s-parameter reference node close

Dear all,

I have a BGA package with solder bumps on top and solder balls on bottom, which belong to power, ground, and signal nets. However, I would like to treat all of them as signals since I want to connect the S-parameter block in the circuit schematic with other models e.g. RDL, PCB.

I did my simulation in CST-MWS. The question is, what sort of reference should I implement? One way to do it is to create virtual PEC planes underneath and above the package. But, aren't those two planes supposed to be physically connected? When yes, how should I connect them?

Later, I want to use my S-parameter for transient simulation. In ADS schematic, the S-parameter block has only one common reference node. How should I treat this node? Shoud I leave it open because it was initially only a virtual plane or should I ground it?

In Cadence schematic, the S-parameter block has pos/neg nodes for every port. Do the different neg nodes of the ports mean anything? In other words, can they be at different potential? And again, how should I treat them?

many thanks,
wlcsp