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.