Well, I make a quick stab at it (don't have much time today)...
Consider a ball grid array package (BGA) footprint that has a nice rows and
columns of little bitty surface mount pads. Since you can't do much routing
on the top layer of the board you have to immediately put a via next to
every pad to get down to the inner layers where there is more room. This is
a fanout, a real short wire and a via for every used pin. Since you don't
want to do it over and over again you can either:
Build it into the footprint (but then you may end up with many unused vias
for unused pins, not desirable) or,
Just route all the fanouts first, tell the CAD system to "LOCK" them in so
they can't be modified or deleted, and then do the real routing.
On boards where the SMT devices are really packed you can't route much on
the surface anyway, so you might as well run a fanout pass on EVERYTHING,
save it somewhere as a backup, and then start routing. Oops, did I just
repeat myself?
Well, hope that helps... have a great weekend.

Jack

        -----Original Message-----
        From:   JoAnn Amerson [SMTP:[log in to unmask]]
        Sent:   Friday, August 28, 1998 12:54 PM
        To:     [log in to unmask]
        Subject:        [DC] And today's question is...........

        I just KNOW I'm gonna get confused by any answers to this but here
goes........

        Could some please explain what a fan-out pattern is?  It seems to be
a series
        of traces and vias leading from a something like a PLCC or quad flat
pack.
        How can you create a standard pattern if you don't know where you're
headed
        until you have a general idea of a board layout.  The final
destination of a trace
        will change from board to board.  Surely one doesn't create several
footprints
        (feetprint?) for the same part?  Does one?

        And if any of you out there in DC-land are using Cadstar AND doing
this,
        contact me.  I gotta learn how!

        'Nuff said on that - I hope y'all have a nice weekend and if you're
an East
        coaster, I hope Bonnie isn't knocking at your door.



        JoAnn L. Amerson
        Design Librarian
        Red Lion Controls, Inc.
        E-mail: [log in to unmask]
        Voice: (717) 767-6961 ext 6308


        Thought for the day:
          Good words are worth much, and cost little.
             -William Feather

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