How do you lap (sp?) a heatsink base

[mjerguy]

I damaged the all copper heatsink base on my CPU--I transported it in my car and the jarring in the car seems to have put some gouges in it.

Have heard about "lapping" the heatsink but how exactly is that supposed to be done? Files, sandpaper, compound, voodoo???

Can someone direct me to a site that explains it or explain it yourself?

Thanks

MIchael Rogers

[supraway]

Start at a medium-grit paper (I forgot the numbering system LOL) and use maybe 4 different grits all the way up to extremely fine grit (1600 -- I think??). BUT on the final sanding, wrap the paper around a piece of glass or something, and sand down from there. When using glass, you get a perfectly flat surface. Then wipe the bottom off with a damp cheesecloth or something. From there, rub in some Arctic Silver in a circular motion, filling in whatever microscopic gaps you may have. DONT TOUCH THE BOTTOM AFTER ALL THIS.

This is probably going a little far, though....

[Hoot]

Depending upon the extent of the scarring of the surface, you may have to start with something like 320 or 400 grit. Ideally, you don't want to overwork the surface any more than you absolutely have to. For an HSF, out of the box with not scarring, I use 400 until I'm confident the surface has no major concave or convex areas. Then, I switch to 600 and eventually 1000. In all cases, use "Wet-or-dry" type paper and keep it wet with water and a drop of dish soap in the water. This keeps the displaced copper (or aluminum) from clogging the pores of the paper. Periodically rinse the paper under the spigot and continue. Many time, with 600 grit, the act of using it reduces its abrasiveness enough to use it like 1000. I do not strive to manufacture a mirror suitable for combing my hair in. That, in my opinion, is a point of dimishing returns. When you are finally happy with the surface quality, as the previous perdson said. Thoroughly dry the HSF, then degrease it with a strong solvent and finally rub some AS into the area of the base where the core will contact it. I use the formica countertop in the kitchen as my reference flat surface. we're not grinding telescope lenses here. :D

73, Hoot

[ButcherUK]

Originally posted by supraway
BUT on the final sanding, wrap the paper around a piece of glass or something, and sand down from there.

Flat surface is more important for low grits as it's a lot easier to make the surface uneven with them. Personally I use glass for everything except polishing.

[DodgeViper]

Lapping, Grinding, or Otherwise Making a Heatsink or Cold Plate Surface Smooth and Flat

1. Use a piece of flat glass as your work surface (it must be very flat, and supported well)
2. Use waterproof sandpaper (sometimes called wet or dry)

3. Use lots of water (several cc at a time, the water flushes the metal particles away, keeps the sandpaper clog free, and makes it stick to the glass)

4. Depending on how flat and smooth your heatsink is, expect to spend 30 minutes or more at this work (I do it on a glass coffee table top in front of the television - you don't even have to watch the work [of course, if the show is bad enough, you can watch the work, and finish faster])

5. Use a relatively light pressure on the workpiece and move it back and forth across the sandpaper; try to keep an even downward force on the workpiece and avoid digging in the edges or corners. Rotate the heatsink 90 degrees approximately (NOT exactly) every dozen or so strokes.

6. If your heat sink is relatively flat and smooth, then 400 grit is a good place to start. If after ten minutes of work you do not see much progress (there are many pits larger than the surface roughness left by the 400 grit sandpaper), drop to a lower number grit (320) or (260).

7. Using finer grit sandpaper will help improve heat transfer: 1500 is certainly fine enough, but since you need to get such fine grades at an automobile parts store (it is used for finishing automobile paintwork), you might as well get a sheet each of 600, 800, 1000, 1500, and 2000. The more coarse grits are easy to find at a paint or hardware store 40, 60, 80, 120, 180, 220, 280, 320, 400, 600. (Grit # 40 will take the portrait off a franc coin in a minute or so; the average heatsink with faint circular milling marks can probably be started at 320.)

8. The work can be completed in less time if you step up through each grade, removing all the pits left by the previous grade before moving to the next finer grit.

9. Each time you change to a finer grit, VERY carefully wash the work surface, your hands, the heatsink, and the sandpaper. Any larger grit left when you start with a finer grade will leave visible scratches.

10. The waterproof (or wet/dry) sandpaper will last a long time. After you are through with the work, wash the sandpaper and let it dry. You can reuse it many times as long as it is not torn.

11. As an example of how fine 2000 grit is, you could use it on Mercedes paintwork and almost not notice an effect other than the missing wax polish.

12. Do not use too much pressure when sanding the heatsink; you want a flat as well as a smooth surface, and too much pressure can cause one edge or corner to be ground down more quickly (that is one reason for occasionally rotating the heatsink a random amount.) Too much pressure may also cause a corner to tear the sandpaper. If you do not tear the sandpaper, one sheet of each grade should last through ten or more heatsinks.

13. Emery cloth is similar to sandpaper, except that the base is cloth, rather than paper. It is ok for the rougher grits, but I think that I will not give as flat a surface when you move to finer grits.

14. Crocus cloth. This is a polishing material similar to emery cloth, but it has jeweler's rouge on the surface rather than emery grit. THIS IS NOT waterproof. I tried it to get a final polish, and added water. All the jeweler's rouge washed off! For a final polish, a soft cloth and a solvent that will remove dirt and grease are about all you need.

15. The method of grinding the heatsink you and I have described is the way flat mirrors for astronomical use are made (except there are machines that do the work now.) A spherical section mirror can be ground in exactly the same way, except that the grinding surface is circular, and the same size as the mirror. (The only additional action used for the flat and spherical mirrors is that the person grinding the mirror, also, after six or eight strokes, walks partially around the work surface to grind from a new angle.) This is probably not necessary for a heatsink, since the method used on astronomical mirrors makes a surface that is smooth and correct to less than 100 nanometers! At that point, thermal compound would not be necessary, and in fact, would have a negative effect. Even just going in a careful manner to the 1500 grit will give you a surface that, when placed on an FC-PGA CPU, will lift the CPU when the heatsink is raised.

16. Penultimate warning: the older CPU's with a relatively large metal cap (about 20 mm X 20 mm) can be very uneven. Sometimes the metal cap may be concave to the extent of a millimeter or two in the center. For something this bad, you will want to start with 280 grit.

17. Final warning: the FC-PGA CPU's, and the SECC2 CPU's have a very small heat transfer surface, about 11 mm X 9 mm. This is the bottom of the silicon slice the CPU is built into. It is laser cut from a single crystal of silicon, and cannot be made any smoother or flatter. DO NOT grind or polish this surface. It will not help, and the CPU gates and channels are less than 1 mm below this surface. Also beware that this will void any warranty on whatever you choose to lap!