Just thought I'd share some of the information I've got from lapping a few heat sinks. Hope you find it useful.
If you're reading this guide you probably already know the benefits of lapping a heat sink but I'll go over it anyway.
Most heat sinks that you buy these days have a rather rough base on them. They are mostly flat, but not always, so by lapping them you get the benefit of a smoother surface resulting in better heat transfer, as well as making sure that your surface is flat.
The heat sink I'll be using in this guide is a good example. This is what it looked like straight out of the box.
It looks ok but have a look at what happens to the themal paste when mounted.
You'll notice that it's thicker at the sides because the base of the heat sink is not flat.
Lapping
You'll need:
A heatsink, CPU, etc.
Some wet-dry sandpaper of 200, 400, 800, 1200 and 2000 grit.
A flat glass surface to work on.
A roll of duct tape.
An old rag.
A couple of hours of time.
Not essential but recommended:
A sheet of grid paper to check the flatness.
Be within range of a television, radio, etc for entertainment while your doing it.
Sandpaper
You need some higher grade stuff than normal sandpaper. Some hardware stores might not stock the 2000 grit so your best bet is to phone around and find out who has what. Try automotive supply shops or specialist carpentry stores as they might have some. If you can't find 2000 grit, just get as near to that as you can.
The prices that I paid ranged from 60 cents a sheet for the 400 grit up to $1.20 a sheet for the 2000 grit. I'd recommend getting around one or two sheets each of the 200, 400 and 600 grit as they last a while, 2 sheets of the 800 and 1200 grit, and 3 sheets of the 2000 grit stuff.
NOTE: If you're only going to lap a CPU then you probably don't need the 200 grit sand paper and probaly only 1 sheet of each because the CPU IHS covers are made from soft metal compared to heat sinks.
Secure the sheet down with tape. The rougher grade sandpaper will require duct tape but with the finer stuff like 1200 grit you can just use normal sticktape. As you can kind of see in this picture, I'm using an old mirror as the glass base.
Lapping Technique
Place the surface to be lapped onto the sand paper and using both hands, gradually start to move the heat sink in a figure eight pattern. It may be difficult at first, particularly with the rougher grade paper, but it should become a steady flow soon enough. Make sure you keep the base flat to the surface and try to avoid letting the heat sink "shudder" across the paper. After a while you should start to see a pattern like this one emerge.
When you get a build up like this it's best to wipe the sand paper clean with an old rag. Just make sure that there is no debri on the paper before you start sanding again.
After a while the heat sink should start to slide easily as the sandpaper begins to wear out. You will also find that as this happens, the heat sink base will begin to smooth out. This is a good oportunity to take your gridded paper or a something similar (newspaper is good) and check your results so far.
600 grit surface - you can see how the edges still have the original surface texture indicating how uneven the surface was from the manufacturer.
600 grit reflection
800 grit surface
800 grit reflection
1200 grit surface
1200 grit reflection
And here is the finished product. You can see it's still not perfectly flat by the original surface showing on one of the edges but it's as flat as it's going to get without using a belt sander.
2000 grit surface
2000 grit reflection
If you have any questions just ask.
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