Case study for RF PCB Material selection

Question:

Does anybody can give some RF PCB material selection guide?

Or maybe give us the tips about how to choose the PCB material while we design RF board?

Answer 1

For most 2.5 GHz or below wireless RF designs we just use FR-4. If the board gets big and loss gets really important then you can get better specified FR4 like the stuff from Isola.

The trick is the layer stackup. For multilayer boards you need to specify the layer stackup carefully as the space between the layers and traces sets the impedance of the traces.

I regularly use simple low cost boards for RF to 2.5 GHz, especially on low cost wireless designs. (I'd probably also use FR-4 for a 5 GHz wireless LAN design too as the RF portion of these things is quite small).

Answer 2

FR-4 Material is OK at 2.45GHz only if your radio can handle significant drift over temperature.  FR-4 is typically used for consumer applications only and the board stack must be tightly controlled.   You need to have expertise when working with FR-4 at 2.4GHz if you hope to get repeatable results.   You need to be careful to make sure your design is sound before you go into production.  You could develop a radio using 5 to 10 prototype boards and everything looks fine.  However you most likely will see a difference in performance in production.   FR-4 is not intended for radio products.

Such as for developing an SWVR meter you will need to use a different material in order to get any kind of accuracy in your SWVR measurements.    I would suggest you look at a ceramic material before you start.  Rogers corporation has been the standard for industrial and specialty designs for a very long time.    I would suggest you look at the RO4000 series.  It is the most cost effective product they have available.  However it will cost you 7 to 10 times that of FR-4 .   It is well worth the cost in order to get repeatable reliable results.

Answer 3

When working for HP building their Network Analyzers we used FR4 type materials up to around 6 GHz. These materials were called GeTek (Now Isola FR408HR) and they had tighter specifications on Er  (Dielectric constant) and Dk (Loss tangent) than plain old FR-4 (which as you suggested can be all over the place).

High accuracy devices need to be calibrated so the calibration would take care of the part to part variations due to the PCB's core material being slightly different – batch to batch as long as Isola type materials are used. Plain old FR-4 would be iffy indeed in production.

We still use these Isola type materials up to about 7.5 GHz today and they can be used to build very good test instruments.

Where I use Rogers materials is where I need lower loss – that is when I am building a spectrum analyzer front end from 3 to 10 GHz or some sort of radio receiver where noise figure (hence loss) is very important – then these more expensive materials are real problem solvers. But they are rarely needed for my work below about 3 GHz (Isola is low loss enough at 3GHz compared to everything else in my circuits – for my work).

Last year I designed and built a Bluetooth Tester (2.5 GHz) that had very decent specifications (0.5 dB accuracies) of both forward and reverse power measurements over time and temperature and it was built quite economically on Isola 408HR.

All my test circuits are built on very economically on plain old FR-4 (the Bare Bones Type) – I don't want to do this for production – but for test circuits it is very economical and I can do a lot of testing for very little cost. (I have a design Idea that is to be published in EDN later this month that shows how to do this).

For high volume consumer type items – I have yet to take apart a Bluetooth device, Wireless LAN or Cell Phone that didn't just use something like Isola for their RF board. So these very high volume devices do just fine to the 2.5 and even 5.9 GHz arena where cost is more important than a dB or so.

This is all not to say that anyone is wrong or right – but we all approach these things from our own “Required Specifications” and “Experience” point and that's what this is – my “Experience” point of view for what sorts of things I have been involved in.

Answer 4

We did a study 3 years ago comparing FR-4 to Rogers 4000 and we found the following benefits when using Rogers.

1. The performance over temperature was much more stable.   The power transfer was consistent as the return loss remained consistant.   Rogers 4000 has a very low insertion loss so it does not fluctuate much.

2. Rogers 4000 has a low insertion loss so it is great when you want to build a low noise figure receiver.   We found that the LNA noise figure was able to come very close to our theoretical calculations, even at 6 GHz.

Rogers 4000 is the best choice when performance is critical.   You have a much better chance of success.   However, most products have used Rogers 4000 are typically low volume and high value items such as test equipment.  

For high volume low performance products such as consumer WiFi hot spot locator you will want to invest your money in engineering to develop a solution using FR-4.