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Why do we clean off old TIM?

post #1 of 12
Thread Starter 
I've been thinking about this a lot lately.

We all know the reason for TIM is to fill the micro holes on the surfaces of the cooler and the IHS to avoid air bubbles. But then here's my question:

Why do we need to clean off the old TIM? Or better yet, Why do we need a thin layer of TIM over the IHS at all?

Because here's the thing: By having that thin layer of TIM between the IHS and the cooler base, it would act as a sort of insulator because it's actually preventing any direct contact. So then why do we recommend that?
post #2 of 12
TIM conducts heat better than air, and air is what you mostly have between the heatsink and IHS if you don't use TIM.
Even the flattest, smoothest heatsinks will not make more than maybe 50% (a guess I admit) actual contact with a IHS, so the TIM fills in all those gaps.

You clean off the old TIM to prevent air bubbles, which again would hurt the ability to transfer heat.
Edited by blupupher - 3/31/15 at 6:36am
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post #3 of 12
Quote:
Originally Posted by invincibler View Post

I've been thinking about this a lot lately.

We all know the reason for TIM is to fill the micro holes on the surfaces of the cooler and the IHS to avoid air bubbles. But then here's my question:

Why do we need to clean off the old TIM? Or better yet, Why do we need a thin layer of TIM over the IHS at all?

Because here's the thing: By having that thin layer of TIM between the IHS and the cooler base, it would act as a sort of insulator because it's actually preventing any direct contact. So then why do we recommend that?

Try it for yourself, check the results, you'll understand why based on numbers.

As has been said, the both IHS and base of the HS are not flat, they will not mate perfectly and contact everywhere. Air is a great insulator so if you have a tiny bit of space not touching, they're actually being prevented from exchanging heat because of the film of air.

TIM is specifically made to transfer heat, it has a lot of tiny particles of the type of TIM you buy. Cooper, aluminium, ceramic, diamond, silver, etc. The TIM is made to have 100% (or damn close) contact between the uneven IHS and HS base.

You can of course lower your temperatures if you lap (tutorial here http://www.overclock.net/t/290130/lapping-your-cpu-and-heatsink) you cpu and heatsink. But TIM is always recommended.
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post #4 of 12
bah..I only wipe and reapply every 2-3 block changes.
usually i just smear it with my finger so everything's covered, smoosh back down/wiggle..and tighten the bolts.
 
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post #5 of 12
I would guess that you wouldn't have a smooth cover of TIM after removing your heatsink, if it's dried up it will chip off a bit. If you apply more you're going to end up with an uneven surface with cured TIM and fresh TIM and chances are you won't have the best possible contact. Though I guess over time it would just cure again, but still maybe not optimal. You can test it yourself and publish the results thumb.gif

And as for why we use it to begin with, if you take your ihs / heat sink to a microscope, they aren't perfectly smooth surfaces, your goal is to minimize any air between the heatsink and the IHS. Without thermal compound, you'll end up with far less contact and more air blocking heat transfer. Would be cool to see a CPU soldered straight on to a water block that said.
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post #6 of 12
I agree on the reasons "why use TIM", as they are perfectly explainatory.

As to "why clear old TIM" I would suggest to consider that over time, most of the TIMs do dry up and become too tough to be again nicely smeared around the surface of the die/heatspreader/heatsink.
Then such blocks of tough, old TIM might actually prevent good contact (that means that the surfaces of heatspreader and heatsink should get as close to each other, as possible) and then worsen the cooling.

I would say that these are pretty compeling reasons to do so.

Sure someone might come up with that this is a conspiracy of the TIM makers to get them more profit, maybe they actually designed the TIMs to be not reusable - maybe that, maybe what. All what matter is, that it is not really a good thing to mix old and new TIM and if the TIM is not well spreadable over the surfaces - off it have to go thumb.gif
post #7 of 12
Actually, its a conspiracy of the TIM makers.... it says so on the internet biggrin.gif

No, the reason the clean off old tim is the same as why you clean out the grease out a frying pan before using it again...
while one could say that using the old fat over and over again gives the bacon that special flavour... after a while it just isnt healthy anymore.

As for contact.. you should use as little TIM as possible, because it actually IS an insulator also, its thermal properties -no matter how outlandish the claims- just cannot beat metal-to-metal contact (although liquid metal tims come close).

The best way to keep tim use to absolute minimum: LAPPING !

Lap both CPU & HS/WaterBlock till they are so flat that the CPU "sticks" to the HS base so that you can lift the CPU up by the HS.
post #8 of 12
As to why we need TIM?

This is actually a pretty smooth factory block here. This is it under a microscope (as best as I was able to show with a camera phone). See those grooves? Those are all insulating air pockets when installing on a CPU. Your CPU looks the same way, lots of craters / valleys ect.






You can get them smoother by lapping yourself, I have a couple blocks that I lapped up to 1000 grit and they are much much smoother than this.

Here is the full block for comparison:

Edited by lmarklar - 4/20/15 at 9:06am
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post #9 of 12
Remember, there are imperfections, even if you lap it (well unless you do extreme lapping which I'm scared to do lol). I suppose your question was why clean it? Not sure how most TIMs age but it's better to put fresh stuff than mix new and old (usually in the tiny grooves if you don't clean well enough). Plus, what if you use a different, better TIM? Would be best to have consistency, so another reason to clean.




Edited by TheBloodEagle - 7/15/15 at 1:20am
post #10 of 12
extreme lapping is nothing to be scared off. Although it tends to become a bit tedious & boring, especially if you go all the way up to VERY fine grit, toothpaste and eventually polishing with a silk cloth smile.gif.
At some point you think your arm is gonna fall off and you'll wish you brought a book to read while going through the motions smile.gif
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