Tim,
        I don't think there are many "rules of thumb" for thermodynamics.
It's a relationship between power dissipation of that part and the
surrounding parts, ambient temperature, thermal resistivity of that
particular layout, and rate of airflow or convection.
        It seems to me you have two choices. Either try adding copper to the
design until the problem goes away or have someone do a thermal analysis.
I'd recommend number two.
        If you're unable to do a thermal analysis, try providing as many
paths for heat to flow out of the system, that is, to the chassis and out
into the air surrounding the unit, as possible. If the board has a ground
plane, add an array of vias under and around the part connected to the
plane. Add copper ground plane on the top and bottom of the board also
connected to those vias. If the board is held by metal card guides, place
the part adjacent to the card guides. That'll reduce the thermal resistivity
between the part and the outside world (the infinite heat sink).
        So that's the approach. Either disperse the heat into the unit by
raising the ambient of the entire unit, or (preferably) find a way to
conduct it out of the unit altogether.

Michael Hiteshew
Lockheed Martin NE&SS-Baltimore
[log in to unmask]
(410) 682-1259

> ----------
> From:         Tim Cables[SMTP:[log in to unmask]]
> Subject:      [TN] 2512 Chip Resistors
>
> Hi,
>
>         We are using these 1W devices at about half their rated capacity
> (.5W) on
> FR4 board material and are seeing discoloration of the FR4. The part is
> capable of reaching about 175 degrees C. FR4 would probably start measling
> well before it reaches that temperature. Has anyone seen any rules of
> thumb
> for adding additional copper to the PCB footprint to spread the heat out
> so
> discoloration or worse would not occur.
>
> Tim
> [log in to unmask]
>
>

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