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Heat Sink Lapping - An Installation Guideby Dan Shumway
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The following article is based on years of experience. It is provided as a free service to our customers and visitors. However, Directron.com is not responsible for any damage as a result of following any of this advice.
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Introduction
Let's start with the basics. Lapping comes from the word lap, which in this scenario means to wash gently against. Lapping is sometimes also called "truing" which the dictionary defines as making exact, correct, or right. So what we're doing is moving a heat sink gently against a flat abrasive surface to correct the surface so it too is perfectly flat.
But my heat sink is already flat, you say. Well, it may appear that way, but the truth is that your heat sink is probably not truly flat. In addition to ridges and valleys from machining the metal it is quite possible that the actual surface of the heat sink is concave or convex.
One test for flatness is to check how reflective your heat sink face is. If your heat sink were perfectly flat all the light that hit it would be reflected off of it at the same angle and give a perfect reflection like a mirror. If it has ridges the light will hit them and angle off in all kinds of directions. This type of surface isn't very reflective at all. Take note that just because a heat sink is reflective doesn't necessarily mean that it's flat. Below is an example:
As you can see, the reflection in this heat sink is refracted slightly, telling you that although the heat sink is polished well, it's not really flat.
So what are the benefits of lapping a heat sink? Well there are plenty, but it all boils down to getting better temperatures (load and idle) as well as improving return from load to idle temperature response times.
Sounds good right? Here's how to do it:
Materials
What you'll need:
- A heat sink
- Light oil or lubricant OR a steady flow of water
- Hard Flat Surface (has to be really flat a thick piece of glass works best)
- 2 sheets each Wet/Dry sandpaper in 320, 600, and 1200 grit
- 2 sheets 2000 grit sandpaper OR a metal polish
- Several soft cotton cloths
- marker
- Chemical Cleaner (nail polish remover, acetone, etc denatured alcohol works best as it leaves no residue)
- Tape
Those with softer skin may also want a leather glove or something to put on the top of the heat sink to pad your hand.
Procedure
First take a good look at the heat sink you intend to lap.
You can't quite make them out but there are machining marks on the bottom of the heat sink, and as you see in the picture, it is only mildly reflective.
The next step is optional but recommended. Remove all the accessories from the heat sink such as fans, power cords, and hold-down clips. If you are uncomfortable with doing so, take the tape and tape items like hold-down clips and power cables out of the way of the base.
After preparing the heat sink, turn it over and clean the base with your chosen chemical cleaner. After cleaning, scribble on the heat sink with the marker. The heat sink should now look something like this:
The marker gives a better idea of just how flat the heat sink is once you start sanding.
Now get ready to sand by taking your flat surface and putting your 320 grit sandpaper on it. (It may be easier to tape the outside edges once started. )
If you opted for oil over a steady source of water skip this paragraph. When using a steady stream of water (like a faucet), place your glass/hard surface in the sink and have the water flowing gently over the sandpaper. I'm not a big fan of this method, but it does work and is cleaner than using oil.
If you're using water skip this paragraph. Place a few small drops of oil on both the center of the heat sink base and on the center of the sandpaper. Spread these drops around the surfaces evenly.
Now gently set the heat sink down on the sandpaper. Start moving the heat sink back and forth along the length of the sandpaper. For best finish, do not use circular motions. (They remove the metal faster but do not result in as nice a finish). Apply pressure, but don't push hard, as this may rip a hole in the sandpaper. Lapping is one of those things where you need patience, not strength or speed. The feeling you're trying to achieve is one where the heat sink is contacting the sandpaper well, but it isn't a dry contact. Adjust the amount of oil or water on the sandpaper to get this feeling. You don't want so much that the heat sink glides effortlessly across the paper.
Continue sanding a little while longer and then gently remove the heat sink from the paper and wipe off the face gently. In some areas the heat sink should look smoother with the marker rubbed off, while other areas may not have been touched yet. Continue sanding with this grit until you get a uniform finish across the face of the heat sink.
As you can see, in the beginning only the outer edges are contacted by the sanding. As you sand, the area becomes larger as the surface of the face becomes more level. Finally before ending with the first grit, only a small strip remains untouched by sanding.
Once you achieve a uniform surface, remove the 320-grit sandpaper and put a sheet of 600 in its place. Prepare it with either water or oil as before. Repeat the process described above as before. This time you shouldn't see the mass change from before because it should be flat now. Gradually sand the surface to smooth it out for. Now it's important to watch for a change in the appearance of the heat sink.
When you finally achieve a 600-grit finish it should look something like this:
Now you could stop here if you wanted as now you have a flat heat sink and probably a lot better finish than you started with, but there are still a couple more steps until that heat sink will shine like a mirror.
The next step in getting a mirror finish is to repeat the previous process, but with a finer grit such as 800 and/or 1200.
Once you have your 1200 grit finish you have two options. You can sand it one more time with very fine grit such as 2400 (be careful when you lift the heat sink off the paper so you don't scuff it), or you can use a metal polish to shine it to a mirror finish. The advantage of using the polish is that the last step is completed very quickly. The disadvantage is that almost all metal polishes contain additives such as corrosion inhibitors and other stuff to shine it up. The problem with this is that those things hinder heat transfer and must be removed with the chemical cleaner.
If you decide to use a polish put a little dab on one of your clothes and gently buff the heat sink face with it until the polish starts to change to a darker color. Then gently wipe it off. Almost any metal polish will do. I personally have had good luck with Mothers Mag and Aluminum Polish, and Flitz Metal Polish.
Whether you use polish or sandpaper for this final step you should end by gently removing any tape from your heat sink (if you used any). Once the tape is removed, clean the heat sink to remove any chemicals, oil, sandpaper particles, etc from the face. Then let it sit to completely dry. Lastly, reinstall anything taken off (fans, clips, etc.) Before installing the heat sink first take a look at the base. It should look like this now
As you can see, the heat sink now has excellent reflective qualities, as shown by the marker top reflection in the picture above.
As a side note, you may find that at first your temperatures aren't any better. Don't worry about it. It takes longer for thermal compound to cure with a lapped heat sink. Allow a minimum of 3-5 days of real use to insure full curing.
That concludes this tutorial. Good luck.
This information has been provided as a reference. Directron.com is not responsible for any problem as a result of properly or improperly following the advice above.
Last Updated: 05/28/04
(c)Directron.com, All rights Reserved.
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