Interesting observation with CPU cooling

I have a couple of CF-30's (K and F models) and noticed that when they are sitting on my lap (bare legs) they get scorching hot but only in specific spots along the bottom plate. Figuring that there might be a way to slightly improve the heat transfer so the entire bottom plate heats up and CPU temperature decreases I started to mess around with the heat transfer compound on the bottom plate where it attaches to the CPU chip.

There is a greyish, crumbly brick of thermal compound that will either stick to the plate or the chip, (or sometimes to both). It is fairly thick, even in it's compressed state. I set up a little test to find out what the normal CPU temperature was before altering things;

Now it is warm in my house (I do not run the air conditioning) so right now the temperature is 28.3 C (83 F). To put a consistent load on the CPU I am running BONIC with two projects running on a Windows 7 OS at 100% CPU utilization. The front of the laptop is elevated 4" above a flat surface so there can be a little bit of air flow under the computer. There is a fan moving the air around in the room.

After 2 hours of letting temperatures stabilize the CPU temperature was 76 C. Running my hand along the bottom of the case there are hot-spots that would be uncomfortable against bare skin for more than a few minutes.

I then removed the bottom plate and scraped off the thermal compound and set it aside. Wiped down the mating area on the heat sink and chip with a cotton swab and a little bit of acetone.

Then I applied thermal interface pads (Dell PN UP755). To fill the gap it took ten pads stacked so the CPU chip and the thermal plate were in contact when the two screws on the center of the bottom plate was tightened down.

I had a use for the old thermal compound; I unscrewed the two heat pipe clips at the ends, the one bent pipe with the clamp at the middle and the entire heat pipe assembly. Under the clamp areas you can see where the heat pipes are supposed to come in contact with the magnesium alloy bottom plate. I rolled the left-over compound out, like you would with pizza dough, until it formed a small stick about six inches long. I broke that in to three pieces and put the material between the bottom plate mating surface and the heat pipe. Each of the three pieces were about 2" long.

Bolting it all together I fired up the laptop and re-started BONIC and "open hardware monitor" to watch the temperatures.

At first it did not look good; the temperature jumped up to 84 C and over the next hour it slowly climbed to 100.0 C. Right when it hit 100.0 C something happened with the compound; the temperature very quickly fell to 62 C where it remains right now.

The entire bottom plate is pretty uniform in temperature, no hot-spots. The resulting temperature is 14 C than what it was before I started to change things around.

1). The gap between the CPU chip and the heat sink is ridiculous. I am wondering if I shim up the heat sink it will close the distance to the chip. I have plastigauge so I can measure the distances and determine how much shimming I would need to do on the heat sink to close that gap up. Decreasing the amount of thermal compound will make cooling a bit more efficient.

2) Adding a heat sink compound to the ends of the thermal pipe really does improve the heat transfer characteristics to the bottom plate. If you are going to pull the bottom plate off for any reason that can be a five minute fix with some thermal compound, even if it is not the expensive stuff (like Arctic Silver).

3) There is some information out there that thermal compounds go through a state change when they are temperature cycled. I will be interested to see if 62 C is the new "normal" or if it will lower the temperature by a few more degrees.

I did this not because I am overclocking the machine but I do some CPU intensive things that will make it run hot over a several hour period (radio propagation analysis stuff). I do not want to burn my lap.

So,
From: 76 C
Before curing or "set" 100.0 C
After curing or "set" 62 C

Materials used: Thermal transfer pads and reusing the old compound to improve heat transfer elsewhere

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Interesting observation with CPU cooling