gEDA-user: QFP fan-out tips

[Randall Nortman <geda-lists@xxxxxxxxxxxxxxx>]

I've got a tiny little 100-pin 0.5mm pitch TQFP chip that needs a
whole lotta I/O connected to it.  I'm doing a 4-layer board (internal
VCC and GND planes), but I'm still getting all tied up in knots, with
vias all over the place.  The connections tend to go off in different
directions -- these aren't pretty 16-channel busses.  Anybody have any
general tips for dealing with this?  I have seen that some layout
programs do automatic fan-out on chips like this.  I could certainly
do that manually, but I'm not sure it will really help a whole lot.  I
still have a lot of signals to connect, and plus several of these pins
are power pins that require their own decoupling capacitor and specify
that it needs to be as close to the pin as possible.  I find I might
have to redefine "as close as possible" as I try to cram more
connections in there.  (Needless to say, the autorouter shrieks and
flees in terror at the sight of this ratsnest, so I am stuck doing it
all manually.)

One thing that would help would be if I could use the internal VCC/GND
planes for routing the odd trace or two, but I have heard that it is a
bad idea to run traces through the planes.  Anybody have any thoughts
on this?

I don't suppose anybody has written any fan-out scripts or plugins?
Would it help?  (I've never done it myself.)

TIA,

-- 
Randall


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