What kind of thermal grease for CPU?

Question

What kind of thermal grease for CPU?

Hi everyone,

I dismantled my iMac G4 17" 1.25Ghz in order to change the DVD inside. And here I made the mistake: it was so dusty inside (I bought it second hand from a chain-smoker) that I decided to clean it. In the process, the thermal pipe got moved. So I tried to move it back into place and realized there were pins to align it. Don't ask how, but the metal clip that holds the double-pipe onto the CPU unclipped itself. So the processor was all visible. I decided to clean off the grease on it and apply thermal paste (arcticsilver 5) instead of whatever was there. And now I think it was the wrong stuff: the machine works but as soon as the workload increases, it freezes. If it sits there, idle, it's happy. But if I launch a Quicktime movie and play an iTunes track (70% CPU usage), it kernel panics after 10 minutes. Launch dnetc and it crashes within 3 minutes (95% CPU usage).

So here is my question: what kind of grease is used between the CPU and its thermal pipe? From what I saw, it was more like a grease than a paste. Does anyone know? I spoke to an Apple Center and the only thing they could tell me is that the machine needs a new logicboard. No way, let's do it myself, but with what compound?

Thanks for your help on that one!
Pierre

iMac G4 1.25Ghz, Mac OS X (10.4.6), no Airport

Posted on Apr 12, 2006 2:17 PM

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Answer 1

Ferd II

Level 10

104,871 points

Apr 12, 2006 8:25 PM in response to Pierre Haberer

Pierre,

The Do-It-Yourself Mac:

Between the thermal pipe and mating surface is a thin coat of silicon paste (known as thermal paste or heat-sink compound). This paste—available at electronics shops for around $5 a tube—helps eliminate air gaps between the surfaces, so heat goes out the pipe rather than into the iMac.

Since it exhibits Kernel Panic behavior, read Resolving Kernel Panics, by Dr. Smoke for some tips.

;~)

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Answer 2

Apr 13, 2006 12:02 AM in response to Ferd II

Thanks Ferd

I had found this info by searching this forum and searching the web and it is wildly available. What I was (still am) asking about was the thermal compound used between the CPU itself and the heatpipe, not between the pipe and the upper part of the casing.
So I'm still on the lookout...

Pierre

iMac G4 1.25Ghz Mac OS X (10.4.6) no Airport

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Answer 3

Androcles

Level 3

536 points

Apr 13, 2006 11:02 AM in response to Pierre Haberer

Go to http://www.applerepairmanuals.com/iMacflatpanel_usb2.0.pdf?AppleManuals=5ef7ab4c6307fb421017af7b3ef4226c

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Answer 4

Apr 13, 2006 12:43 PM in response to Pierre Haberer

Surely thermal paste is thermal paste. The Apple Service Manual, that Androcles links to, (Mirror here) doesn't mention different types.

It's all just a paste of a good conductor (i.e. Silver).

mrtotes

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Answer 5

Androcles

Level 3

536 points

Apr 13, 2006 4:32 PM in response to Adam Coates

I actually looked it before... and there really isn't a significant difference to warrant using an expensive or inexpensive paste... considering performance.... price is another issue...

Be very generous with the paste... Best of luck!

p.s. I bought the cheap silicone based one and it worked fine...

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Answer 6

Apr 14, 2006 2:34 AM in response to Androcles

Ok, I got it fixed. Here's how:
First I looked at the PDF mentioned earlier. Again, because I had found it before. And there is nothing about the CPU.
So I made measurements and research on the Web about G4 cooling. The thermal pipe on the CPU side doesn't have a flat finish - unlike the bridges that need thermal paste. Also, the chip itself is very slightly above the square formed by the CPU. Originally, I was using Arctic Silver 5 only on the chip. Then I remembered that when I removed the thermal pipe from the CPU - I will never do it again, I promise! - there was a sort of grease almost everywhere.
Research showed that thermal paste was not the compound to use because it's too thick and because of the surface of the thermal pipe. Instead, you have to use thermal grease (which is usually white) because it's more liquid and stay that way. You apply it generously on the chip and on the borders of the CPU. It makes a seal between the whole CPU and the pipe, which means the surface in contact is 4 times bigger, with a hot spot in the middle (the CPU) and a less warm surface around it.
Then I reassembled the iMac, booted up, let it run for an hour doing a bit of iTunes work. Then I extracted the tracks of a CD, fine. Then I launched dnet which is great for CPU testing: after 20 minutes of work with a 97% CPU usage, the iMac is still happy. I stopped here because I didn't have time to test further, but I will let it run tonight. And if I get the time, I may replace the "no-brand" thermal grease I bought by some Arctic Silver Ceramique.

iMac Sunflower Mac OS X (10.4.6)

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