streaks are copper bits ground from the IHS.

I lapped the CPU slowly & carefully over the course of a day. It was deemed complete when the straight edge consistently showed no gap across the center of the IHS. It was difficult to achieve complete flatness. A steady slow stroke, even pressure, and regular rotation of the CPU was employed. Even so, as the following pictures show, a bit of curvature appeared around the primeter, though it might be more accurate to say it remained. My estimate is that about 85~90% of the center is flat; the perimeter slopes away a tiny bit.

Center of the IHS against the straight edge, after completion of lapping.

Across the center again, CPU turned 90 dgrees.

Image of a 1366 CPU without the IHS.

Approximate position of the die beneath the lapped IHS.

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