Mail Archives: geda-user/2015/09/14/10:12:52
On Sep 14, 2015, at 7:16 AM, Stephen R. van den Berg (srb AT cuci DOT nl) [via geda-user AT delorie DOT com] <geda-user AT delorie DOT com> wrote:
> DJ Delorie wrote:
>>> What you want to do is have tons of vias and peel back the mask to
>>> expose more copper to remove the most heat possible.
>
>> It's debatable whether bare (shiny) copper or soldermask can dissipate
>> heat better. I've heard both ways, although I'm partial to the
>> argument that a matte black mask would be the best emitter.
>
> There are some common misconceptions here (as a physicist, I think
> I can speak with some authority; I don't claim to be infallible though):
>
> The colour of the material, or even the material itself which is
> on the outside (touching the air) does not matter at all with
> regard to heat radiation.
For temperatures of a few hundred Kelvin, the relevant wavelengths are in the infrared, so the visible color is indeed irrelevant.
>
> The reason why it's called a black-body radiation is not because
> it radiates better when black. No, they call it that because a black
> body does not reflect anything *extra* besides the radiation it already
> emanetes.
Not true. Emission and absorption are determined by the same function of wavelength, commonly called “emissivity”. The higher the emissivity, the darker the material will appear at the given wavelength. If you had a material for which this was not true, you could construct a machine that would violate the second law of thermodynamics.
>
> Does the material matter then at all? Yes, it does, but only for
> two reasons:
> - If you cover it with soldermask, then the soldermask will carry off
> a little bit more of the heat to neighboring soldermask. To maximise
> this effect it will suffice to simply have no soldermask clearance
> so that the soldermask touches the sides of the copper.
> - Spots which are not covered with soldermask, which are either
> tinned or raw copper will not help in radiating more than
> soldermask covered spots, but they *do* help when receiving
> incoming radiation. I.e. they will reflect radiation coming
> from the outside better than spots covered in soldermask.
>
> So, what is best:
> a. White soldermask.
> b. Soldermask merely touching the sides of the copper (if you
> want to spread the heat to surrounding soldermask as well).
> c. Leaving as much reflective metallic area uncovered.
> d. Making the metallic area as shiny as possible (tinned is better
> than raw copper).
>
> But, to summarise, it will only help reflecting incoming radiation,
> it will not matter one iota for primary heat radiation.
Not true. Polymers generally have higher emissivity in the infrared than shiny metal. We exploit this in spacecraft design by covering surfaces we want to keep cold with “silver teflon” tape. The outer teflon layer is highly emissive in the thermal infrared, but transparent to most of the incoming solar radiation. The silver backing has very low emissivity, so it reflects the incoming radiation.
In space engineering, we sometimes capture this effect by defining an average emissivity, ε, over thermal infrared wavelengths, and and average emissivity, α, over the solar spectrum. We call α “absorptivity” to distinguish it from ε, but the real distinction is just the wavelength range, not the physics.
When you’re not operating in a vacuum, coating the bare metal with a polymer will very likely improve radiative cooling. It is, however, an additional thermal resistance between the metal and the air, so it will impair conductive and convective cooling. Detailed calculation and measurement is the only way to resolve this.
John Doty Noqsi Aerospace, Ltd.
http://www.noqsi.com/
jpd AT noqsi DOT com
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