# Thermal conductivities, why are we using thermal paste? Why can't we make it better?

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Via: http://en.wikipedia.org/wiki/List_of_thermal_conductivities

So copper is one of the top thermal conductivities with ~385 (W* m^-1 * K^-1), what is that unit watts per meter Kelvin? Shouldn't it be meters squared? It should be, copper can eat up 385 watts per square meter * degrees kelvin.

Back to my post, it lists thermal paste as 8.89 (W/mK). This makes me want to imply that we could make cpu blocks and heatsinks out of something with a thermal conductivity just above thermal paste. Because the paste can only pass off so much wattage that means we only will ever need to get rid of that many watts in our heatsink because we can never see more wattage than the TIM lets through.

Is this reasoning correct?

Also, someone looking into making me a pure diamond cpu block. Thermal conductivity = 2000
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#### Cavi Mike

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It has nothing to do with its thermal properties compared to metal - it's about its thermal properties compared to the air that actually comprises the vast majority of space between a dry CPU and heatsink when pressed together. Neither surface is perfectly flat nor are they perfectly smooth. The liquid paste fills those gaps and increases the rate at which heat can transfer from the CPU to the heatsink.

Thermal paste 101 dude.

#### SkeeterSkeeter

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Quote:
Originally Posted by Cavi Mike

It has nothing to do with its thermal properties compared to metal - it's about its thermal properties compared to the air that actually comprises the vast majority of space between a dry CPU and heatsink when pressed together. Neither surface is perfectly flat nor are they perfectly smooth. The liquid paste fills those gaps and increases the rate at which heat can transfer from the CPU to the heatsink.

Thermal paste 101 dude.
Well yes I know what it is for. Look at my OP. Try to answer my question; not explain how TIM works I know that already.

Because TIM can onyl transfer so much wattage. Why do we make our heat sinks out of such high thermally conductive materials

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Quote:
Originally Posted by Cavi Mike

It has nothing to do with its thermal properties compared to metal - it's about its thermal properties compared to the air that actually comprises the vast majority of space between a dry CPU and heatsink when pressed together. Neither surface is perfectly flat nor are they perfectly smooth. The liquid paste fills those gaps and increases the rate at which heat can transfer from the CPU to the heatsink.

Thermal paste 101 dude.
This.

Also, we are using metals ... Liquid Pro anyone?
Quote:
Coollaboratory Liquid Pro is the first heat conduction paste that consists of 100% liquid metal alloy. It is liquid at room temperature (like mercury), but it is absolutly nontoxic and has a high moistening ability for several materials. The Coollaboratory Liquid Pro is especially recommend for nickel-plated copper.

Coollaboratory Liquid Pro contain no non-metallic additives (like silicone, oxides etc.) at all. It also does not contain any solid particles. Due to these properties, Coollaboratory Liquid Pro surpasses the best high performance heat conducting pastes by a multiple.

#### Cavi Mike

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Quote:
Originally Posted by SkeeterSkeeter

Well yes I know what it is for. Look at my OP. Try to answer my question; not explain how TIM works I know that already.

Because TIM can onyl transfer so much wattage. Why do we make our heat sinks out of such high thermally conductive materials
Even if I were to believe you, your understanding of "square meter" is also a problem. The space between the CPU and heatsink is so small that it's negligible the difference you get between a liquid metal and any other type of liquid(like thermal paste). All that matters is you don't want a gas(air) separating the two.

#### DaveLT

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Yes and No
W/m-K means thermal conductivity per every meter-kelvin. Confusing unit but most pastes should conduct very well anyway
What's really important is thermal resistance, this is where good pastes split bad pastes apart instantly

That also doesn't mean whether it will be that good or not because paste with higher viscosity is going to work not as well as a lower viscosity with high mounting pressure
Having higher viscosity means with high mounting pressure the paste won't spread correctly and might even form thermal paste "gaps" where instead of the IHS touching the heatsink it's touching the thermal paste
What that means is that in areas where thermal paste shouldn't go will make results worse regardless whether the paste's thermal resistance is low or not because perfect contact = 0C/W
E.G the deepcool z9 has a excellent thermal resistance of 0.058C-in2/W but it's VERY thick making it hard to spread but works best for poor contact and generally loses VS deepcool z5 on great contact pressures

#### LavishB

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Quote:
Originally Posted by Cavi Mike

Even if I were to believe you, your understanding of "square meter" is also a problem. The space between the CPU and heatsink is so small that it's negligible the difference you get between a liquid metal and any other type of liquid(like thermal paste). All that matters is you don't want a gas(air) separating the two.
If that were all that matters Ivy Bridge tim wouldnt lose so terribly vs Sandy Bridge solder.

#### SkeeterSkeeter

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Quote:
Originally Posted by Cavi Mike

Even if I were to believe you, your understanding of "square meter" is also a problem. The space between the CPU and heatsink is so small that it's negligible the difference you get between a liquid metal and any other type of liquid(like thermal paste). All that matters is you don't want a gas(air) separating the two.
What do you mean I don't understand a square meter?

What am I even trying to convince you of? I asked a question...

Ok I don't want air/gas separating the two. Because air/gas has a terrible conductivity. So that's why we use TIM high conductivity and can fill in the holes. Ok so to my question. If this TIM can only allow so many watts through it (based on its properties) does that mean that the heat sink is limited to the conductivity of the paste because the paste can only pass it as much as it can and the heat sink can only let off as much as it gets?

#### DaveLT

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Pretty much correct. That's why silver solder should be used for processors from now on

#### SkeeterSkeeter

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Quote:
Originally Posted by DaveLT

Pretty much correct. That's why silver solder should be used for processors from now on
So I can throw my copper heat sink away and build one out of a material with just slighty more conductivity as my paste and I should get the same cooling effect?

#### DaveLT

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NO i meant from the die to the IHS

#### SkeeterSkeeter

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Quote:
Originally Posted by DaveLT

NO i meant from the die to the IHS

Ok, so if the heat resistance of the TIM is higher than that of the heat sink. Then throw away the heat sink and make one with a thermal resistance close to that of you TIM? Because the lower resistance heat sink can do nothing better than the TIM because the TIM only gives it so much wattage to work with? Meaning a lower resistance heat sink on a high resistance TIM just makes it so the heat sink is "ready" for more heat though none ever gets to it through the TIM.

(no I am not going to throw my heat sink away).

#### Artikbot

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Quote:
Originally Posted by SkeeterSkeeter

So I can throw my copper heat sink away and build one out of a material with just slighty more conductivity as my paste and I should get the same cooling effect?
No. The heatsink needs to have good conductivity, because the distance the heat has to travel is in the order of the centimetres. You have at best a handful of microns worth of thermal paste.

#### SkeeterSkeeter

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Quote:
Originally Posted by Artikbot

No. The heatsink needs to have good conductivity, because the distance the heat has to travel is in the order of the centimetres. You have at best a handful of microns worth of thermal paste.
So in the processor-solder-lid-TIM-heat sink layers which is the weak link and why? Which one could be made better?

#### DaveLT

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You don't use TIM all over your chip, that's a bad idea. All TIM is supposed to do is fill up air gaps, won't you get it?!

#### SkeeterSkeeter

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Quote:
Originally Posted by DaveLT

You don't use TIM all over your chip, that's a bad idea. All TIM is supposed to do is fill up air gaps, won't you get it?!
Why in the lower world do people think I don't know what TIM is for!

What is the weakest link in the cooling chain for air cooled CPU's.....

Say, "we changed the solder to more silver" because its the weakest link. We added more copper to TIM because its the weakest link. Which layer in the cooling lets the least amount of heat through to be put off into the air?

I mean from your end, wth do you think I am asking?

Its like every word I type on here people think I mean the exact opposite.

#### PCCstudent

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My feeling is that I think our OP sees the TIM product as a "bottleneck" in the transfer of heat between two very thermaly conductivly metals. I had never thought of TIM in this way.I remember pulling off an older P4 cpu (older Dell machine) and the cpu actualy had a pad similar to single sided tape to be used between the cpu and the heat sink,this was new to me also

#### Quasimojo

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I can sympathize with the OP. I think most of us have been in the situation where we have trouble conveying a particular concept bouncing around in our head to words on a page.

If I understand the question correctly, I would say that the CPU and heat sink (the parts with the best heat conductivity properties) do make contact, so that contact provides the best heat transferrance. However, that contact is also not uniform across the surface, so thermal compounds are required to fill any gaps (as others have noted, and I'm not saying you don't understand this).

That being said, the "weak link" is the thermal paste. I think your question should be, why don't we use a better base for thermal pastes, like copper, to bring it closer to the potential of the heat sink. Do I understand you correctly?

If I've completely missed your point as well, my apologies.

#### SkeeterSkeeter

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Quote:
Originally Posted by Quasimojo

I can sympathize with the OP. I think most of us have been in the situation where we have trouble conveying a particular concept bouncing around in our head to words on a page.

If I understand the question correctly, I would say that the CPU and heat sink (the parts with the best heat conductivity properties) do make contact, so that contact provides the best heat transferrance. However, that contact is also not uniform across the surface, so thermal compounds are required to fill any gaps (as others have noted, and I'm not saying you don't understand this).

That being said, the "weak link" is the thermal paste. I think your question should be, why don't we use a better base for thermal pastes, like copper, to bring it closer to the potential of the heat sink. Do I understand you correctly?

If I've completely missed your point as well, my apologies.
omg, mind meld. (or w/e that vulcan thing is)

I guess that is my question.

But to answer it we use silver, the highest value conductivity we can get and actually use. I mean we could put diamond dust in it, or helium II, but those cost to much and helium II is a gas I think...

Ok so if TIM is the weakest link, does that mean everything else down the chain from TIM (read above TIM toward heat sink) can be removed and replaced with less conductive parts because the TIM only lets so much through to work with, thus lower conductance materials can still handle what the TIM gives out?

#### SkeeterSkeeter

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Quote:
Originally Posted by Quasimojo

I can sympathize with the OP. I think most of us have been in the situation where we have trouble conveying a particular concept bouncing around in our head to words on a page.
Its like when you text "idk" to someone who does not know what it means. They respond with "idk?" meaning what is "idk" not I actually don't know.

Same thing happens when trying to convey emotion via text.

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