Is multilayer RF pcb possible?

Hello,

Can we design a Multi layer RF pcb, does it work actually?
I want to see the performance of RF multilayer pcb, please share if you have any references regarding this.

Thanks in adv.
KNR

What frequency are you looking for?

It is possible. At RF frequencies, the traces become transmission lines and they have to be impedance controlled. That means that if you bury an RF trace in a 50Ω system, you'll have to design the trace as a stripline, with controlled width of trace and heights of dielectric material both above and below the trace to give a 50Ω stripline. Of course, traces on the outside layers become microstrips, with controlled width of the trace and the height of the dielectric below the trace to maintain the 50Ω.

Yes, it is possible, but it is not an easy task. Depending on the criticality of the design, PCB tolerances may need to be tightly controlled, thus significantly increasing the cost. Plus, troubleshooting a problem PCB batch can be very difficult (you buried a lot of traces). Therefore, you might want to stay away from burying RF traces, if you can and keep all critical components as microstrip.

Now, if you are talking about putting RF traces on a multilayer PCB (and don't intend to bury any traces), this is quite easy. I have done this on multilayer PCBs, including HDI PCBs, quite often with success. You just clear out parts of the inner layers that you need to make the impedances work for you.

it is most common for a top RF layer, Ground plane under it, intermediate layers, another ground plane, and then an RF layer on the bottom. You ONLY have to go thru the board from one side to the other with via holes, that you can arrange gound vias around and mostly keep a 50 ohm impedance.

Putting a buried RF layer is MUCH more harder. I have tried for a delay line once, but it was pretty high loss...higher than expected.

I personally would not try a multilayer RF board much above say 4 Ghz.

Thanks for ur texts..
My pcb size has been restricted in size so It's becoming difficult for me giving RF tracks on top layer, so it's forcng me towards one more inner layer for RF routing.
Please tell if u have any experience regarding losses and any other considerations to be taken care in the same.

knr said:

My pcb size has been restricted in size so It's becoming difficult for me giving RF tracks on top layer, so it's forcng me towards one more inner layer for RF routing.

Click to expand...

RF traces should have priority for outside layer routing, especially if the primary focus of the board is RF. Don't ask, but demand to be given the required space on the outside layers. If you don't, then you will have no one to blame for the board not working except yourself. And when it doesn't work and you can't get it to work, then YOU will be blamed for this debacle by the people who denied you your needed space in the first place. Believe me on this, because I have seen it happen.

If you have any EM simulation software then you can try it..