# Thermal Paste alternatives

#### Rbeckett

Joined Sep 3, 2010
208
I have several SSR's, a power supply, and 2 pancake style cooling fans I am mounting in a control box. The SSR's are all 25 and 40 amp units and the power supply is a 12VDC 5A SMPS. I understand the need for using thermal paste to transfer excess heat, but all I have been able to locate is in very small quantities in a squeeze tube. I am going to need several grams or more and would like to obtain a working quantity of the correct paste without having the issue of waste from so many small tubes. No matter how you squeeze those tubes a small amount remains behind and is an expensive waste. The total voltage applied to the AC outputs on the SSR's is 115 VAC (House current/voltage in the US) at less than 15 amps if all were turned on and operating at the same moment. This is not even possible due to the switching scheme I am using with rotary switches to select the SSR's and components to turn on. So what can I substitute that will work properly, stay within budget, and not create a huge mess when handled? TIA for any help you folks might be able to provide.

Wheelchair Bob

#### mcgyvr

Joined Oct 15, 2009
5,394

#### crutschow

Joined Mar 14, 2008
31,126
How about a 3.5gm tube such as this.

Remember that you want to use a very thin layer of thermal compound, just enough to fill the gaps between the heat sink and the device. A thicker layer increases the thermal resistance since the compound thermal resistance is always higher than the metal it is in contact with.

#### mcgyvr

Joined Oct 15, 2009
5,394
^^^ yes 99.9999% of the time ANY hand applied paste is applied too thick.
We actually have a specially milled "spatula" that is used to apply a consistent layer to some schottky diode modules we use or we just use thermal pads and forget about the messy paste.

#### bountyhunter

Joined Sep 7, 2009
2,512
How about a 3.5gm tube such as this.

Remember that you want to use a very thin layer of thermal compound, just enough to fill the gaps between the heat sink and the device. A thicker layer increases the thermal resistance since the compound thermal resistance is always higher than the metal it is in contact with.
+1 Read that over carefully. In most cases, the paste does more harm than good for that reason. You actually have to use a straight edge to scrape off the excess and make sure it only remains in the surface holes. In reality, paste doesn't really help much even under the best circumstances and hurts if too much is used. It also makes a mess.

#### bountyhunter

Joined Sep 7, 2009
2,512
^^^ yes 99.9999% of the time ANY hand applied paste is applied too thick.
We actually have a specially milled "spatula" that is used to apply a consistent layer to some schottky diode modules we use or we just use thermal pads and forget about the messy paste.
It is surprising how bad (thermal resistance) those pads are. Some are are really thick and some even have an internal layer of Kapton tape to prevent "puncture" that can allow the device to contact the heatsink. I did the research on those Silpads and showed my boss how bad they were and they stopped using them. They only started using them in the first place when the guys on the production line whined about how messy the grease was. But after seeing how bad the silpads were, they told them to shut up and use the grease.

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#### crutschow

Joined Mar 14, 2008
31,126
+1 Read that over carefully. In most cases, the paste does more harm than good for that reason. You actually have to use a straight edge to scrape off the excess and make sure it only remains in the surface holes. In reality, paste doesn't really help much even under the best circumstances and hurts if too much is used. It also makes a mess.
Wikipedia quotes an informal test where the thermal resistance between a heat plate and heat sink decreased from .66°C/W without, to 0.5°/W with thermal paste, about a 30% improvement. I think that would be typical between common components and heat sinks. Interestingly the type of paste didn't seem to make much difference.

#### mcgyvr

Joined Oct 15, 2009
5,394
It is surprising how bad (thermal resistance) those pads are. Some are are really thick and some even have an internal layer of Kapton tape to prevent "puncture" that can allow the device to contact the heatsink. I did the research on those Silpads and showed my boss how bad they were and they stopped using them. They only started using them in the first place when the guys on the production line whined about how messy the grease was. But after seeing how bad the silpads were, they told them to shut up and use the grease.
Just depends on which one you pick and what you can live with.. There are a few in the .9deg C/W or less range.. A "over applied" hand applied paste can exceed that easily.
But yes..thermal paste when applied properly (only a couple mils thick at most) should have a better thermal conductivity..

#### #12

Joined Nov 30, 2010
18,224
Here ya' go. One ounce of thermal goop, GC brand. I probably bought it at www.mouser.com

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#### bountyhunter

Joined Sep 7, 2009
2,512
Just depends on which one you pick and what you can live with.. There are a few in the .9deg C/W or less range.. A "over applied" hand applied paste can exceed that easily.
But yes..thermal paste when applied properly (only a couple mils thick at most) should have a better thermal conductivity..
Our power converters always used TO-3 transistors. The silpads we could get back then more or less doubled the thermal resistance from transistor case surface to heatsink compared to a properly greased mica insulator. That was a huge deal because the transistors dissipated a lot of power. My bosses' initials were JFB and we always said it stood for Just F------- Barely when it came to design headroom....... so the added temperature rise on the transistors was not a good thing.

#### bountyhunter

Joined Sep 7, 2009
2,512
Just depends on which one you pick and what you can live with.. There are a few in the .9deg C/W or less range.. A "over applied" hand applied paste can exceed that easily.
But yes..thermal paste when applied properly (only a couple mils thick at most) should have a better thermal conductivity..
The number that sticks in my brain for a properly greased mica insulator on a TO-3 transistor is something like 0.3 - 0.4 C/W. I think the silpads were about twice that ballpark.

#### THE_RB

Joined Feb 11, 2008
5,438
I just buy heatsink paste in 50g tubes. Then cut them open to get the last gram out.

On larger devices it's not as critical especially if they have good flat heatsinks and large bolts or multiple bolts. You can get away with no paste at all, I often use large block devices like SSRs with no thermal paste, (as Crutschow said in post #7 it only makes a 30% heat difference, and likely less difference with large well-bolted devices). However I also use oversized heatsinks on anything large, and run the component far less than it's max heat dissipation.

#### ErnieM

Joined Apr 24, 2011
8,346
I bought a tiny tube from Radio Shack decades ago, and still have 3/4 of the tube left.

A little dub will do ya is literally true here.

I recently re-compounded my PC's CPU (using the "special" CPU stuff) and one interesting hint the manufacturer offered was to use an old credit card as the spatula, and work the grease into the heatsink.

#### Rbeckett

Joined Sep 3, 2010
208
Thanks RB.
I had originally considered using none, but was trying to "do it right" so I thought it would be better. I am including two 12 volt pancake fans with heat sinks extending outside the control box and not using any where near the capacity of the SSR's. So I might just forgo the goop and just make sure I have sufficient air flow across that part of the control board. That should be more than adequate for dissipating any heat the SSR's might generate.

Wheelchair Bob

#### JayArr

Joined Mar 11, 2013
14
You can buy thermal compound from a plumbing store for about $20 a QUART. It's thinner than the electronics stuff but I've been using it for 10+ years without problem. The outrageous price from electronics supply houses is just a plain olf rip-off. #### Meixner Joined Sep 26, 2011 117 You can buy thermal compound from a plumbing store for about$20 a QUART
What is it called and what do plumbers use it for ?

#### THE_RB

Joined Feb 11, 2008
5,438
Thanks RB.
I had originally considered using none, but was trying to "do it right" so I thought it would be better. I am including two 12 volt pancake fans with heat sinks extending outside the control box and not using any where near the capacity of the SSR's. So I might just forgo the goop and just make sure I have sufficient air flow across that part of the control board. That should be more than adequate for dissipating any heat the SSR's might generate.
...
Yeah the goop is a lot more critical on something small like a TO-220 FET that gets quite hot, as all that heat has to get through a tiny thermal connection.

But the SSR have a metal block back with a very large area to conduct the heat out, and the heat will be quite low in temperature compared to say a single transistor. The small TRIAC device etc inside the SSR will have goop there, attaching it to the back of the SSR, but the back of the SSR is basically a heatsink so really you are attaching one heatsink to another heatsink. Larger area, lower temp differential, much less critical a task.

I wouldn't lose any sleep over it, just connect it up with the fans etc and do some tests. If it runs hot in any way you can add the goop later.

#### gerty

Joined Aug 30, 2007
1,302
I used to work for Carrier Air Conditioning as a maint tech, down on the assembly lines they used thermal compound by the 2.5 gallon bucket. They would spread it liberally on the crankcase heater, then stick it in a cast iron compressor.
The heater would warm the compressor oil on cold days, allowing it to start easier in the heat pump mode.
The heater element was tubular, about 5/8 dia and 4 inches long.
They would coat the entire heater surface, but a dab in the hole in the compressor and insert the heater.

#### JayArr

Joined Mar 11, 2013
14
What is it called and what do plumbers use it for ?
Plumbers or welders use it to coat something they are heating to stop the flow of heat. Silicon will transfer heat well but in a large mass it will "suck up" a lot of heat. For example, if you're soldering a copper pipe that's 10 feet long you don't want to heat up all ten feet before the solder melts so you spread the paste around the pipe a foot away and it's easier to solder. Welders use it to keep surrounding equipment or components from catching fire, discolouring, peeling paint. Autobody guys smear it thick in a circle around where they need to weld a patch to keep the heat from spreading out through the panel and warping it.

Silicon is cheap, it's only the electronics industry that pays through the nose for it, marketers use all sorts of lingo to sell it but it's mostly snake oil, sort of like the myth of Platinum Monster cables.