I believe the initial concern of lead protrusion was due to the way all the
boards were once loaded into backplanes.  Each board plugged into .5"
spacing on connectors did not allow much for component height on one side of
the board and lead protrusion on the back side.  Also keep in mind, as you
stated, the potential of shorting.  The shorting which occured with this
backplane assembly was leads coming through the board and touching metal can
components. Therefore when the mechanical dimensioning was conducted it
resulted in a protrusion not to exceed about 0.060".

The other issue for lead protrusion was the method in which components were
installed into the boards.  Old pick and place equipment and DIP inserters
had the capability of what they use to call crunching the components.  The
implication being many times leads would bend beneath the component and not
go through the board.  To catch this defect, the requirement was put in
place that all leads must protrude the board and be visible on the bottom
side.  I also think that one of the other issues for lead protrusion was
through hole reliability, if any hole problems existed.

To have leads protrude longer than the spec calls for is all dependent upon
your application.  If it will be assembled into a backplane then spacing
issues must be addressed.  If it will be assembled into a chasis then the
clearance needs to be defined and you lead protrusion can be adjusted
accordingly.

Hope this helps.

Leo Lambert
EPTAC Corp.
At 04:11 PM 11/15/96 +0000, John McGee wrote:
>
>Greetings Technetters-
>
>In the interest of streamlining a process, we're experimenting
>with straight-thru assembly of leaded components in mixed-tech products.
>The proposal on the table is to allow protrusion of up to 3.6mm (0.15").
>This doesn't conform to existing IPC requirements (we're building Class 2
>products).
>
>What should be our concerns, other than potential shorts?
>Why is Class 3 so stringent at 1.5mm max?
>
>
>Thanks for your help.
> 
>
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