Re: Using a PCB as a heatsink

On Fri, 16 Feb 2007 01:35:36 +0100, Fred Bartoli
<fred._canxxxel_this_bartoli@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> Gave
us:

MassiveProng a ?crit :
On Fri, 16 Feb 2007 08:09:29 +1300, Terry Given <my_name@xxxxxxxx>
Gave us:

John Larkin wrote:
On Thu, 15 Feb 2007 04:33:03 -0800, MassiveProng
<MassiveProng@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:


On Thu, 15 Feb 2007 05:35:18 +0000, Eeyore
<rabbitsfriendsandrelations@xxxxxxxxxxx> Gave us:


And a solder mask adds a minute amount of thermal resistance, not
enough to worry about.
I'd have thought so myself.
A blanket is a blanket is a blanket. Period.

It's like resistors in series. A couple of mils of solder mask in
series with inches of air. And the mask material is a much better heat
conductor than air. So the mask adds a small fraction of a percent to
overall theta, not enough to matter. It probably improves radiation a
bit, more than it impedes conduction.

John
I've never looked at solder mask in this respect, but I did the calcs
once on powder-coating. Its a surprisingly good blanket (after all, its
plastic). The conclusion was: dont powder coat heatsinks. So we anodised
it instead - "it" being a flat Al plate

disclaimer: I had no idea they were going to be powder coated until I
saw one; some marketing guy decided he wanted everything black.....

Cheers
Terry


If you make sure to have it grit blasted before the anodize, you'll
maximize its thermal efficiency. Matte black IR paint (not just any
***) will make it radiate AND thereby convect to the surrounding air
even better still, further maximizing the capability of the sink.

The finish extrusion typically places on the Aluminum is usually too
smooth for my tastes, but I used to make fully traceable Black Body
calibration sources for NIST, so what do I know?


So you should have no pb giving us some emissivity figures and back of
the envelop convection/radiation ratio at usual temperatures. Right?

Our tests were such that a matte finish surface of any media
radiated IR better than a polished or glazed surface will. It is also
in just about any CRC physics book around. Surface emissivity
listings tend to refer to surface quality and many times give two
figures. Polished, and a rougher surface quality figure.

I tend to prefer figures to "taste".

I prefer common sense to your need for hard documents or formulae.
Pretty basic physics.

It all depends on how the face being observed is finished.

We cut grooves in ours so nothing "looking at" the black body source
was ever actually pointed at a perpendicular surface, and a device
"looking at" the surface could also be at other than a perpendicular
angle and still get the same reading.

We had others where the target was heated air that were essentially
a coil of nichrome heating wire wrapped around a basketball
(literally) then doped up with fiberglass/silica "paper mache" <sp>.
After that dries, we split the halves, and remove the ball, and apply
more silica medium to the inside of the sphere., then the two halve
get assembled back together with a 2" silica tube about ten inches
long on it. That assembly gets baked. We suspend that in am
insulated cabinet, and provide NIST with a source that can do 3000 C
at an emissivity of 0.995, and hold/keep temperature within a few
degrees of setpoint.
That's pretty awesome!

On the benchtop sources, getting a uniform temperature across an 6"
plate sized surface with a single heating element source is not an
easy task. The ingot is several inches thick, and by the time it (the
heat source) soaks out to the front surface, all but the last half
inch at the outer edges of the ingot are evenly radiating. Especially
since it gets recessed in a case so that no local air currents play
upon it.
.