Symmetric PCB stack-up for phased-array antennas

Question

We all know that a symmetric PCB stack-up is always recommended since it makes manufacturing easier and more reliable. However when designing PCB for antennas application this became quite a difficult constrain. For instance, let's consider a conventional patch antenna (maybe stacked for bandwidth reason) as radiating element in a phased-array. In this scenario we have very different requirements in terms of thickness since on one side of the PCB we have a patch antenna that requires fairly thick substrates but on the other side we have an RFIC which due to the ball-to-ball distance of the BGA packaging requires a thin dielectric so that manufacturer can drill vias that are thin enough to fit the BGA.

How do antenna designers overcame this issue? Are there some sort of best practice for this kind of situation? or should we just try to find a different radiating element that is more suitable?

Answer 1

As you said, ideally you would like a symmetric stackup to reduce the chance of the board bowing, also called potatoe-chipping in some industries.

For many designs this is not possible, such as for a complex multilayer PCB with power, digital, and RF (including radiators) on them.

So we try to do a balanced stackup as much as possible, recognizing that the un-balance in the stackup that remains (because of the radiator design) may cause the board to bow. We then fabricate some of these (call them prototypes) and see what bowing occurs.

If the bowing is more than allowed, then one approach is to add fixturing in the manufacturing process to hold the board in place during the fabrication process.

Sometimes we may add layers, or redo the stackup (and routing) to improve the symmetry of the stackup. But more layers mean highers cost and higher weight, which may not be acceptable in some applications.

On a recent design, we reduced the layer count from 28 to 18 in order to reduce cost and weight (30% savings), and utilized additional holding fixtures as mentioned above to keep the residual bowing within spec.

Answer 2

We all know that a symmetric PCB stack-up is always recommended since it makes manufacturing easier and more reliable.

Well, it's recommendable for something like a 4-layer board with both-side component placement and no special needs. This isn't something that's always applicable!

However when designing PCB for antennas

... not in that case.

In this scenario we have very different requirements in terms of thickness since on one side of the PCB we have a patch antenna that requires fairly thick substrates but on the other side we have an RFIC which due to the ball-to-ball distance of the BGA packaging requires a thin dielectric so that manufacturer can drill vias that are thin enough to fit the BGA.

Well, you identified that you don't need a symmetric stackup!

How do antenna designers overcame this issue?

By not using a symmetric stackup when not possible.