At 11:22 AM 11/22/99 -0800, Douglas McKean wrote:
>[...] the equation for Heat Conducted From Surface to
>Air.  It's the following ...
>
>Q = h * (T1 - T2) * A
>
>h  = Heat Transfer Coefficient
>T1 = temperature in degrees C
>T2 = temperature in degrees C
>A  = area in square meters.
>
>h is given several values given still or turbulent air.
>
>Still Air      h = 23 to 28
>Turbulent Air, h = 85 to 113
>
>One square inch is 0.0064 square meters.
>
>Dissipation in Watts for 0.0064 square meters,
>an ambient of 20C and a raised temp of 40C
>(a temp differential of 20 degrees C), this
>works out to be 2.94 Watts. Worst case it
>and say 3 Watts.

Now, take that square inch of copper PCB surface and drill plated holes
through it, and add copper to the other side. Suppose the holes are 0.050
inches in diameter and are spaced 0.100 apart. I'll assume there are 100
holes. The total area of the holes is .2 inches squared, so the surface is
reduced to 0.8 square inch on one side, but to this we add the 0.8 square
inch on the other side, which now has a low thermal resistance to the first
side, and the area of the hole walls, which is another 1.0 square inch. The
total surface has become 2.6 square inches, which alone would explain why
experiment showed greatly improved heat dissipation with holes drill in the
board/heat sink. However, the holes will also have an effect in increasing
air turbulence, so we would expect the value of h to increase. Easily, it
might double.

In the other direction, cooling by radiation would also increase because of
the use of both sides of the board; it is more difficult to quantify this;
but my sense is that the increase in pure radiative cooling would be
greater than a factor of 1.6. But radiation is less significant than
conduction/convection.

So I would expect heat dissipation to increase by a factor of five from
drilling those holes and using both sides of the board.

I've asked SMT magazine if they were the ones who printed the article about
this technique of improving heat dissipation; I could not find it on their
web site index, which was not readily searchable. In any case, the
information in the article, as I recall, had been somewhat mangled by
either poor writing or poor editing and some of the technical information
necessary to assess the reported results was missing.

Next opportunity, I plan to design a test board and verify the prediction.

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Abdulrahman Lomax
P.O. Box 690
El Verano, CA 95433