Hi,

 I would disagree that it varies "Much" but you do need to do a little math.

 Knowing your PCB material you know the thermal conductivity, that is your
key parameter. This lavers of copper on internal layer don't help much
though for a small part, like an SOT-23 they will help spread the heat.
Spreading the heat gives a greater area cross section with which you apply
to your thermal conductivity formula.

 My experience with an Al backed board is that those thin copper foils do
not conduct out from the part very well, many .25 inch or so. While thermal
conductivity of FR4 is about .3 w/m k  and copper is 1000 time that, the
cross section of a copper foil is only 1% that of a 10 x 10 mm square
through the board. So the thermal resistance through a PCB is in the same
order as the thermal resistance through a solid copper plane! (Someone
please check my math!)

You can take two VERY important design steps:

 1. Use a thin PCB laminate.

 2. Use a thermally conductive laminate.

Then, if your device is small (sot-23 type size copper on top AND internal
layers will help spread heat.

 Use LOTS of tiny vias to tie your top side copper (directly under the part,
not around the sides) to both internal planes and bottom side copper. I am
assuming you have a very thin insulating layer between bottom side copper
and the Al plate so bottom copper to Al thermal resistance will be very
low.. The many thin walls on the vias will suck down heat well. Avoid the
idea of big holes filed with solder. Solder has about 10% or the thermal
conductivity of copper. You want a lot of little vias each with a copper
wall running top to bottom.

 With a DPAK sized device on .5mm thick thermally conductive laminate and
with a lot of tiny vias I was able to get about 1C/Watt from DPAK tap to
Aluminum backing. Now think about that. That means I can dump 40 Watts from
a DPAK without any type of mounting hardware, screws, clamps, or epoxies.
Just regular old SMT reflow of the part.

Bob K




-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Wayne Thayer
Sent: Tuesday, April 22, 2014 9:12 AM
To: [log in to unmask]
Subject: Re: [TN] Seeking : Thermal Resistance onto PCB surface

Hi Roland-

It can vary a bunch, depending on how much metal is in the pcb and how that
metal is arranged.  Most designs for thermal transfer depend on thermal vias
to do most of the work.  You can get fast ballpark answers with "back of the
envelope" thermal transfer coefficients for PCB laminate, or even better,
grab a board that's similar and just measure the performance.  If you need
answers within a few degrees, then the thermal analysis can get considerably
more complex.

Wayne Thayer

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Roland Jaquet
Sent: Tuesday, April 22, 2014 4:06 AM
To: [log in to unmask]
Subject: [TN] Seeking : Thermal Resistance onto PCB surface

Dear Technetters,



I am confronted to an unusual challenge I am not able to solve. Can someone
help, if you please



We are seeking : Thermal Resistance onto PCB surface (14 layers) standard
approx.. 1.8 mm thick



Between



External Cu of approximately 30 to 35 metric microns covered with ENIG



And



Aluminum "Alodinized" 1200 or  Anodized in black



Thank you for your help





Meilleures salutations

Best Regards

Roland





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