Heart wrote:

I don't know if this is an old message, but I don't remember seeing it a
month
ago, so here goes :

> Actually, what I was hoping to do was use 4 oz. copper on one side and
> 2 oz. copper on the other.

I suppose a circuit board fabricator can do this, but it will incur
extra processing, so expect
it to add at least the cost of an additional layer.  Actually, many
houses may refuse to do it,
as they need to plate both sides at the same time to get the plating in
the holes.  The through
hole plating requires both sides of the hole to be open to get even
plating through the
length of the hole.  To have different plating on the two sides of the
board, they either have
to start with different thicknesses of foil on the two sides, or plate
one side longer than the
other.  The second method would interfere with hole plating, as well as
requiring that they
plate the whole panel to 2 Oz., stop, clean, mask one side off and then
plate the other side
for 2 more Oz.  I suspect they would worry that the 2nd 2 Oz. layer
might not adhere well
to the first layer.

>  The 4 oz. side would have the heavy power stuff and the 2 oz side
> would have the surface mount I.C.'s and control circuits.  Talk about
> short signal paths from logic to power MOSFET's, it would be the
> thickness of the board. We have used bus bars in the past for TO-220
> designs and plan to at least to connect some of the sources of the
> MOSFET's.

<snip>

> At any rate, with my proposed system I could imagine 50 watts
> continuous especially for 4 oz. copper.  BUT, I realize these are just
> a bunch of numbers and formulas.  50 watts on the 4 oz. copper would
> mean a temperature gradient of 30 degrees C and 2 oz. would give 60
> degrees C over a distance of just 1 mm.  Can an FR4 board handle that?

I didn't completely follow the snipped description of the heat sink, but
I believe
you were going to have a copper heat spreader and an aluminum heat sink
on top
of that, all on the same side of the board.  I think the whole board is
going to run hot,
so there won't be such a big gradient.  The board, itself can handle
these temperatures,
in general.  The weak spot is the foil adhesion, and these temperatures,
over time,
may well degrade the epoxy film that adheres the foil, and the
transistors might
start to come loose from the FR4 substrate, pulling the foil with them.

Anyway, unless you have forced air, and plenty of it, applied to the
heat sink,
it will not be at ambient temperature.  I just think that the whole
thing will run
hot, there are a lot more subtle heat paths that are less conductive
than the
copper and aluminum, but they add up.  I did some work with relatively
low
power preamps running in hard vacuum, and did all the thermal analysis
based
on copper foils, as I couldn't find any thermal info on the FR4.  It
turned out
that peak temperatures of the hot components were way below predictions,

because the FR4 (.062" thick) apparently conducted heat at least as well
as
the 1 Oz. Copper.

> Does anyone know the thickness of the copper on a plated through
> hole?  The cross sectional copper area will of course depend on the
> diameter of the hole.  Also, its thermal properties would be greatly
> effected if it were full of solder.

The plated through hole copper is generally a little less than the outer
layer.
They start with 1/4 or 1/8 Oz copper foils, drill holes in the laminate,
then
plate both the outer layers and the holes at the same time.  Because the

holes start out with practically no copper after the seed and
electroless
plating, the outer layers stay about 1/4 to 1/8 Oz ahead of the holes.
Supposedly, the smallest holes are starved for plating due to the long,
thin tunnel formed by the narrow hole.

> What am I missing here?  Any comments appreciated.  This has to be
> done right for high volume production, 10,000 per quarter.

In the absence of any test data generated through the same PC board
process, if
not the fabricator in question, I think you have to just do the best
engineering you
can, and then test its performance.  If you will be making 40,000 a
year, you definitely
MUST spend the effort to test the process and determine the heat flow,
etc.
I think the laminate manufacturers may be able to help you with what
happens
to the foil adhesion under high temp exposure and thermal cycling.  They
had
to do all this for the military, and larger commercial customers.
Possibly, there is an IPC or other document that at least covers some of
this
territory.

Jon



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