Abd ul-Rahman Lomax wrote:
>
> 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.

Um ...  It appears without any sort of rigorous analysis
that there's a min/max at work here.  I.e., there'll
come a point where too many holes won't help.  Loss of
too much material and heat conduction paths being
interfered with too many holes.

Just a suggestion, but it might be worthwhile to specify
the exact orientation of said board - horizontal or vertical.
I'm *assuming* you mean horizontal.  Now without proper
convective paths, the bottom plate will do little as
a second dissipative surface.  Start adding holes, use
an approx virtual hole of only 60% air flow through
the hole (I think that's about right), and that's
when air flow begins to make effect.  Start adding
too many holes or holes that are too large and you
may end up as I stated earlier.

In which case, the vertical position of said board might
not benefit from the holes being oriented horizontally.
Which in that case, would be interesting to see the
effect of heat dissipation on a vertically oriented
board with no holes.

I'm not all that sure of the temperature differential
across the horizontal solid plane due to convective
air currents.  But, I worked with the temperature
differentials across a vertically oriented surface
trying to dissipate heat.  It can be quite large
severely effecting or determining IC placement for
thermal management.  Put two vertically mounted
boards in close proximity to each other and they
can greatly effect one another.  Almost in a
odd feedback set-up.

Just some thoughts.  - Doug