At what data rate should we consider PCB via back drilling? How about flex PCB?

Question

Hello community member,

I have a 4 layer PCB. At what data rate should I consider via back-drilling? Also do we back-drill 3 layer flex PCB? Thanks.

Answer 1

For a 4-layer with signals on the outer layers, never. 3-layer flex… also never.

The whole point of back drilling is to remove stubs on the outer parts of vias, that if left undrilled, would cause reflections and thus degrade the signal. This would happen only on a via that connects to an inner layer. Vias to connections between outer layers do not have stubs.

Whether you need to back-drill or not depends entirely on the signal integrity goal you have for the signal on a particular via to an internal layer.

More here: https://resources.pcb.cadence.com/blog/backdrilling-in-pcb-manufacturing-2

You determine the need for stub removal based on physical simulation of your board. For a digital signal you use something like Hyperlynx; for an RF signal you’d use some other tool that does 2-d or 3-d field simulation.

That said, I’ve built many 6- and 8-layer boards using fast interfaces and RF and never saw the need to back-drill. I avoid the need to do that by routing critical signals on the outer layers only, preferably with no vias at all.