Re: gEDA-user: QFP fan-out tips

[DJ Delorie <dj@xxxxxxxxxxx>]

> I've got a tiny little 100-pin 0.5mm pitch TQFP chip that needs a
> whole lotta I/O connected to it.

Are you set for 8/8 rules?  And minimum size vias for the fab you've
chosen?

> 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?

Route critical nets first, let the autorouter do the rest.  Leave
enough space between chips for routing.

> 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 route power inwards under the chip, with vias to the power planes and
the decoupling cap on the other side.

> 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?

Depends on the speed of the signals.  Putting a fast signal on the
ground plane, for example, not only adds two vias, but forces the
return current for both signals (the rerouted one, and the one it's
trying to pass) to go around the hole you've just created.


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