Specctra - Burried via

[engcif]

Hi,

We have been working on a 6 layer + 2 plane layer PCB. PCB contains a
BGA package microprocessor and SMD components and several types of 603
pack passives. I have organized the stackup of PCB layers in orcad
layout. The problem is:

I have to use burried vias ( vias defined only in the inner layers and
cannot be seen on top and bottom layers) to fully route the PCB. I have
defined the padstacks of burried vias in orcad layout and then routed
the board using Specctra. In specctra I chose smart route. After
routing I have noticed that only one type of via was used which touches
on every inner layer including top and bottom. None of the burried vias
were exist. In several tries by changing padstacks of vias and defining
on different inner layers nothing improved. Specctra kept routing the
board using a single type of via.

I belive that my via definitions are correct but how can I force
specctra to use burried vias. If I can use burried vias, I can route
wires over vias in other layers. Also specctra can give better results
because the space on the board will increase. This is because specctra
stucks itself by adding more single type of via.

I am waiting your answers. Please post me anything you know about
burried vias.

 

[salmon]

Hello engcif,

It's been a while since I used Specctra but I used to love using it. It
was the best Autorouter around at one time. I'm not sure how it
interfaces with Orcad so assume your problem is only with Specctra and
not Orcad.

Have you tried re-defining the buried layers using a do.file once you
are in Specctra? It may be that the Orcad buried layer rule is not
getting passed to Specctra properly.

Also, it may be that 'Smart' route is to blame. Have you tried routing
without the smart route option being on?

All the best.