Can I reuse fairly new thermal paste?

leenhia

Reputable
Mar 9, 2015
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I just got a new cpu and motherboard.I installed cpu on motherboard 3-4 days a ago but I had to take it apart because of bent cpu pin. Can I just reuse thermal paste since it's only been 3-4 days and I haven't really use my pc cause I was still doing troubleshoot on it?
 

Darkbreeze

Titan
Moderator
No. If the cooler has been removed, you need to completely clean both the CPU lid and bottom of the heatsink using isopropyl alcohol and a lint free cloth or coffee filter, and then reapply new paste. Reusing previously applied paste will allow bubbles to occur between the contact surfaces and generally results in poor heat transfer. It can also cause paste to squeeze out from between the two surfaces since it's already been compressed towards the edges once.
 
Except for the new formulations that harden or dry out, thermal paste doesn't "wear out" with use. So in that respect you can re-use it.

The problem is the thermal paste is only meant to fill in microscopic pits and valleys when the CPU and heat sink mate with each other. If the old paste is contaminated in any way with dirt, hair, or anything that can sit between the CPU and heat sink preventing the two from sitting flat against each other, it'll be worse than having no paste at all.

Edit: Bubbles in the paste shouldn't be a concern because if you're applying so much paste that bubbles can form, you're doing it wrong. You don't want a layer of paste between the CPU and heatsink like a layer of mayonnaise in a sandwich. You want maximum metal-on-metal contact, with the paste only filling in microscopic pits. If you're doing it right, there isn't enough paste for bubbles to form in the first place. It's the dirt, dust, or hair preventing the two metal pieces sitting flush with each other that you have to worry about.
 

tech-wreck

Honorable
Jan 8, 2014
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depending on the type used, it does harden over time (i've had to chip it off with a plastic scraper before) but 3-4 days it should still be pliable. as long as it's not oozing down the edges it should be fine. when you get it up and running monitor your cpu temps and if it gets hot then re-do it.
 

Darkbreeze

Titan
Moderator
What happens with air bubbles is, when they burst, you are left with areas that have no thermal paste, therefore heat transfer in those areas is diminished. It's a bad idea, is not a standard procedure and the fact that you're indicating it's ok when every experienced builder I've ever met, talked with or read commentary from all concur that it's a procedure that should not be used. It's just a very bad idea. In a pinch, you could do it for a short amount of time, like a day or two, until you can get replacement paste, but I HIGHLY recommend not doing it.


Not to mention if it had been run for longer than a couple of hours prior to being removed, the paste has already cured to some degree and won't ever flow correctly into the micro-pores the way it's intended to.
 

Pinhedd

Polypheme
Moderator


Hi,

Thermal paste "can" be reused in the sense that reseating the heat sink without applying new thermal paste will still provide sufficient heat transfer to prevent damage. It's not the best idea, but it will do in a pinch.

If you're not overclocking, it won't be the end of the world. However, if you're looking to keep your CPU temperature low the paste should be reapplied as soon as convenient.
 

As I stated, if you've got so much thermal paste that bubbles can form, you're doing it wrong. The vast majority of the heat transfer is from metal-on-metal contact. The coefficient of thermal conductivity of aluminum is about 200 W/m-K. For copper it's about 500 W/m-K.

The coefficient of thermal conductivity for thermal compound is about 0.5 to 5. So it's much worse than metal-on-metal. Don't be taken in by marking about the coefficient for diamond or silver. When you mix those materials in a suspension, the net coefficient is a harmonic mean, meaning it conducts more like the worse conductor - the filler - than the diamond or silver that's mixed in.

For comparison's sake, the coefficient for air is about 0.024. If you do the math, if a heatsink is sitting on a CPU with 1/3rd of it's surface area metal-on-metal, 1/3rd thermal compound, and 1/3rd an air bubble, 99.3% of the heat transfer is via metal-on-metal, 0.6% via the thermal compound, and 0.07% via air. So when applying thermal compound, it's crucial to maximize metal-on-metal contact.

Which is why if you've got so much paste that a bubble can form (i.e. it's like a layer of mayonnaise on a sandwich), you're doing it wrong. All that paste is probably doing more harm than good. It is in fact possible to run a computer with no thermal compound between the CPU and heatsink with little ill effect, provided there is good metal-on-metal contact (the compression straps help increase metal-on-metal contact).
https://www.youtube.com/watch?v=RAr2wKZ_nes
(And before you go off about it being an "old" cooler CPU, it's a P4 2.4GHz w/ 60 Watt TDP. Modern CPUs probably idle cooler than that one did.)

The reason dirt or hair or dust in the thermal paste is deadly is because it tilts the heatsink relative to the CPU's heat spreader, preventing metal-on-metal contact.
 

Darkbreeze

Titan
Moderator
If you reuse paste, you're doing it wrong. Period.

I'm well aware, at least as much and probably more, of all the facts you've stated above, and I don't dispute any of them, except that paste shouldn't be reused. The fact that when you pull two parts apart you have no control over neither the adhesion or grouping of the remaining past can clearly lend itself to a less than desirable orientation of where that paste is now at and how it will go back together.


You can't simply smear it around again or you'll be introducing contaminants into the equation, which you don't want to do, plus considering the fact that most pastes will have already cured to the point where its unlikely to still be fluid enough to work with if you did. Clearly you shouldn't have air bubbles after an initial application if done right, but sticking the two parts back together after having pulled them apart could not by any stretch of the imagination be either considered done right or controllable.


As said several times now, it will work in a pinch, for a short time, but I would certainly not intend to leave it that way.
 

Zahirrudin

Commendable
Jan 25, 2017
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i used to reclaim back thermal paste...must clean both heatsink and IHS surface...apply back the reclaim one...should add new one because the reclaim amount surely will be less...should be no problem...still on my delid i7-7700k CPU...and working fine...

it rarely happen..because at that time i'm short of thermal paste after deliding my cpu...normally I apply all new paste...
 

Darkbreeze

Titan
Moderator
"Re-claiming" thermal paste is a bad idea. The thermal paste loses some of it's beneficial properties, depending on the type of paste, such as the ability to flow and transfer heat well, over time as it gets old. Many of them harden, others simply break down much as almost any component or mixture does after prolonged exposure to heat or many heat and cool cycles. If you go through the trouble of removing a heatsink of any kind and cleaning the surfaces, it's absurd to not apply fresh thermal interface material when basic formulas are as cheap and plentiful as they are.
 

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