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Using a PCB as a heatsink

E

Eeyore

Jan 1, 1970
0
MassiveProng said:
Most of it at this level will/would get handled by thermal
conduction cooling via the leads into surrounding substrates.

That is why I think this would work so well. It has two soaking
elements instead of one, and if they were all on the main PCB the
whole mass could warm and actually degrade the thermal abatement
desires. PCB substrates are fiberglass and epoxy, and don't move
thermal energy very fast.

Really ? I've heard it claimed thar FR4 counducts not a lot worse than
aluminium.

Graham
 
E

Eeyore

Jan 1, 1970
0
MassiveProng said:
With emissivity the apparent color is not as important as the
surface quality.

We painted our black body calibration sources with a matte black
paint and got near .98 plus emissivity. Average Human skin is also at
.98

Both are due to surface texture. The bare aluminum ingot has a smooth
surface, and some of the IR generated inside when it gets heated get
reflected back inside. When it has the matte finish, it has many many
more angles to radiate from the surface at. They are little tiny
scratches that add up to more actual surface.

IR and a smooth, polished , flat surface do not get along. It
reflects back inside the medium, and any that does radiate only
radiates from the surface in a perpendicular ray. The matte surface
allows the heat in the medium to radiate away from its surface at
several angles of incidence. A denser IR "flux" emanates.

I certainly agree about surface finish.

I spent some time too on 'turbulent air flow' heat sinks. They work far better
than ones where the air flow is laminar.

I've *removed up to 30% of the material in force cooled heat sinks and got a
result that's 20% *better* !

Graham
 
E

Eeyore

Jan 1, 1970
0
MassiveProng said:
respect a few holes in the pcb will allow extra air flow.

In this case, if anything, s bunch of plated vias would assist in
the conduction cooling process,

That too. I like to have some of them around surface mounted regulators for example.

but not much on convection in a box with no forced air. With forced air, it would add
a little.

With some holes you can get convection 'through' the pcb. I've done this where I've had
pcb mounted vertical heatsinks for the likes of TO-220 devices.

Graham
 
J

John Larkin

Jan 1, 1970
0
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
 
J

John Larkin

Jan 1, 1970
0
Heat radiating from a hot surface conducts into the air. Using the
single term radiates to air is fine. IR does the work either way,
from within the hot medium. How do you think heat conducts to air
down at that little boundary layer? Does it shake hands with the
surface? No. IR is the thermal engine of ALL matter. We wouldn't be
here otherwise.

Are you saying that infrared photons are the mechanism by which heat
is transferred from a pcb surface to the surrounding air? Or that IR
is the way heat is transferred "within the hot medium"?
It DOES matter when what WE are after is a maximized system for
removing heat.

Thermal design by hunch and opinion is like any other design by hunch
and opinion: probably wrong.
Shall we discuss micro-air gaps under large IGBTs? Next thing,
you'll be telling me that doesn't matter either. There is an entire
industry that says you're wrong.

I never mentioned micro air-gaps under large IGBT's, so you would be
in error to say I ever did, much less that I got it wrong. Hell, I
don't think I've ever seen an IGBT in person.

John
 
T

Terry Given

Jan 1, 1970
0
John said:
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
 
R

RHRRC

Jan 1, 1970
0
That's funny. Looks to me like exactly what these guys did with one
of their one watt LEDs... you know Luxeon...

They even put it in a little plastic tub.

http://www.amazon.com/TerraLUX-LED-Replacement-Maglite-Flashlights/dp...- Hide quoted text -

- Show quoted text -


Yes - highly creditable source you refer to.
According to their blurb they are 25 times brighter than a Krypton
bulb.

Facts: from luxeon datasheet typical output for 1W emitter at 350mA
(typical Vf 3.42V) for a white typical 45 lumen, minimum 30 lumen: for
a warm white typical 20 lumen, minimun 14 lumen. all at Tj=25degC.
Derate down to 68% of these figures at Tj=120 degC. Linear
interpolation between these figures.

The bulb they are 25 times brighter is (say typically) a KPR102
flashlamp bulb -700mA, 2.4V, 15 lumen.

According to their blurb the batteries last 10 to 15 times longer

According to their blurb the led last 100,000hrs.
- better let Lumileds know since they claim much less.

In practice they run the led (very variably) in the 150mW region *not*
the 1W or more the poster requires.
 
M

MassiveProng

Jan 1, 1970
0
Really ? I've heard it claimed thar FR4 counducts not a lot worse than
aluminium.

Graham
You're kidding right?

Al is one of the best thermal conductors. That's why we use it for
heat sink media.

Don't see many FR4 or G10 heat sink fins out there.

No. PCB substrates are specifically designed to be rigid,
non-conducting (electrically) mediums. As a rule, most electrical
insulators are also fairly poor thermal conductors. Of course, silica
is one exception.

The fiberglass component moves heat very slowly. The epoxy binders
make it a bit better. The material is also meant to withstand high
temperatures, like ovens and frying transistors. The cladding
adhesive releases before the board substrate burns.
 
M

MassiveProng

Jan 1, 1970
0
I certainly agree about surface finish.

I spent some time too on 'turbulent air flow' heat sinks. They work far better
than ones where the air flow is laminar.

Yes, which is why both added vias (large with small annular rings),
as well as staggering the placement heights would make this design
work quite well.
I've *removed up to 30% of the material in force cooled heat sinks and got a
result that's 20% *better* !

Yep. even slow moving fans work if the design is done right. All
depends on power levels of course. It gets harder to say these things
as the heat actually really does start being exhibited by a part for
any given case size. Air or not. But for low power dissipation
levels, it is very easy to abate small thermal generations.
 
M

MassiveProng

Jan 1, 1970
0
Are you saying that infrared photons are the mechanism by which heat
is transferred from a pcb surface to the surrounding air? Or that IR
is the way heat is transferred "within the hot medium"?

IR is the engine for all heat. When you pass current through a wire
to make a soldering iron's tip hot, it is IR energy that heats up
those mediums, and it is IR energy that "conducts" into the air
streaming up off that hot tip.
Thermal design by hunch and opinion is like any other design by hunch
and opinion: probably wrong.

There was no hunch, but it seems we do have a chump in the thread
now, chump.

It is MORE than a fraction of a percent, and it is YOU that issued
the hunch. Accusing me of it makes YOU the chump.
I never mentioned micro air-gaps under large IGBT's, so you would be
in error to say I ever did,

It is the SAME kind of thing, mr. resistors in series.
much less that I got it wrong. Hell, I
don't think I've ever seen an IGBT in person.

OK, dipshit... Think HUGE HOT FET. The interface between the FET
and the sink is crucial.

Having the mask on a PCB meant for abating heat is NOT optimal, and
represents MORE than a "fraction of a percent" of degradation.

Hunch that, hunch boy.
 
M

MassiveProng

Jan 1, 1970
0
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
shit) 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?
 
F

Fred Bartoli

Jan 1, 1970
0
MassiveProng a écrit :
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
shit) 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?

I tend to prefer figures to "taste".
 
M

MassiveProng

Jan 1, 1970
0
MassiveProng a ?crit :


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.
 
J

John Larkin

Jan 1, 1970
0
IR is the engine for all heat. When you pass current through a wire
to make a soldering iron's tip hot, it is IR energy that heats up
those mediums, and it is IR energy that "conducts" into the air
streaming up off that hot tip.

That's demonstrably silly, and useless besides. Air is almost
perfectly transparent at thermal wavelengths (easy to demonstrate) so
radiation is not the mechanism by which a hot object transfers heat to
the surrounding air... any IR leaving a hot surface zips right through
the air without heating it.

Example: a square cm of blackbody-quality pcb surface, 25 deg C above
room-temp radiative ambient, radiates only about 17 milliwatts of
thermal IR. That's tiny compared to convection.

John
 
E

Eeyore

Jan 1, 1970
0
Terry said:
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.....

Marketing !!!!!!!!!!!

Aaaaarrrgggghhhhhhhhh !

Graham
 
M

MassiveProng

Jan 1, 1970
0
Hi - I'm going to be making a board that has a large number of high
power (1W) LEDs. I'd ideally like to use the PCB as a heatsink to get
rid of all the heat. How much surface area/watt do I need to allow?
Thanks!

-Mike

You need to give the actual package you are using as the coupling
characteristics play into it.
 
J

jasen

Jan 1, 1970
0
Don't see many FR4 or G10 heat sink fins out there.

No. PCB substrates are specifically designed to be rigid,
non-conducting (electrically) mediums. As a rule, most electrical
insulators are also fairly poor thermal conductors. Of course, silica
is one exception.

I had the impression that alumina and AlN were better,
silica is one ingredient in the fibres of FR4
 
R

Rich Grise

Jan 1, 1970
0
Really ? I've heard it claimed thar FR4 counducts not a lot worse than
aluminium.


Do you mind if I ask from whom you heard this? It doesn't sound very
realistic to me.

Thanks,
Rich
 
E

Eeyore

Jan 1, 1970
0
Rich said:
Do you mind if I ask from whom you heard this? It doesn't sound very
realistic to me.

Some dim and distant application note.

Graham
 
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