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July 2002

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Date:
Tue, 9 Jul 2002 11:41:54 +0800
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Hi, Frank,

We at ST Aerospace have the same problems of how to extract heat from
chips, conduct it to the card edges and through the chassis to cold walls.
We opted to create a grid of PTV's in a coper fill area immediately below
the chips (where ship configuration allows for this). The PTV's then
connect only to two 2 oz thermal planes. A similar pattern of PTV's are
then punched around the card edges to transfer the heat from the thermal
planes to the interface with the chasses. At present, we fill these
"thermal Vias" with DuPont CB101thermally conductive epoxy, but later
calculations have shown that only a slight increase in copper plating
thickness in the holes has the same K value as this epoxy.I am trying to
pursuade the designers to move the thicker copper and do away with the
epoxy filling process.

To comment on sage Werner's suggestions, the reliability of completely
filling of vias with copper or solderd copper wire depends on the substrate
material. FR4, for example, has quite a high LCTE in the Z-axis. If the
holes are solidly filled with metal with a lower CTE, and the boards are
operating in harsh enviroments with large temperature extremes, there is a
danger of connections with inner layers being fractured. That's our theory,
anyway.

A few tenths of a mil increase in hole plating thickness using, say, 20%
ductility copper, promises greater reliability and replaces the equivalent
conductivity of an epoxy-filled hole.

If further heat extraction is required, devise an overhead metal framework
and extract heat over the top of the board by bridging the gap between
frame and chip with high K value thermal pads such as are available from
Fujipoly or Thermagon.

Peter



Werner Engelmaier <[log in to unmask]>    09/07/2002 09:42 AM
Sent by: TechNet <[log in to unmask]>

Please respond to "TechNet E-Mail Forum."; Please respond to Engelmaier

              To:  [log in to unmask]
              cc:  (bcc: DUNCAN Peter/Asst Prin Engr/ST Aero/ST Group)
              Subject: Re: [TN] PCB heat dissipation








Hi Frank,
You did not say where the heat is going to once it gets to the bottom; that
is important. The best heat transfer top-to-bottom is obtained by plating
the
PTVs as close to shut as possible. Alternatively, you can take larger PTHs
and solder Cu-wires into them and then cutting the wires flush. Filling
PTVs
with solder is less efffective, particularly since you cannot guarantee
that
all PTVs will be 100% filled.

Werner Engelmaier

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