Hi Joe,

*Some* IC packages have the case used for
something other than ground.  It's far better
to have an overall policy of "heatsink to
ground plane isolation" than not.  Especially
when dealing with alternate parts that may
differ in case construction.

The second thing that comes to mind in the
general case is unwanted or unexpected
'ground loops' - return current from a
device that does not go where you want
it to go.

Say you have a motor driver circuit carrying
several amps being driven by a TTL logic
circuit that operates below an amp or so.
Mixing these return currents through the
same ground plane could spell disaster.

Let's say you trace the return current for
the driver to one spot on the board. Then,
the case (which can act as a lead for the
device's return current) is connected to
the heatsink.  The heatsink is connected
to the ground plane.  You may have mixed
grounds - high current motor ground (analog)
with low power sensitive control circuit
ground (digital). You wanted the trace
to handle the return current for the
device.  But, now it's not going there.
It's going through the ground plane.

It gets more complicated sometimes when
there's 3 grounds (connected at one common
point) on one board - analog, digital,
and signal.


----------
> From: Joe Reichert <[log in to unmask]>
> To: [log in to unmask]
> Subject: [DESIGNERCOUNCIL] Heatsinks to Ground Planes?
> Date: Monday, September 22, 1997 11:57 AM
>
> Could anyone give me some insight as to why heatsinks would be isolated
> from ground planes?  My first thought would be to minimize emission or
> absorption of noise, but I've never had a problem with this...that I
> know of, anyway!
>
> Also, has anyone had any experience with using a copper layer to
> transfer heat from a heatsink to...(for instance an aluminum case).  The
> heatsink is applied directly to the copper layer and a few inches away
> the copper layer is attached to an aluminum case via a bracket.  (the
> aluminum case is sealed closed).  Any insight would be greatly
> appreciated!!!
>
> Thanks,
> Joe Reichert
>
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