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Date: Mon, 14 Sep 2015 15:16:29 +0200
From: "Stephen R. van den Berg (srb AT cuci DOT nl) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
To: geda-user AT delorie DOT com
Subject: Re: [geda-user] 5630 LED footprint
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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.
-- 
Stephen.