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Heatsink Mounting Paste Question.

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Ash Small

Sat Jul 16 2011, 02:38PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

I just been doing some Googling to identify some heatsink paste that I aquired with a box of assorted junk.

It would appear that this one, which is silicone free, consists of zinc oxide powder in a synthetic oil, according to the safety data sheet. (most others appear to be zinc oxide in silicone based oil).

Pure zinc oxide nano-particles are a few $'s per pound:

yet heatsink compound is several $'s per gram.

Does anyone have any idea why this is and whether mixing zinc oxide powder with oil will work just as well as commercial heatsink compound?

(I think I'm going to have to try this)

EDIT: I imagine an oil with a low vapour pressure and/or high boiling point would be best, something like diffusion pump oil or some of the brake fluids/hydraulic oils, also one that doesn't absorb moisture, but I'm open to suggestions.

Wikipedia says silicon grease or fractions of mineral oil.

Bored Chemist

Sat Jul 16 2011, 06:53PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

"Does anyone have any idea why this is"
Because they can.
Who wants to risk frying the expensive new CPU by experimenting with a cheap alternative to the thermal paste when you can just buy the "official" stuff?

Ash Small

Sat Jul 16 2011, 09:52PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Bored Chemist wrote ...

.Who wants to risk frying the expensive new CPU by experimenting with a cheap alternative to the thermal paste when you can just buy the "official" stuff?

You have a valid point there, but when I'm using nearly Â£10's worth of paste every time I change a Â£1 2N3055 it's time to try making the stuff myself.

Further reading has informed me that most of the heat is condicted by the oil itself, the 'filler' just improves the heat conductance a bit (and presumably helps keep the oil in place).

Sulaiman

Sun Jul 17 2011, 11:22AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Ash, the purpose of the thermally conductive paste is only to fill the voids between the two metal surfaces, a thick layer is worse than a thin layer. It's not like cement with bricks.

Practice putting a small blob in the center and attach to the heatsink then take off the transistor to see how much is required so that all of the metal is covered with as little as possible 'leaking out' around the edges. Some folks smear the grease all over, some (me) just put a single blob or line to reduce air bubbles being trapped.
Also, due to the viscosity of the paste it's better to tighten the transistor in stages, leaving a little while between tightenings to allow the paste to flow.

The excess that squeezes out is wasted, worse, it becomes a place for any contamination blowing around to stick to, possibly causing an electrical leakage path.

The 'best' interface material is a thin soft metal sheet that deforms under pressure to completely fill any voids with excellent thermal conductivity. Often used with igbt blocks etc.
Not worth it for a 2N3055 though!

I guess, though I don't know, that if the oil is very thin it will creep out across the metal surfaces leaving a dry powder film ... not good.
'dried up' thermal paste definitely causes failures.

Pinky's Brain

Sun Jul 17 2011, 07:29PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
Location:
Posts: 837

You can get phase change pads pretty cheaply for PC CPUs ... you could just cut them up.

They don't really have the best thermal conductivity, but they are almost impossible to mis-apply and have the best long term reliability (minimal thermal pumping).

The large metal block heatsinks we use for switches have very large thermal resistance any way (CPU heatpipe heatsinks can be pretty impressive in this regard) so in the end the slightly higher thermal resistance of the pad isn't a big deal.

Bored Chemist

Sun Jul 17 2011, 09:17PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022

Assuming this

is right then it's the additives in thermal epoxy and grease that do the job.

Ash Small

Sun Jul 17 2011, 10:40PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Bored Chemist wrote ...

Assuming this

is right then it's the additives in thermal epoxy and grease that do the job.

I appreciate your input, Chemist, but Wikipedia says this:

"In compounds containing suspended particles, the properties of the fluid may well be the most important. As seen by the thermal conductivity measures above, the conductivity is closer to that of the fluid components rather than the ceramic or metal components"

Sulaiman

Mon Jul 18 2011, 06:01AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

I think of it as the particles mostly filling the voids between the metal surfaces and the oil/grease filling the space between the particles.

Ash Small

Mon Jul 18 2011, 04:25PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Sulaiman wrote ...

Ash, the purpose of the thermally conductive paste is only to fill the voids between the two metal surfaces, a thick layer is worse than a thin layer. It's not like cement with bricks.

.

Pinky's Brain wrote ...

You can get phase change pads pretty cheaply for PC CPUs ... you could just cut them up.

.

When I said I used nearly Â£10 worth of paste changing a 2N3055 I didn't use it all on the transistor, I also used it between the plates that the heatsink is constructed from, to improve thermal conductivity there.

Some did get squeezed out though, but I thought it was best to use slightly too much than too little.

I'm building some similar, larger ones for a bridge rectifier, and that will probably use up several syringes full.

I may use what I have left on the stud diodes themselves, and try making some for in between the heatsink plates.

I know a stack of plates isn't ideal at all, but I think with sufficient contact area all the heat will get dissipated.

klugesmith

Mon Jul 18 2011, 05:47PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

I've used zinc oxide sunscreen paste as a thermal conductor, but never measured its properties.

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