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Date:	Mon, 14 Sep 2015 09:28:41 -0400
Message-ID:	<CAM2RGhQrNT-NbXSsuVb_Wjzgd1HWhDR21kLC357wA+yx7bTWKQ@mail.gmail.com>
Subject:	Re: [geda-user] 5630 LED footprint
From:	"Evan Foss (evanfoss AT gmail DOT com) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
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On Mon, Sep 14, 2015 at 9:16 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.
>
> 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.
>
> 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.

Thank you that was a satisfying read.

> --
> Stephen.



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