How to Lap a Heatsink and CPU Guide.

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How to Lap a Heatsink and CPU Guide.


Most overclockers and computer aficionados strive to push their systems just that bit faster; an 800 MHz or even 1 GHz overclock just doesn’t cut it, we always want just that bit more. As we push our systems further and faster it produces more and more heat, and this heat can destroy your CPU and cause damage to other parts of your system. Just as we endeavour to achieve that extra 10 or so MHz, we attempt to battle the force of heat caused by the extra CPU speed and increased Vcore. 1 or 2°C could be the difference between life and death for our CPU and system. To battle heat, we'll go to almost any lengths.             
We will buy the best case with the best airflow the latest and best Coolers and Heatsinks we can find, find the best fans with a high CFM and static pressure, and the newest and best Thermal Compounds. Even after all this there is still more we can do.
Or perhaps you may just want to reduce the temps in your current system.
Well what can we do?    Well you can do what is known as “LAPPING”.
"Making the Heatsink and CPU surface PERFECTLY FLAT. Or smoothing the surface by sanding, grinding etc." The aim is much more to get the Heatsink/CPU FLAT not shiny, shiny just looks nice.
Tip: Do not use Car polish, Brasso or the like as these can leave a (WAXY) residue or other impurities that are very hard to fully remove and can inhibit the final results.

So why would you want to lap?

CPU’s and very few if any Heatsinks are perfectly flat, They may look flat to the naked eye but usually have many high and low points and grooves and marks left by the machining process.
You can test this easily by laying a razor blade or something similar ( very straight and flat ) across the surface and holding it up to a light source, this will help you see the highs and lows as the light shines between the blade and Heatsink and CPU.  There is also the ink test, you can put some ink ( Just a couple of drops mixed with water) onto the glass then place the Heatsink/CPU on top, this will highlight the highs and lows, with the different shades of the ink
Lapping helps even out the high and low points that cause an uneven contact area between the Heatsink and CPU. This in turn gives you a better contact between the CPU and Heatsink, and the better transference of heat from the CPU to the Heatsink, leading to lower CPU temps.

The razor blade test

Note:  These highs and lows can cause air bubbles which cause a failure in the Thermal Compound. This can also be caused commonly in liquid and cheap silicone type compounds. Under pressure and heat they expand creating voids and early failure of the compound. Air is also an insulator, so in effect your sink is riding on a cushion of air. This effect will impact performance and greatly decrease the long term reliability of your mount.
Even after lapping a high quality Thermal compound is required as there are still very small grooves that air can get trapped in. They just are not visible and will cause less of a problem. I highly recommend an extremely good TIM such as the IC Diamond extreme performance Thermal Compound.


What you need.

                                                                                                                                                                                                   Sandpaper.                                                                       Wet/Dry                    
       400    Grit Super Fine Wet & Dry
     600    Grit Ultra Fine Wet & Dry
     800    Grit Ultra Fine Wet & Dry
   1000    Grit Ultra Fine Wet & Dry
     1500    Grit Mirror Fine Wet & Dry
            If you like you can go higher but this should be enough to achieve that the FLAT finish you require.
    2000-2500   Grit Mirror Fine Wet & Dry, if you want an extremely shiny/mirror look finish.

A very flat surface, a piece of glass 20cm square and ¾-1cm thick
The black CPU cap that came with the CPU to help cover and protect the contacts
Masking Tape or similar
Black marker pen
Distilled Water (preferably but not essential)
Hand dish-washing liquid, Without citrus based additives if you can. As pure a dish-washing liquid as you can get.
Isopropyl alcohol 90%+ Pure for cleaning, soft lint free cloth and Coffee filter papers
Time, 1 - 3 hours. depending on finish required
You should be able to get all this for around $15 - $25
You should be able to get it all from the Hardware store, Auto store, Supermarket, and Chemist
A camera is always good as you can record what you have done.

The lapping process.

Firstly clean the CPU/Heatsink very well using the Isopropyl alcohol and lint free cloth or coffee filter paper.
Tape a piece of the 1000 grit sandpaper to the glass. Wet it well and add a couple of drops of dish-washing detergent, this just helps the Heatsink/CPU glide better, and stops it catching or jerking on the sandpaper, this can cause an uneven and undesirable finish.
Mark the Heatsink or CPU with the black marker pen with 2 or 3 lines in each direction (#) and a cross through the middle, from edge to edge and corner to corner, then hold the Heatsink by the side, and slowly move the heat sink up and down the paper. Be careful not to apply any pressure to the heat sink, you want its own weight to do the work for you, with the CPU apply very very light pressure (use pretty much just its own weight as well), slight and even pressure. Do this about 5 times then turn the Heatsink or CPU 90° and repeat. Do a complete rotation. Then have a look at the surface and this will give you a good idea as to how much you have to lap it and where the biggest problem areas are.
Note: With the CPU make sure you protect the pins/contacts with the cover that originally came with your CPU and some tape to prevent any water or metal fillings getting in there. This can be disastrous and destroy your CPU. So be very careful. Also when doing the CPU, work in a static free environment. Using an anti static wrist strap or a non-carpeted area.
Once you know where the areas that need the most attention are and how much work you have to do you are ready to start.
Start with the coarsest grit first, the 400 Grit. Tape sandpaper to the glass and wet as above.
Place the Heatsink or CPU on the wet sandpaper. Hold the Heatsink or CPU by the side, and slowly move the Heatsink or CPU up and down the paper as described above. Do not lift up when you get to the end of the sandpaper pull it back the same way you pushed it. Making sure you are holding the Heatsink or CPU flat and even at all times, do not rock them back and forth.
After about ten passes across the sandpaper, rotate the heat sink 90° (that's a quarter of a turn) and continue to sand until you've made a complete rotation. I continue to do this until I've made 10-15 complete rotations, stopping every couple of rotations to re-wet the sand paper, and reapply the dish-washing liquid. I find after the first 2 or 3 Grits 5-10 rotations were enough. Clean the Heatsink/CPU regularly during this process with clean water and coffee filter paper. After the 400 grit you should have about 75% - 80% of the copper showing.
This is where you have to keep an eye on it yourself, all Heatsinks and CPU’s are different and may require more or less work than described above.

Repeat the process using the 600, 800, 1000, 1500 grit sandpaper, and 2000/2500 if you want that real shine. In the end, you will have a nice, mirror like reflection on the Heatsink and CPU base.
This shows the progression from start to finish with a CPU. The last 2 pictures show a nice True 120 and what can happen if you go too far with HDT Coolers.

When you are happy with your results clean thoroughly with water then Isopropyl alcohol and lint free cloth and allow to dry thoroughly. Do a final flatness test just to be sure you are happy with the results.

That is it, well done, you've just lapped your Heatsink and CPU and created a flatter surface for better heat transfer. You should get about a 3°C - 5°C drop in CPU temps, though this will vary depending on whether you did just the Heatsink, CPU or both. I personally have achieved improvements of over 7°C after lapping both the Heatsink and CPU in highly overclocked systems. With all other conditions exactly the same. I also found the core temps can even out over the cores in Quad core CPU's, meaning where you may have a temp variation of 6°C - 8°C and sometimes more between the cores before lapping, that variation can drop to 2°C - 4°C or better across all the cores after lapping your CPU.
For example, lapping load temps across the cores could be 57°C /64°C /59°C /62°C. After lapping they may be 55°C /58°C /56°C /57°C.
I have Lapped well over 50 CPU's and Heatsinks and even water blocks and every time I have seen an improvement in CPU temps.
Have fun and good luck.
Warning: While lapping is fairly simple and hard to get wrong if you are careful and take your time (Do not rush), you must still take full responsibility for any accidents, including sanding the Heatsink/CPU so the surface becomes uneven and ineffective or like in the picture above, or destroying your CPU!
Also note that lapping will void your warranty on the CPU and most likely on the Heatsink as well.

Now get yourself some great Thermal compound so all your hard work achieves the best possible results. Then enjoy.

I use, sell and highly recommend Innovation Cooling, IC Diamond Thermal Compound.

Please feel free to contact me if you have any questions.
Happy computing.
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