Posted: January 18, 2004
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You bought the pinnacle of air cooling technology, but you want to make sure you're getting the best contact possible; you want to lap and polish your heatsink. This guide will show you how to do just that and get the most from your heatsink.
This guide uses a Thermalright SLK-900U heatsink, but the lapping technique applies to pretty much any heatsink.
The base of this heatsink is flat, but the finish leaves a lot to be desired. It needs to be lapped to get a smooth surface that will give you more surface area and better heat transfer. This article will guide you through lapping your heatsink like a pro, so get ready to make a shopping list and start sanding!
What you need:
• A flat, thick piece of glass
• Wet or Dry sandpaper
• Duct tape
• Rotary or orbital buffer (optional)
• Aluminum polish (optional)
• Pure denatured alcohol
• 100% cotton terry towels
The first thing you need is a piece of glass that is large enough to work on and thick enough not to warp when you use it. My piece of glass is about 36" long, 12" wide, and 1/4" thick.
The next is sandpaper. Since these heatsinks are flat from the factory, you can start with 400 grit paper, progressing to 600, 800, 1000, 1500, and ending up at 2000 grit. The sandpaper used for this guide is made by 3M and can be purchased from auto parts stores, body shop supply stores, and anywhere else that has automotive touch up paint/bodywork supplies.
Let's start lapping! Get your sandpaper ready and place your sheet of glass on/in the sink or somewhere you can run water over it during sanding. Start the water flowing over your glass (it's not necessary to have it blasting, just enough to keep the area clean) and place a sheet of 400 grit paper on it, let it get wet so it will stay on the glass.
Now it's time to prepare the heatsink. Since the black metal bracket on the SLK-900U will hang down and get sanded in the process, I used duct tape to hold it up out of the way and protect the paint.
Before you start sanding, let me cover one very important point, how to sand. You'll get different opinions on this technique, but to achieve the smoothest finish with the most contact area, you must sand lengthwise, not in circles. While a circular or swirling motion is good when you're trying to remove lots of material fast, it's not good for lapping a heatsink. You want to sand back and forth along the length of the base. Look at the picture of the taped heatsink above, you would sand it left to right (for square bases, just pick a direction and keep it that way).
Begin sanding with the 400 grit paper and continue until the base is flat and all the machine marks are gone (lines in an arc across the base). How do you know when the base is flat? It's not very obvious on the 900U because it's already so flat, but when you start to sand you will see places that aren't getting touched by the paper, when the surface is evenly sanded it's flat. This will probably occur before the machine marks are totally gone.
You're ready to switch to 600 grit now. Basically, you just sand it until the 400 grit finish is gone and it's an even 600 grit finish. Continue to 800, then 1000, and 1500 before moving to the final grit (2000).
Now for the 2000 grit paper. Start sanding like you did with the other grits, but do this for a longer time, until the paper has broken down (it happens in about five to ten minutes with the 3M paper pictured above). After the paper has become noticeably smoother, use less pressure on the heatsink until you hand is just guiding it back and forth and not applying downward pressure.
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