Atomic guide to CPU lapping

Justin Robinson | Jan 23, 2009 4:41 PM

We give you the low-down on how to get your CPU and heatsink flatter - and running cooler - by lapping it to within an inch of its silicon life!

In the mad enthusiast rush to squeeze more clocks and speed out of our gear, there are a few hurdles along the way that hold us back, one of the largest being heat. Heatsinks draw it away from the CPU, and radiate it, but it is in this step that they sometimes fall down.

Assuming ours is a perfect world, every heatsink would have a perfectly flat base, and every Integrated Heat Spreader (IHS) on a CPU would also be flat, needing only a smidgeon of thermal goop to provide the material needed for the very small gaps between them. Sadly, our assumption was a little misguided, and neither of those things exist.

The IHS on a CPU is nearly always extremely convex (raised in the middle, and falling off towards the outer edges), and is made of copper coated with nickel. We’ve chosen the Core i7 920 chip (mostly because we’re crazy, and have no respect for misbehaving tech) and a
Thermalright Ultra 120 Extreme 1366 cooler for this experiment, and we’ll show you not only how to get great results for little money, but what we managed to achieve.

Running our CPU at idle, we recorded the following temps at idle, and at load under OCCT after a period of twenty minutes:

Core i7 920 - Idle

Core i7 920 - Load

Click on to see just what we do to it...

What you’ll need
Naturally, you can’t just jump into something like this without preparation, but being the helpful lads we are, we’ve done some of the work for you.
• Wet/Dry sandpaper in 400, 800, 1200 grit (you can get up to 2000, but this gives diminishing returns for the effort used)
• Flat piece of glass or mirror – has to be a very flat surface, and clean
• Willing subject, aka a CPU or heatsink
• Small glass or bottle of water
• Strong duct tape, and some scissors

After you’ve assembled your array of tools to get the job done, the first step is to clear the CPU of all thermal goop. Once you’re finished, cut some duct tape and completely cover the underside of the CPU, leaving only the IHS bare.

The next step is just over the page…

Take your sheet of 400 grit wet/dry sandpaper, fold it in half lengthwise and cut down the middle with the scissors. This will help you with movements, as well as get you the most out of the paper you have. Take the half-sheet of sandpaper and use the duct tape to secure it to your piece of glass, and don’t be afraid to use as much tape as you want – a heatsink will want to move this around quite a lot. Sprinkle just enough water on the paper to wet and lubricate it, but too much will loosen the tape – you don’t want to give the CPU a bath, after all.

Turn the CPU so that the IHS is facing down towards the sandpaper, and grip it firmly. Start at either the left or right of the sandpaper (depending on your preference), and slowly slide the CPU back and forth ten times, each return trip counting as a single time. When you’ve completed ten, give the CPU a spin by ninety degrees, and repeat. After only a few strokes, the text will wear away, and you’ll start to see copper poking through.

We’re still lapping on the next page…

Periodically check the IHS of the CPU, and make sure that you rotate the ninety degrees the same direction every time. Keep using the sandpaper, making sure you’re sliding the CPU as straight as you can manage. The piece may wear out (shown by it being smooth to the touch), so replace it if there is no resistance when sliding back and forth. Add small amounts of water as needed. You’ll know you’re getting close to finished with the 400 grit when all the nickel (silver) is worn away from the top of the CPU, like in this following shot.

We’re finished with the 400 grit sandpaper for now, leaving us with a very scratched but decidedly coppery CPU. The silver parts around the edges are a part of the beveling of the CPU, which doesn’t contact the heatsink, so don’t worry about those (though AMD users won’t have these, as the IHS is relatively straight-edged). Here’s what our CPU looked like:

Keep reading to see what to do next…

Once we were finished with the 400 grit, the next step up is the 800. Follow exactly the same steps with this as before, taping down and using only a small amount of water. You’ll know you’re finished with this grit once the deeper scratches left by the previous grit have mostly been rubbed away.

Moving from the 800, you can move up in steps of four hundred all the way to 2000, though we stopped at 1200 simply due to the fact that going that far isn’t worth the time spent. This will take you a while, so make sure you have the TV on, or some beer to drink (just not too much, it makes it…tricky). Once you’re finished with the 1200 grit, you should have something very flat, with only a few incredibly fine scratches.

To give an idea of just how flat this has made the CPU, here’s a side-on shot:

What will we do next?...

Not content with just the CPU, we did the same to the base of our Thermalright Ultra 120 Extreme, removing the nickel base and exposing the copper, making it as flat as needed to mate with the CPU. You can even see our chosen location (piece of glass on a kitchen bench), and this can really be done anywhere comfortable.

All the lapping completed for both the CPU and heatsink, the tape was removed from the CPU and it was cleaned with a slightly damp piece of paper towel. The heatsink can simply be run underneath the tap, and left to dry (though not getting water in the fins is recommended, because it takes ages!). Don’t run the CPU underneath the tap, it might be water but that doesn’t make it water cooling! For proof of just how flat these two have been made, here’s a shot without any pressure or thermal paste, just gravity:

So it’s all finished, but what did it actually do?

Lapping is an exercise not only in willingness to move a piece of metal or silicon back and forth for a drop in temps, but in patience for the thing to be over. Our best advice is to take your time with it, and expect to spend at least two hours per piece, rushing this means potentially making the bases worse in the long run. So long as you keep turning the same ninety degrees, keep up the same pressure and ‘laps’ of the sandpaper, you’ll get a very good result.

Final Results
We recorded an average temperature drop at idle of 2.25 degrees celcius, and an average drop at load of 2.5 degrees. While this might not sound like too much, it can be the difference between stability and crashing, and is a great way to increase the cooling potential of any heatsink, or even a waterblock. So long as you’re careful, lapping is a great way to play with your tech, and will grant you some extra overclockability to boot – just forget about your warranty!

Core i7 920 Lapped - Idle

Core i7 920 Lapped - Load