One of the first tasks was to prepare the base of the heatsink before attaching it to the CPU. The heatsink/fan is an AVC Mega Cool 60mm x 70mm with a 60mm fan and retails for £10. I dismantled all the hardware from the fin block and removed the phase change thermal interface compound (that is the modern high-tech replacement for the old white heatsink paste that gets everywhere). Most heatsinks come with a pad of it already stuck to the contact area, these days. It is like chewing gum when at room temperature but when heated, it thins out and provides the desired film thickness. Here is a photo of the base before work started. Note that there are very few scratches incurred in removing the chewing gum!
Note also that the photo is around three times larger than the real thing. The darker area on the right is the 'step' that is needed to clear the cam-box, which houses the mechanism activated by the locking-lever, because it stands a little higher than the top of the CPU.
To prepare the block, we need to remove the coarse scratches and surface grain which reduce the overall area of the CPU core that actually comes into contact with the heatsink. These 'grooves' mean that we have to rely on paste to fill in the valleys to ensure the best heat transfer. It would be altogether better if the 'grooves' weren't there in the first place.
In the diagrams below, the paste is shown in green and the heatsink in grey. This is where 'Artic Silver'® thermal compound gains its reputation because it has a higher thermal conductivity than the classical white zinc oxide pastes. Since there are many valleys to fill in, an improved paste will offer a better performance.
However, the lower diagram shows substantially less compound between the CPU and heatsink, only costs 30 minutes of effort to do and is likely to be three or four times as effective at transferring heat from the CPU core than high conductivity pastes. In fact there is so little paste in the gap that its thermal characteristics become less critical and zinc oxide paste is entirely satisfactory. It is also much easier to clean off when dismantling and re-assembling the CPU/Heatsink.
So how do we achieve the required 'flatting' of the heatsink face? 'Wet or Dry' paper is the way to go. Depending on the coarseness of the finish on the face, choose the first grade of paper to produce about half the amount of coarseness. Don't go straight away to the finest grades since it will take all day to remove any substantial amount. I would suggest that the required grade should take about ten minutes' work to remove the scratches from the previous grade. You can test the coarseness produced by rubbing the edge of the heatsink with the next finer grade to get an indication.
Here are a few tips that should help the work go smoothly:
- Use the paper wet. Add a drop of washing-up liquid to half a pint (250ml) of water and use this soapy water to wet the paper frequently. The soap will help stop the paper clogging while you work.
- Look at the progress each thirty seconds to see how well you are performing the task. Wipe the sludge off the face and assess if all the scratches from the previous stage have been removed.
- If they have, change to the next finer grade (or the next but one). When you get to 800, the job is nearly finished.
- Place the whole abrasive sheet on a flat firm surface and arrange to clamp it or tape it down to stop it moving. It is possible to hold the heatsink with one hand but a smoother action is obtained by using two hands. Wrap the fins in cloth or tape to save your hands.
- Rub the face up and down the paper in the same direction that the original scratches are running. Don't rub in circles.
- Scrub up the job in the sink when the paper phase is complete. Remove all traces of the black sludge and do the same for your hands. You don't want to get these coarser particles carried over to the next stage so clean off the work area too.
- Find a piece of hard corrugated cardboard (motherboard box divider?) about the same size as the paper. Fix it down showing the non-shiny, ie. the porous side and pour a little Brasso® onto it and carefully start to rub the face up and down taking care not to let the edge dig in.
- Solvol Autosol (Used by motorbikers to polish their shiny parts) is very good as is Tormek Honing Compound (used by woodworkers to hone their plane blades and chisel to 'razor' sharpness by polishing the bevel edge. Being pastes, these will probably be easier to work with than the liquid polishes like Brasso. I used the Honing compound on mine because it was close at hand.
- Don't use silver cleaning wadding nor any of the others on a duster wrapped around a finger or pad because we need to keep the surface flat. A surface that is domed or hollow is probably going to perfom worse than the original finish at getting the heat out. Such defects can also put the CPU core at peril by resting only on the edges and risking them getting chipped off. The design range of applied force by the heatsink clip onto the CPU is 12 to 24 pounds - POUNDS not ounces! That's why keeping it flat is important.
- The target is to obtain a shiny but not perfect surface. Some scratches are needed to provide canals for the excess paste to get squeezed out. Coat the heatsink base in the contact area with a thin layer and wipe it off leaving a thick 'smear' behind. Coat the CPU core sufficient to be unable to see the printing on its surface. Assemble immediately after inspecting the surfaces closely to ensure freedom from pet hairs, grains of dust, flakes of skin etc.
The paper hasn't yet been wetted with the soapy water and you should use about a teaspoonful (5ml) at a time. Do take care not to let the edge dig in and tear the paper. Also, be careful if you go over the edge of the paper, it is easy to 'scoop' a lump out of the edge.
This photograph above shows the base about half way through the first grade flatting and the right hand side is already there while the left side shows a little more work is needed. The diagonal lines are smears from wiping away the sludge to reveal the surface.
Here is shown the final grade completed. All the coarse grooves have been taken out and there is a haze of fine scratches from the last 800 grit paper. There is a slight reddening at the top of the picture.
Here is the corrugated cardboard polishing pad with the liquid or paste abrasive on its surface. Again, the face should be rubbed up and down in the same direction as before. Just visible, top right, is my little squeeze bottle of soapy water solution. (Don't make the solution too strong, it will form a stiff foam that stops the cutting action of the grit. Also note that it has nothing to do with this step!)
Use a paper towel or tissue to clean off any metal polish and inspect the result by holding it to the light.
![Checking the result of polishing[1]](w5100243.jpg)
Now you can see why there was a red blush at the top of the previous picture! Below, I focus on the surface of the heatsink.
![Checking the result of polishing[2]](w5100244.jpg)
Here you can see the fine scratches I mentioned previously. They actually help the situation by providing the means for the paste to flow out of the centre and thus become as thin a layer as it can.
Halfords sell a pack of Wet & Dry papers in mixed grades which have 3 x 240 grit, 4 x 400, 4 x 800 and 4 x 1200 grit. Fifteen sheets in all for about £4.00. I would avoid the 240 grit paper initially since I doubt any suppliers are issuing items with a 120 scratch size. But if the 400 grit is seeming slow to work, it may be worth trying the 240 as long as it is finer than the face scratches.
So, this how the whole thing looks. For the inquisitive, I have listed the items from top to bottom:
