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September 1998

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Subject:
From:
"Nelson, John" <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Sat, 19 Sep 1998 10:47:31 -0400
Content-Type:
text/plain
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text/plain (64 lines)
Steve, there are as many reasons for x-outs as there are board shops
times the number
of operations that each board shop performs.   Just a few examples:
        1) When the lamination press operator accidentally bangs two
panels together
        and one gets a nick in the copper, the board ends up with an
open that fails
        electrical test and that board is x'd out.
        2) When a drill bit breaks off in a hole, the drill machine will
stop and the operator can
        replace the bit and restart the machine.  The panel is good, but
that one circuit with
        the bit lodged in the hole cannot be saved.
        3) When the print operator aligns the artwork a bit
mis-registered only those holes
        that are slightly off center due to drill wander will break out
of the pads and the rest will
        be good.  Some boards pass test, and some get x'd.
When you tell the board shop no X's they have to eat the cost of all
rejects so your price per
image goes up.  When you accept multiple X's your assembly cost goes up.
The trade-off is
yours to make and negotiate.

The opportunity is yours, however, to have products designed that give
the board shop high yields.
Most designers don't consider plating thief as a part of design.  If the
board shop asks to add thief
the answer is usually that either the designer is too busy working on
the next generation or the
design is frozen and can't be tampered with for fear of crosstalk or
changes to signal timing.
Another isssue that affects yields is spacing.  All designs are run on
an autorouters.  Routers accept
rules up front about minimum conductor widths, pad sizes and spacing.
When a designer starts the
router with a minimum space of say, 5 mils specified, the computer
starts running the first track
at 5 mils from the nearest pad.  When it finishes running it doesn't go
back to find out that there were
no more lines run and that that first track could have been spaced out
further to reduce the risk of shorts.

Efforts made to review designs for maximum uniformity of density and
maximum spacing between lines
and pads will pay off in improved yields and reduced costs.

Ps. If you bring your Duck to Virginia I'd love to drool all over it,
but I quit biking 15 years ago when my
        last one was stolen and I don't need to get hooked on an
expensive hobby again.

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