On Mon, Sep 14, 2015 at 6:12 AM, John Doty <jpd AT noqsi DOT com> w= rote:

On Sep 14, 2015, at 7:16 AM, Stephen R. van den Berg (srb AT cuci DOT nl) [via ge= da-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 mas= k to
>>> expose more copper to remove the most heat possible.
>
>> It's debatable whether bare (shiny) copper or soldermask can d= issipate
>> heat better.=C2=A0 I've heard both ways, although I'm part= ial 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 th= ough):
>
> 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 a= re 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.=C2=A0 No, they call it that because a b= lack
> body does not reflect anything *extra* besides the radiation it alread= y
> emanetes.

Not true. Emission and absorption are determined by the same functio= n of wavelength, commonly called =E2=80=9Cemissivity=E2=80=9D. The higher t= he 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 m= achine that would violate the second law of thermodynamics.

>
> Does the material matter then at all?=C2=A0 Yes, it does, but only for=
> two reasons:
> - If you cover it with soldermask, then the soldermask will carry off<= br> >=C2=A0 a little bit more of the heat to neighboring soldermask.=C2=A0 T= o maximise
>=C2=A0 this effect it will suffice to simply have no soldermask clearan= ce
>=C2=A0 so that the soldermask touches the sides of the copper.
> - Spots which are not covered with soldermask, which are either
>=C2=A0 tinned or raw copper will not help in radiating more than
>=C2=A0 soldermask covered spots, but they *do* help when receiving
>=C2=A0 incoming radiation.=C2=A0 I.e. they will reflect radiation comin= g
>=C2=A0 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
>=C2=A0 =C2=A0want 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
>=C2=A0 =C2=A0than 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 =E2=80=9Csilver teflon=E2=80=9D 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 emis= sivity, so it reflects the incoming radiation.

In space engineering, we sometimes capture this effect by defining an avera= ge emissivity, =CE=B5, over thermal infrared wavelengths, and and average e= missivity, =CE=B1, over the solar spectrum. We call =CE=B1 =E2=80=9Cabsorpt= ivity=E2=80=9D to distinguish it from =CE=B5, but the real distinction is j= ust the wavelength range, not the physics.

When you=E2=80=99re not operating in a vacuum, coating the bare metal with = a polymer will very likely improve radiative cooling. It is, however, an ad= ditional thermal resistance between the metal and the air, so it will impai= r conductive and convective cooling. Detailed calculation and measurement i= s the only way to resolve this.

I bet one way comes out a lot better than the other though, such that= a rule of thumb is possible for most circumstances.=C2=A0 I've heard b= oth as well and tried to sort this out in the past, I just took another loo= k:

=C2=A0 * I just read that= convective cooling is the by far the dominant term at normal operating tem= peratures (with atmosphere). =C2=A0

=C2=A0 * I think convection is ultimately driven by conduction acro= ss the material boundary? =C2=A0

=C2=A0

= =C2=A0 * From experience, I think polymer insulates much more than metal

So If the above is true I think the polym= er would squash the dominant form of dissipation and bare metal is better.<= /div>

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