Re: Using a PCB as a heatsink

On Wed, 14 Feb 2007 18:03:10 -0800, MassiveProng
<MassiveProng@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Wed, 14 Feb 2007 08:13:19 -0800, John Larkin
<jjlarkin@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> Gave us:

On Tue, 13 Feb 2007 23:22:41 -0800, MassiveProng
<MassiveProng@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx> wrote:

On Wed, 14 Feb 2007 07:01:11 +0000, Eeyore
<rabbitsfriendsandrelations@xxxxxxxxxxx> Gave us:



Mike Noone wrote:

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?

How many ? Use some simple logic and work out how hot your pcb is going to get.
Hint: 10W is a lot in an enclosed space.


Make small PCBs for each LED (1 sq cm ea or less), and mount those
above the driver board like a third of a cm or more with pins or stiff
wires, and the airgap will do a lot of cooling with a bit of flow.
Leave both sides of the tiny boards fully cladded on one or both sides
(of course negate shorts), and leave them bare ie no mask so they
radiate their heat better. Heavy gauge copper or SPC wire (solid)
would allow some heat conduction as well.

Bare copper has a thermal infrared emissivity near zero; it's hard to
find anything that radiates worse.

I do not agree with the number you "selected" to use, but yes, any
polished surface will have a lower emissivity. I forgot to mention
grit blasting the PCBs to make the finish matte.


Do you really grit blast pcb's?


Bare copper with a matte finish has a higher emissivity than any
type of copper under an insulative blanket of solder mask.

I'd have to try that. I'm a bit dubious. But radiation is a minor heat
dump mechanism at the sorts of temps that pcb's usually run at.


Heat conducts through the device leads or package, to the PCB copper
and substrate, and then radiates to air. If it has an added layer of
solder mask, that radiation will be less.

Heat doesn't raidiate to air, it conducts and convects to air. Heat
radiates *through* air.

And a solder mask adds a minute amount of thermal resistance, not
enough to worry about.

John

.

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