Heatsink compounds (thermal greases)

[Doug_Ford]

This discussion cut out of this thread. . . .

Originally Posted by Gerald D

If you replace that alu heatsink/ground plate with a copper heatsink/ground plate that would look smart

Or you could take that money and buy a new house. I can't believe the price of copper lately.

[domino11]

Doug/Gerald,
The high cost of the copper plate would be incredible over in Canada. I would stick to the aluminum plate and invest in some thermal transfer grease or some sort of thermal transfer pad to reduce the thermal junction between the heatsink and the drivers. This should be done even if you have one of the expensive copper plates.

[gmessler]

Here is a good source of thermal grease.

http://www.tigerdirect.com/applicati...&sku=C283-1147

[Gerald D]

Use the grease very lightly. It is only meant to fill the air gaps and is not supposed to stop the metal surfaces from touching each other. The grease conducts heat better than air, but not as good as metal. Since I am not using the Gecko near its limit, I don't bother to use any grease at all.

[domino11]

Gerald,
Dont worry about using the grease liberally. Any excess that is not needed will just ooze out from the sides a little and can be wiped up with some alcohol. The purpose of the heatsink grease or any other thermal pad (pads come in various materials but look like thin rubber sheets), is to fill the many microscopic voids in the two surfaces to maximize the thermal contact area. You will always get better thermal performance with some kind of thermal interface material than with just a raw material interface. Even with carefull machining and lapping to mate surfaces, thermal interface material (grease or pad) will improve the thermal performance. (Sorry for the long answer but this is the kind of stuff I do at my day job.)

You are right that you can usually get around using it for this type of application, but I really like to keep my electronics as cool as possible. Remember the fault conditions, us electronics guys like to over protect our stuff. Its probably from years and years of stuffing our hoses and pipes with copper. Personally I hate heat sink grease but thermal interface pads are great things. Just a little more expensive.

[Richards]

My experience makes me lean towards using Gerald's recommendation of using a minimal amount of grease. I've had to replace a bunch of CPUs over the years when my customers thought that if a little is good, more is better. The springs that held the heat sink to the CPU weren't strong enough to squish out the excess grease on the failed CPUs and the grease actually ended up insulating the CPU from the heatsink.

On the back of the package of a tube of heatsink compound that I bought at Radio Shack (part number 276-1372) is this message: "Use a thin coat of heat sink compound between the heat sink and semiconductor to enhance heat transfer. For best results, the heat sink and semiconductor should have a tight mechanical bond."

[Gerald D]

When Heath mentioned a "thermal pad" that looks like a "thin rubber sheet" that did not give me a good feeling. I can just imagine someone believing that thicker is better and putting in 2 sheets . . . . . Then went to google and found this doc which summarises as follows:

The quality of the Thermal Interface Material (TIM) compound is also very important. Manufacturers of thermal grease or paste use various components to achieve good thermal conductivity and proper spreading characteristics. These components have a direct effect on the consistency and durability of the compound. The following should be noted:

- Ensure that the proper amount of compound is used.

- Too little thermal compound reduces the thermal conductivity. The compound can dry out and prevent adequate heat transference to the heatsink. This can cause damage to the processor. An excessive amount of thermal compound can cause some of the material to leak out at operational temperatures, potentially causing contamination or failure of other components on the motherboard.

- Low-quality compounds often harden and dry out too quickly. When they dry out, they are no longer adequate thermal conductors.

- Low-quality compounds can become fluid and leak at the operating temperatures of a system.

- AMD Does not currently recommend using thermal compounds that are electrically conductive. Electrically conductive thermal compounds include most silver and gold pastes. These types of thermal compounds can leak out and contact vulnerable elements of the processor or motherboard (such as capacitors, resistors, pins, etc.), which may cause shortcircuits

or damage the processor or other components.

- Before use, ensure that the thermal compound has not reached its expiration date.

[domino11]

Gerald,
Yes you are right when talking about excess compound when referencing processors and small semiconductors. The case I was referring to was a hard shelled assembly like a gecko that when screwed or bolted down to the heatsink or plate would easily squeeze out any excess compound. Now I am not talking about loading it on in layers but if you just put a single coat with a small stiff brush on the back of the unit then you dont have to worry. Processors are a different animal and most processor manufacturers use what is called a phase change material. This thermal infterface material looks like a thin piece of rubber but when it is installed between a processor and heatsink, the heat from the processor in use actually changes the state of the material and it will conform to the two surfaces and establish a very good thermal interface. The only drawback with these is that if your remove the heatsink for any reason, then a new phase change pad need to be used. The old one is no longer good. This is different than the thermal interface pads I was referring to.