Steve,
        Talk to your casting supplier about what kind of tolerances are
reasonable.  The guy's doing the casting work are alot smarter (usually)
about how to do "their job" than you are with how to do "their job".

        IPC board flatness specs are concerned with matching coplanarity of
the board with components mating with the board; not the coplanarity of the
finished product.

        If the board support is a machined screen printer support, 0.005" is
a reasonable, but very tight tolerance.  You will have to pay for your
tolerances, so its good practice to give suppliers as much tolerance as
possible.  You need to evaluate how much tolerance your design will
accommodate.

        If the support is a final assemble mount, I would imagine most
assemblies could easily tolerate as much as 0.050" out of plane with the
Datum.  I've seen large boards sag as much 0.300" when they come out of the
oven and a stiffener wasn't used (causes lots of problems).  These large
boards will flex enough to still fit into their chassis.  On the other hand,
boards that match a tiny LED to a faceplate require a much tighter tolerance
with the chassis mounting points.

        It all means you will have to get a calculator, evaluate the end use
of the Aluminum casting and add up your tolerances.

(Luckily there are few easy answers.  Otherwise, you and I would be out of a
job!)

Good luck!
Ryan G.

> -----Original Message-----
> From: Steve Collins [SMTP:[log in to unmask]]
> Sent: Monday, February 07, 2000 10:27 AM
> To:   [log in to unmask]
> Subject:      [TN] Flatness Specification for PCB's & Supporting Devices
>
> An Engineer just came to me and ask if I could find out what the flatness
> spec
> would be called out in a Aluminum casting that supports a printed circuit
> board.
> Any takers on this or suggestion where to start. We call out a flatness
> (Bow &
> Twist spec per IPC-D-275) but I'm not sure if this is sufficient for
> casting
> operations.
>
> Steve Collins
>
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