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I've got a project that features an array of ultraviolet LEDs on an aluminium board. This board shall be screwed to a heatsink.

My problem is that I'd like to apply thermal paste to improve the thermal conductivity, but I don't know how.

The area is approximately 220 mm x 440 mm and in total twelve screws are distributed evenly over the board.

How would you apply the thermal paste in this case? For a CPU I'd know the answer, but the surface of a CPU heatspreader is way smaller and I guess the dot technique won't work here.

I think covering the whole area is not a good idea because there will be many air pockets left. Perhaps still dots to give the air in between space to get out?

Any advice?

Thanks!

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The question of pattern is an open one (and likely not a conclusive one i.e. infinitely many patterns are equivalent), but the use of patterned paste is a well trusted and documented one. Here's an application note for example:

Using screen-printing templates to apply thermal grease in IGBT modules | Infineon

 Screen-printing template for an EconoPACK™ 3 baseplate

A toothed trowel could also be used, making a grooved surface (see: tile mortar / mastic trowel). The grooves need to be fairly shallow, of course; not like a hardware store trowel!

(Although come to think of it, maybe you could spread an even layer of paste to start with, then use such a trowel to scrape furrows out. Repeat crosswise if a square-dot pattern is desired. Would be a bit messy, but could be an option...hmm!)

The resulting furrows might trap air, but those voids should never be wider than the groove spacing.

There is still the question of permissible voids. This depends on size of the void, thermal conductivity of the aluminum PCB, and power level. Heat can flow sideways through the PCB to nearby grease spots, raising the temperature of that source (i.e. a source directly above such a void) by some nominal amount. It might well be that your power level is low enough that it's highly noncritical, and a bit smeared on here and there would do.

Although I would guess a PCB in free air would do well enough in that case, not needing a heatsink at all. Perhaps your power level is pretty aggressive, on the order of a kW say; in that case, you may want to take thermal measurements and verify that you're getting the performance you require.

Keep in mind also, the grease only smooshes down so far, at some distance from the screws. How much, depends on the stiffness of the board (so, thickness and material). Paste is only a modest conductor by itself, with its main value being to displace thin layers of air (which is a particularly poor conductor in thin gaps). A full calculation might involve device size and power, and the conductivity and thickness of the PCB, grease, and heatsink; mind that the sink itself need not have a uniform temperature, so is worth including in the calculation/simulation. (A full simulation is probably more effort than it's worth; just try a few paste strategies and see what measures best. Do apply power gradually, so as to avoid overheating things in case a setup performs poorly.)

There's also the option for gap-filling pads, which are made from a tacky rubber material with good thermal conductivity. Self-adhesive materials are also available, providing a near-permanent bond between pieces. Since a PCB of this size, and average thickness, will be modestly flexible, such an assembly could be rolled together without too much trouble. (Board flex can take up larger-scale errors in flatness, and the pad takes up smaller-scale errors or surface roughness.)

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  • \$\begingroup\$ Second the gap pad option. An area this large without a specialized bracket or something to maintain even pressure is asking for voids. Thermal grease is designed to be used under high pressure in very thin (single digit micron) layers. \$\endgroup\$
    – vir
    Commented Jun 30, 2023 at 22:22
  • \$\begingroup\$ Thank your these details. I should have mentioned the power level which will be in the 300W range. \$\endgroup\$
    – Korbinian
    Commented Jun 30, 2023 at 23:59
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thermal paste with an straight (not warped or bent) al-PCB and good screws would be evenly distributed all over the board and if everything is OK, the paste layer would be so thin that it would hardly consume one little tube of paste.

I would do this routine:

  • Apply some drops on the base surface
  • Put the LED board on it and while applying pressure, jiggle (shake) the LED board so it spreads and makes a reasonably thin layer.
  • Take it up, apply some drops to dry parts
  • Put the LED board back and jiggle it again while fastening the screws.
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Dot technique sounds like a bad technique as you want a very thin layer of paste between the heatsink and board. Plus, the dot technique requires a lot of pressure between the surfaces for complete flow. Unless the PCB has a brace, you won't get the pressure required between the fasteners to evenly spread the paste.

We use a plastic scraper (putty knife) available from hardware stores in the paint section to distribute the paste on large area heatsinks (240 x 120 mm). As much as you can, roll the board on the heatsink to reduce trapped air. I would place a 1 mm shim on one end of the board and install fasteners on the opposite side of the shim, working your way towards the shim. Take care not to stress the board when the fasteners get close to the shim. When you're approximately 3/4 the way towards the shim, remove the shim.

An interesting article examines the effectiveness of various paste patterns.

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