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Selecting PCB material for RF

01 Nov 2012  | Steve Hageman

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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 6GHz. While this may not be too important and not worth the extra cost if your circuit operates at less than 6GHz, at 10GHz the losses are even less and FR-4 really starts to show its weakness.

These really high-performance materials work well up to the 20GHz-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 multiGHz 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 7GHz 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 10GHz 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 2.5GHz 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.

[1] Here we will define RF as 100MHz to 6GHz 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, 2012.
[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-ohmtransmission 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.,

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.

To download the PDF version of this article, click here.

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