Quote:
Originally Posted by
doyll
The CPU chip, the heat source, does not touch the entire area of IHS. And the IHS doe snot spread heat any better than a good cooler base. The result is only a square or rectangle in center area of IHS is radiating heat.
Something like this
The more TIM that is applied and the bigger the area it covers, the less pressure there is and the thicker the TIM remains. And as the TIM does not transfer heat as well as copper / metal, the thicker the TIM the less / slower the heat is to leave the chip.
Hope that makes sense.
I hope i won't sound offensive but, no, it makes no sense to me. If it did, the difference between the pea and spread would be much greater. Heat doesn't transfer in perfect vertical fashion. The heat will transfer first in more surface of IHS, than what the pea method leaves to imagine and then to the cooler. Just like if you heat a pan, the heat will not remain just in the center of a pan where the flame is concentrated, but will heat up the "walls" of the pan too. This because the heat travels best within the same metal, than between two metals or 2 different substances anyway. The result, is why if you touch the walls of a pan, you will burn your hands, despite the walls not being in contact with the kitchen flame. Because heat, travels through the same metal very well. Maybe less heat than in the part directly hit by the flame, but still hot enough to burn you. The result in a CPU, is that the entire IHS will be radiating heat, just more in the center, exactly like a pan. Because, like you say, the TIM, despite being in the center of the flame, isn't as good in thermal transfer as to completely eliminate the horizontal thermal spread in the IHS, because metal to metal, always beats metal to TIM. So, at the end, the theoretical advantage of the pea, that eliminates all air, is in reality shrunk, by the larger area of contact that the spread method has. The more the IHS surface that is coated, the more the heat transfer, because heat transfer isn't limited to the center of the IHS, but to the periphery too.
Or to put it in more "scientific" terms, heat in conduction, spreads through gradient:
Where k is the thermal conductivity of the material,
A is the cross sectional area,
THot is the higher temperature,
TCold is the cooler temperature,
t is the time taken,
d is the thickness of the material.
http://formulas.tutorvista.com/physics/heat-transfer-formula.html
Why does the heat transfer to the rest of the IHS too and then radiate to the CPU cooler?
- The K, thermal conductivity of the IHS metal, will always be higher than the K of TIM (metal to metal is better than metal to TIM and even better than metal to air). This explains why the heat will also move horizontally to the periphery of the IHS.
- THot-TCold (DeltaT) means, the heat will always go from hotter to colder point and the bigger the difference, the quicker the transfer. So it can't just "bypass" the rest of the IHS in order to go to TIM and then to the CPU Cooler. The rest of the IHS is cooler than the center of the IHS too. So heat will move to the periphery of the IHS too... For the same reason that heat can't bypass entirely the pan's external walls and just heat up just the food in it.
- d is inverse proportional to the heat transfer. So, the thicker the TIM, the bigger the d, the worse the heat transfer. Which explains why thick TIM is bad.
I think this explains both why there isn't a spectacular "win" of the pea method and why thin is good. EVERY little corner of IHS, will heat up and partecipate to the heat transfer (increasing "A" - the cross section area), as long as that corner of IHS is in higher temperature than the CPU cooler base above it. Which will always be the case, because metal to metal, beats metal to TIM.
Had it been otherwise, the pea method would give surprisingly better results than the spread method, for eliminating those pesky air pockets that act as insulators (lower K, higher d). But it doesn't, because the lowly IHS periphery, counterbalances for good part this effect, contributing to the heat transfer much more than a non coated surface of the pea method (coated IHS to cooler is much better than non coated IHS to cooler, in the periphery).
If only the TIM had better K (thermal conductivity), than the metal itself, would the pea method shine. But alas, the metal to metal is always more efficient than metal to TIM. So despite the source of the heat being in the middle, the rest of the IHS, offers better K (the same as the entire IHS) than the TIM. And this why the entire IHS will partecipate to the heat radiation towards the CPU cooler. (passing through 1 material vs passing through 2 materials).
It also explains why liquid metal TIMs, perform much better than any synthetic TIM, despite the latter being applied with the "perfect" pea method. (much bettter K, increased A compared to simple pea method, no air).
Because, at the end, coolers aren't made of plexiglass...
EDIT: To put it in 2 words:
- Pea method, eliminates the air, allowing for low d, but has decreased A (areas not covered by TIM, will transfer heat much worse than any air pocket in the spread method).
- Spread, has higher d (air), but larger A.
Result: Instead of a crushing victory for pea, the differences are negligible and possibly within statistical error...