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I am assembling this PC http://www.codinghorror.com/blog/2007/07/building-a-pc-part-i.html

Of course, I'm in Geek Heaven. :D

I installed the heat sink fan in the wrong direction, and now I want to remove and reattach it.

It came off easily enough.

Now do I remove the old paste from the top of the microprocessor and the bottom of the heat sink or do I just put additional paste and that's it?

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Yes remove ALL of the old paste. The best way to remove old past is to use Q-tips and isopropyl alcohol/googone. –  jmreicha Oct 19 '11 at 13:35
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I would not use googone as it will leave an oily residue. Polish the cpu with a paper towel to finish the process. –  music2myear Oct 19 '11 at 16:22
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2 Answers

up vote 7 down vote accepted

If this is a new paste you just put, you only want it to be a uniform thin layer on top of the CPU or the heatsink (don't put it on both surfaces), so you don't need to scrape it all off completely. I usually use an old credit card as a spatula to spread the thermal paste thinly and evenly. You can use a similar soft plastic tool (you don't want to damage either the CPU or the heatsink).

If this is an old paste, it is better to scrape it off and put a new one if you removed the heatsink from the CPU.

If you managed to make a big mess, it is sometimes easier to just scrape it off and re-apply. Some pastes contain silver particles, therefore it is slightly conductive. You don't want that paste on anything else except the surfaces it is intended to be as it might create a short circuit.

Update: According to Intel's instructions on how to apply thermal paste, soft, dry cloth or tissue should be used to wipe off previously applied/used thermal paste. My opinion is Isopropyl alcohol, Googone, or other substances are risky because they can either contain too much water or be too chemically aggressive. Soft, lint-free cloth works just fine.

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Is cotton buds ok to wipe off previously applied/used thermal paste? –  Alpine Oct 19 '11 at 14:41
    
Thermal paste is usually thick, especially old one. So choose cotton buds that are "hard", not the "puffy" soft ones, otherwise they will disintegrate, leaving sticky cotton fibers all over the place. Otherwise they are ok. –  haimg Oct 19 '11 at 15:35
    
as long as you clean up all the lint junk that could get in-between there. A lintless wipe like a good microfiber towel can finish up. –  Psycogeek Oct 19 '11 at 15:37
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Remove the old paste before you do anything. You can use rubbing alcohol (denatured iso-propanol). Try not to use Acetone (nail-polish remover) as it can damage the PCB in case of a spill. I'd suggest applying a rice grain size and letting the HSF do the work. The purpose of the TIM is to fill in the gaps, not act as a barrier between the surfaces.

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