Determine the number of pcb layer

[khiworld]

Hello.

I just read the article which is "3 Steps How to determine/calculate the number of pcb layers."
I woudl like to know the reason why 2 layer PCBs for interfaces with controlled impedance (USB) is not recommended.

Thanks.

[robertferanec]

There are at least two big main reasons why controlled impedance tracks are hard to do on 2 layer PCB:
- it is very hard to have a solid GND plane on 2 layer PCB
- GND plane is far away from the signal layer (this for example means, that it is very hard to find dimensions such track width and gap which could be realistically used on the PCB .. usually to meet required impedance, the dimensions would be huge)