At 06:41 PM 11/15/99 -0500, Bob Landman wrote:
>I have a comment also.  This is often done but I don't have any
>calculations to back up the practice.  It seems to me that this
>surface, whether 1 oz or 2 oz or whatever, is a black body radiator
>and that is what is "cooling" the heatsink tab on the regulator.  It
>isn't convective cooling so putting holes in the plane makes matters
>worse as you have reduced the surface area of the radiator.

A practice which is verified by experiment does not need to be "backed up"
by calculations. I did not make up the hole idea; I took it from an article
which appeared, as I recall, in SMT magazine showing the increase in heat
disippation from the use of holes, from actual measurements. It was
substantial, but I do not recall the exact figures. An increase in power
disippation for the same heat rise, as I recall, was something like a
factor of five or ten. Larger holes were better than smaller holes. Maybe I
can find the article....

It does seem to me that holes would reduce radiative cooling; certainly
radiation normal to the surface would be reduced, but this effect would
only hold at angles to the surface such that the other side of the holes
could be seen (i.e., one could see, from that angle, all the way through
the board). The loss in radiation would rapidly decline as the angle
deviated from normal to board surface. I don't think that it would be a
large effect, overall.

In the contrary direction would be two effects: the absolute surface area
of copper would be increased (and thus there is an increased area in
contact with air), and there is improved heat transfer between both sides
of the board (the idea I had in mind includes having copper plane on both
sides). Thus even radiative cooling might be increased, since there is
radiation from both sides of the board.

I don't know why Mr. Landman would say "it isn't convective cooling." I
design boards for space flight where the environment is a vacuum and,
definitely, we have to be much more careful about cooling because of the
lack of atmosphere. Ultimately, all the cooling is radiative.

For the rest of us, if convective cooling were not significant, why in the
world do I have a fan in my computer's power supply? The fan does not
increase radiation, and it does not increase conduction, in only increases
convection.

If I have the terms right, there are three kinds of cooling: radiative,
conductive, and convective. Putting holes in the board may reduce radiative
cooling a little, but it will probably increase conductive and convective
cooling. (When I originally wrote "convective," I meant "through the air"
and thus was including both air conduction and convection.)

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