There are RF based PCBs that are 2 layered too, I accept that but we are not talking about them here, although the answer is simple and universal to actually apply to 2 layered PCBs too.
On Google you will find solutions to this issue like RF signal integrity, interference etc but these are the common answers that you will find on the internet. I will tell you something that I experienced practically while designing.
NOTE: For complex RF circuits there is no way but to make 4 layered pcb because of the routing complexity involved, but when you are dealing with simple rf circuits like subGHz RF frontends like cc1101, cc10L, RF microcontrollers like MKW41Z, MKW21Z or from SAM series from Atmel etc then there is no need to make 4 layered boards but that is where the catch lies and read on to find out about it.
A lot of RF based ICs require 50Ohm impedance tracks. There are some that don’t need it because the matching circuit or tuning circuit is already there inside the module itself (like CC3200MOD), but when working at IC level where you solder RF based ICs with crystals and everything then quite a few of them need the 50 ohm impedance track. This is where it gets interesting.
If you open up any microstrip line impedance calculator (I am using AppCAD) and put in the values then you will realize where the problem lies. Let me explain this to you with an example –
This is the window where you put in the values for various parameters and the calculator gives you the value of the impedance. The parameters are –
- Width of the microstrip
- Height or thickness of the PCB(FR4 material to be exact)
- Thickness of the copper
- Dielectric (which is a constant for different materials, you get to choose from a drop down list below, just choose FR4 )
- Frequency (2.4GHz in our case)
- Units (mm in our case)
Lets look at common values for a 2 layered PCB design –
This is the screen shot of the Fusion PCB service from Seeed Studio. The common thickness values for PCB are 0.8 and 1.6mm. The thickness of the copper has options of 1oz, 2oz which translates to 1.4mils and 2.8mils respectively that translates to 0.03556mm and 0.07112mm respectively. Now these are the values that are fixed depending on your fab house, what you can play with is the width of the stripline. So plugging in the combinations you will realize the following –
- Length has no effect on the impedance of the strip line (as it should!!)
- As thickness of FR4 reduces then impedance reduces.
- Increasing the thickness of copper also reduces the impedance (not by much as the change in thickness is not much.)
- You can change the impedance a lot by playing with the width of microstrip.
The fourth observation is what is crucial and the cause of problem here!!.
At the optimal values the thickness of microstrip that gives you the value close to 50 ohm is 50.20 ohm at 1.4mm. However 1.4mm is TOO much for thickness for smd capacitor and inductors of sizes 0603 or 0402 or 0201 which are the preferred sizes for tuning circuits. I will show you an image of what a PCB i am designing looks like –
The thick trace in the middle and towards the end are 50 Ohm impedance tracks. But I have designed it cleverly (the actual thickness here is 1.25mm actually), if you change the trace with to 1.25mm and click on the unrouted wire from the left capacitor to the right capacitor then you will realize that it will short the capacitors terminals!!.
Now if you look at a thickness of 4 layered pcb between the top and the second (Ground) layer, you will notice it is usually from 0.2mm to 0.32mm.
So, if you plug in these values in the impedance calculator you get a significantly lower value for the width of the microstrip!!.
And that folks is one of the reasons why RF pcbs are 4 layered !!.