Mail Archives: geda-user/2015/09/14/13:32:38
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On Mon, Sep 14, 2015 at 6:12 AM, John Doty <jpd AT noqsi DOT com> wrote:
>
> 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 =E2=80=9Cemissivity=E2=80=9D. 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 =E2=80=9Csilver teflon=E2=80=9D tape. The outer te=
flon 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, =CE=B5, over thermal infrared wavelengths, and and av=
erage
> emissivity, =CE=B1, over the solar spectrum. We call =CE=B1 =E2=80=9Cabso=
rptivity=E2=80=9D to
> distinguish it from =CE=B5, but the real distinction is just the waveleng=
th
> range, not the physics.
>
> When you=E2=80=99re not operating in a vacuum, coating the bare metal wit=
h 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.
>
I bet one way comes out a lot better than the other though, such that a
rule of thumb is possible for most circumstances. I've heard both as well
and tried to sort this out in the past, I just took another look:
* I just read that convective cooling is the by far the dominant term at
normal operating temperatures (with atmosphere).
* I think convection is ultimately driven by conduction across the
material boundary?
* From experience, I think polymer insulates much more than metal
So If the above is true I think the polymer would squash the dominant form
of dissipation and bare metal is better.
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<div dir=3D"ltr"><br><div class=3D"gmail_extra"><br><div class=3D"gmail_quo=
te">On Mon, Sep 14, 2015 at 6:12 AM, John Doty <span dir=3D"ltr"><<a hre=
f=3D"mailto:jpd AT noqsi DOT com" target=3D"_blank">jpd AT noqsi DOT com</a>></span> w=
rote:<br><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;borde=
r-left:1px #ccc solid;padding-left:1ex"><span class=3D""><br>
On Sep 14, 2015, at 7:16 AM, Stephen R. van den Berg (<a href=3D"mailto:srb=
@cuci.nl">srb AT cuci DOT nl</a>) [via <a href=3D"mailto:geda-user AT delorie DOT com">ge=
da-user AT delorie DOT com</a>] <<a href=3D"mailto:geda-user AT delorie DOT com">geda-=
user AT delorie DOT com</a>> wrote:<br>
<br>
> DJ Delorie wrote:<br>
>>> What you want to do is have tons of vias and peel back the mas=
k to<br>
>>> expose more copper to remove the most heat possible.<br>
><br>
>> It's debatable whether bare (shiny) copper or soldermask can d=
issipate<br>
>> heat better.=C2=A0 I've heard both ways, although I'm part=
ial to the<br>
>> argument that a matte black mask would be the best emitter.<br>
><br>
> There are some common misconceptions here (as a physicist, I think<br>
> I can speak with some authority; I don't claim to be infallible th=
ough):<br>
><br>
> The colour of the material, or even the material itself which is<br>
> on the outside (touching the air) does not matter at all with<br>
> regard to heat radiation.<br>
<br>
</span>For temperatures of a few hundred Kelvin, the relevant wavelengths a=
re in the infrared, so the visible color is indeed irrelevant.<br>
<span class=3D""><br>
><br>
> The reason why it's called a black-body radiation is not because<b=
r>
> it radiates better when black.=C2=A0 No, they call it that because a b=
lack<br>
> body does not reflect anything *extra* besides the radiation it alread=
y<br>
> emanetes.<br>
<br>
</span>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.<br>
<span class=3D""><br>
><br>
> Does the material matter then at all?=C2=A0 Yes, it does, but only for=
<br>
> two reasons:<br>
> - 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<br>
>=C2=A0 this effect it will suffice to simply have no soldermask clearan=
ce<br>
>=C2=A0 so that the soldermask touches the sides of the copper.<br>
> - Spots which are not covered with soldermask, which are either<br>
>=C2=A0 tinned or raw copper will not help in radiating more than<br>
>=C2=A0 soldermask covered spots, but they *do* help when receiving<br>
>=C2=A0 incoming radiation.=C2=A0 I.e. they will reflect radiation comin=
g<br>
>=C2=A0 from the outside better than spots covered in soldermask.<br>
><br>
> So, what is best:<br>
> a. White soldermask.<br>
> b. Soldermask merely touching the sides of the copper (if you<br>
>=C2=A0 =C2=A0want to spread the heat to surrounding soldermask as well)=
.<br>
> c. Leaving as much reflective metallic area uncovered.<br>
> d. Making the metallic area as shiny as possible (tinned is better<br>
>=C2=A0 =C2=A0than raw copper).<br>
><br>
> But, to summarise, it will only help reflecting incoming radiation,<br=
>
> it will not matter one iota for primary heat radiation.<br>
<br>
</span>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.<br>
<br>
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.<br>
<br>
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.<br></blockquote><div><br></div><div style=
=3D"">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:</div><div style=3D""><br></div><div style=3D"">=C2=A0 * I just read that=
convective cooling is the by far the dominant term at normal operating tem=
peratures (with atmosphere). =C2=A0</div><div style=3D""><br></div><div sty=
le=3D"">=C2=A0 * I think convection is ultimately driven by conduction acro=
ss the material boundary? =C2=A0<br></div><div>=C2=A0</div><div style=3D"">=
=C2=A0 * From experience, I think polymer insulates much more than metal</d=
iv><div><br></div><div style=3D"">So If the above is true I think the polym=
er would squash the dominant form of dissipation and bare metal is better.<=
/div><div><br></div></div></div></div>
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