What PCB material do I need to use for RF PCB desigin ?

Is plain old FR-4 (also known as "Glass Epoxy") PCB material suitable for use in RF designs[1]? This question comes up time and again. Many say no, fewer say yes - who's right?

As I have published before [2], I have been using FR-4 material for years to build not only protoboards, but wireless radios, RF test fixtures and RF test equipment. This is not to say that FR-4 does not have limitations, but when you understand the limitations you can make better cost/performance tradeoffs for all your designs.

So what are the limitations of FR-4? Er (dielectric constant) stability from lot to lot and over frequency is one of them [4]. Loss is another, then there is the concern of lead-free processing temperatures and perhaps thermal conductivity as even low-power RF can consume a lot of power if the active circuits are biased to provide very high linearity.

Since many FR-4 materials are not not really specified for RF performance the Er can and will vary from manufacturer to manufacturer and from lot to lot; sometimes Er is not even specified by some material suppliers! Does this all mean that FR-4 and "Glass Epoxy"-like materials can't be used for RF?

So how does one pick a PCB material? It depends on many factors, some of which are:

• Product Cost
• Circuit design, which drives: required impedance stability
• Signal Loss tolerance
• Operating temperature (Temperature expansion, stability over temperature, etc)
• Heat sinking ability (even low power RF can dissipate a lot of power)
• Soldering / Assembly Temperature (Lead Free)

Some of the above may be applicable to your project and some may not. Then there are the choices of material itself,

• Plain old FR-4, with a higher loss and not tightly controlled Er[2]
• Better specified Er[2] FR-4 derivatives (these may have better loss also)[3]
• Specialized low-loss RF Materials with well specified Er values and much lower loss

It is a simple matter to go through the data sheets and make a spreadsheet that compares these items above one by one for comparison.

This can lead to a bewildering array of options, especially for the person new to RF. I have seen many cases where people new to RF have used expensive exotic materials for even low-frequency non-critical applications simply because someone said that the application was "RF" and they went into "Over-Specify Mode" just to be safe, but is this really needed?

Let's look at some real world applications:

• High Volume / Cost Sensitive
• Cell Phone
• GPS Receiver
• RF Remote Control
• WLAN, Bluetooth, ZigBee et.al. Transceiver

• Low Volume / High Performance
• Test Equipment
• Really high speed – bleeding-edge designs

In the high-volume cases you will be hard pressed to find anyone using really exotic materials in the under 6-GHz world. Take apart all the items that I just mentioned and you will find materials that look just like regular old FR-4. In the low-volume but high-performance category you will find board material that again looks like FR-4 and you will find higher-frequency materials, especially when the operating frequency exceeds 6 GHz.

In the low-volume cases, performance may be paramount and the circuit designs might be more complex. Many of these products do use a tighter specified type of "Glass Epoxy" or exotic RF materials. Mainly for their repeatability and for the trace losses.

How does FR-4 really perform?

Figure 1: My standard FR-4 RF prototype board. I use these 2-inch-long quick PCBs to prototype all sorts of filters, amplifiers and other RF circuits – very handy to have around and they only cost a few dollars each.

In addition to my standard FR-4 prototype boards (Figure 1) I also make quick turn prototypes on Rogers RO4350B material [5] (a low-loss, high-GHz material) so I compared the two for insertion loss (S21). I started with a 2-inch Coplanar Waveguide Over Ground structure and, using the same connectors, I measured trace loss over a 130 to 7000-MHz band. I then scaled the data so that it would be in dB loss per inch. The connector losses were not de-embedded because they represent very little of the loss and both test boards had better than 25-dB return loss so there wasn’t any appreciable mismatch loss to account for (Figure 2).

Figure 2: Measured loss per inch of plain old FR-4 and very high-frequency Rogers RO4350B material. The Rogers material does have less loss, but even at 2.5 GHz the FR-4 holds its own at less than 0.3 dB loss/inch.

If you were building a 2.5-GHz Bluetooth module and the RF traces were about an inch long total – would you really care about a 0.3-dB signal loss, especially in light of the fact that the antenna matching circuit will probably exhibit more loss than this? Probably not. Even if you used Rogers RO4350B with its loss at 2.5 GHz of 0.13 dB/inch you would only be saving 0.17 dB.

Antti 

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Skype :raypcb26