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:
Some of the above may be applicable to your project and some may not. Then there are the choices of material itself,
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:
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.
Sensitivity to Physical Parameters Most even slightly experienced engineers know that the Er of FR-4 is wildly variable not only between vendors but with frequency. This is true, but how does this variability stack up against the other obvious variable – like board thickness? Figure 3 compares all these variations.
Figure 3: The sensitivity to Er and board thickness are compared on FR-4 over a range of values. First the sensitivity to trace impedance with varying Er was tabulated over the commonly thought of values for Er (4.3 to 4.9). Then the sensitivity to trace impedance with varying the board thickness was calculated. The line length to get a 45 degree phase shift at 6 GHz was likewise recorded for each parameter.
As can be seen, the result is that while Er does indeed affect trace impedance, its sensitivity is proportional to the square root of Er. So for a huge Er change of 4.3 to 4.9, the total trace impedance changes only 2.7 ohms peak to peak. It should be noted that on my 2-inch test board that I have been building for years, I have not seen this level of variation which suggests that the lot-to-lot variation of a single FR-4 supplier is quite good and way better than the worst-case numbers commonly quoted.
What is more interesting however is the impedance variation due to board thickness. Figure 3 also shows this for a 5% thickness variation in a nominally 59-mil thick PCB. The trace impedance changes 3.5 ohms peak to peak.
So the board thickness variation causes the calculated trace impedance to vary more than the wildly variable Er values that are commonly quoted.
Another interesting fact about Figure 3: I tabulated what length an electrical 45 degree line would need to be for each case at 6 GHz. The electrical length changes with changing Er, but not with changing board thickness. If your circuit design incorporates distributed elements made from transmission lines to build circuits like filters or matching networks, then you are calculating line lengths to make certain electrical lengths to synthesize the equivalent capacitive or inductive elements. These lumped elements will change value with changing Er, but not so much with PCB thickness changes.
The Er of FR-4 also varies with frequency as shown in figure 4.
Figure 4: It is well known that the Er of plain old FR-4 varies with frequency. This composite data from various sources suggests that the variation is from about 4.7 at low frequencies to less than 4.4 at high frequencies.
Now that we have analyzed some of the performance parameters and trade-offs of plain old FR-4 we have found that the loss isn't all that bad under 6 GHz, and the many times unspecified and wildly varying Er is sometimes troublesome – especially if you are trying to use distributed circuit elements in your designs – but otherwise FR-4 can work in even high-performance RF circuits.
Incrementally Improving on Plain Old FR-4 The first thing that can be done to improve on the generic FR-4 is to use a FR-4-like material that has a specified and controlled Er range. This material won't be called FR-4 but will be made out of the same type of “Glass Epoxy” technology. PCB shops typically like these materials because they process with the same FR-4 manufacturing flow. There are many manufacturers that supply materials like this [3] and you should also make sure that the completed board uses material that can survive the high-temperature lead-free assembly processing temperatures, if you have this requirement also.
The Er of these materials is usually also much more stable with frequency than FR-4, so if the Er was plotted as shown in figure 4, the Er line would be very much flatter. Most of these materials have an Er of around 4.4 to 3.9 or so.
Your PCB shop can then probably design the required single or multilayer stack-up and supply the effective or Design Value for Er that should be used in your design, or they can generate this data for you if you tell the target trace width and reference plane spacing, etc.
Some of these improved Glass Epoxy board materials also have better losses than the generic FR-4 so that can be a plus also.
The thermal conductivity of these improved FR-4 materials is usually about the same as plain old FR-4. The thermal conductivity of your finished PCB will be more influenced if you use multiple layers and flood copper ground planes on all layers.
The big advantage of these improved RF-specified Glass Epoxy materials is that they are only incrementally more expensive than plain old FR-4 and many of them are rated for lead-free soldering temperatures, something that plain old FR-4 usually can't do.
High-Performance RF PCB Materials The next step up in improving on FR-4 is to use a high-performance material like the mentioned Rogers RO4350B[5] and others. As can be seen in figure 2, the RO4350B PCB loss is less than half the loss of FR-4 at 6 GHz. While this may not be too important and not worth the extra cost if your circuit operates at less than 6 GHz, at 10 GHz the losses are even less and FR-4 really starts to show its weakness.
These really high-performance materials work well up to the 20-GHz-plus range and have a very stable and repeatable Er. The Er of these materials is also usually much lower, being on the order of 3.6. As with the higher-grade “Glass Epoxy” materials, the Er is essentially flat with frequency.
If your circuit design uses distributed elements or matching networks in the multi-GHz range then there is really no better choice than these types of materials for lot-to-lot consistency.
As an added bonus these materials are not usually based on glass epoxy, but instead have a ceramic filler which really improves the thermal conductivity.
Many of these materials can also survive lead-free assembly temperatures very well.
All of this performance comes with a cost however – your board cost, to be specific.
Another option to building a multilayer PCB with all high-performance material is to build a hybrid Glass Epoxy/high-performance material type board. This is where you use a material like the high-performance Rogers RO4350B on the outside layers – where the RF components and Microstrip traces are – and use a lower-cost Glass Epoxy inside where the power and control traces reside.
This Hybrid-type construction works out quite well and can save a substantial amount on your board costs. Be sure to check these details out with your board supplier though to be sure that the materials you want to use are compatible with each other.
So what's the verdict? Well I hope that I have shown that plain old FR-4 or improved Glass Epoxy can indeed be used at all the common RF/wireless frequencies up to 7 GHz or more. If plain old FR-4 won't work for you for some reason, then you have the option of using a high-frequency, better specified “FR-4 like” Glass Epoxy material that won't ruin the budget. If total loss and circuit stability are of paramount importance to you, or if you need to go above 10 GHz where FR-4 is really getting pretty lossy per inch, then you can always use the exotic high-performance microwave materials.
Currently the most popular wireless RF frequencies are around the 0.3 to 2.5-GHz range, and FR-4 will work just fine at those RF frequencies in many applications, especially when considered in light of the other variable parameters like board thickness, which has a greater effect on trace impedance than does even a widely varying Er.
References [1] Here we will define RF as 100 MHz to 6 GHz as these are the frequencies where the overwhelming majority of commercial wireless work occurs today.
[2] Hageman, S. “Make a quick-turnaround PCB for RF parts,” EDN December 15, 2010
[3] Improved FR-4 Glass Epoxy like materials are available from many manufacturers, such as: Isola Group, Nelco and Arlon MED to name a few.
[4] Er or “Epsilon sub R” is the dielectric constant of the material. This value along with the dielectric thickness sets what the trace widths need to make 50-ohm transmission lines on the PCB. 50 Ohms is the most common interconnect impedance, but 75 ohms is also popular with our video and broadband cable friends.
[5] Rogers RO4350B is a very popular material for high-performance wireless designs, but there are many others. I mention it specifically because I can get 24-hour prototypes made from it at very low cost and this works out well for me. www.rogerscorp.com, www.sunstone.com
About the author Steve Hageman is a confirmed “Analog-A-Holic” since about the fifth grade when he built his first short-wave receiver. After acquiring his first his first Apple ][ computer in 1982, Steve has always enjoyed marrying Software to Analog Hardware to build useful measurement systems. Since then Steve has had the pleasure of designing such diverse products as: Modular Data Acquisition Systems, Switching Power Supply Test Systems, Radio Receivers, RFIC Test Systems and most recently Software Defined Radios for Wireless Testing and Spectrum Analysis. Steve may be reached via his website at www.AnalogHome.com or his blog at AnalogHome.BlogSpot.com or The Practicing Instrumentation Engineer.
That started so: there was the G10, which was a pcb material produced with relative tight tolerances, also electric tolerances wee trustable. But is was discovered that –horribile dictu et auditu — G10 is flammable! Therefore came the FR4, which looks li
Boy we all forgot about G10 so long ago… Fire is an issue and I'm sure many buildings have been saved because of the flammability ratings on Electronic Components. That's a good thing. Of note, I have been building the little test board of Figure 1 (and
I’m sure every engineer worth something has let out the smoke out of at least one component. And some of us working in high voltage and semi high power have had the fortune of letting the smoke out of the entire board. While this is a very rare occasion or
Ahhh yes – Nothing lets the smoke out like power circuits! And when this happens, nothing gets everyone's attention like the smell of a melting PCB! 🙂
for High Frequency Roghers Material is better, if need good Rogers 4350 or 4003 PCB I Recommend： http://www.pcbsino.com http://www.pcbsino.org
Is it ok to use FR4 for digital signals like 2.5GHz or 5GHz,where atleast the 5th harmonic is required?
Hi.I have a question.My task is i need to design rectangular microstrip patch antenna at low frequency.The frequency that i used is 700MHz.My question is,what are the material that is suitable for low frequency?i appreciate your help
Is it fine to use FR-4 material as a substrate at 700MHz frequency?I really appreciate your help.I need to do design for my project
“Hi, Regarding RF Microwave PCB related information please link to http://www.atechcircuit.com/products/microwave-rf-pcb“
“Great article, Steve.nnWe put together a page on our RF PCB capabilities and put together a simple overview of some generally recommended RF materials and bonding materials to use for various applications and industries including consumer electronics, m
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