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The cost and performance difference between two radio frequency printed circuit board (RF PCB) materials is substantial and adds up quickly. For a price difference as low as just $25 between two material choices, the losses or savings add up to $25,000 for every 1,000 boards. Since most boards are stacks, that cost difference for 1,000 boards becomes $100,000 if the material is used for four layers in a stack made of eight layers.And that is just for the cost of materials. Some materials, which use non-standard manufacturing techniques, can cost even more in terms of labor to produce. The goal of this article is to help you reduce cost without sacrificing performance for your target application.
Important Performance Parameters: Dielectric Constant and Dissipation Factor
The most important parameter is the dielectric constant. It is a measurement of a material’s ability to store electrical energy in an electric field. However, as frequencies climb into the RF and microwave ranges, the given dielectric value on a material’s spec sheet is not necessarily constant. More expensive materials have dielectric constants that are stable over broader or at higher ranges. Always discuss with your vendor and PCB manufacturer if the listed dielectric constant value holds true for your target application. The next consideration to make is the dissipation factor. A lower dissipation factor results in lower losses at higher frequencies. This is especially important for high power applications.
Most Affordable: FR4
FR4 is a glass-reinforced substrate, and technically a “class” of material, not a specific material. It offers reliable results for low-performance applications at an affordable cost, such as consumer electronic devices. Such popular use has led to a solid understanding of its electrical and mechanical properties, making its performance and limits well understood. The manufacturing methods used when working with FR4 are also reliable and inexpensive compared to other material types, further keeping costs in check.However, it has two major drawbacks. The dissipation factor is four times as high as performance-grade materials. Its performance varies widely over broad temperature ranges, making it unsuitable for stressful applications, such as military, industrial or space.
Balanced Mid-Grade Option: RO4000
Designed by Rogers Corporation, RO4000 offers considerable performance increases over FR4 but uses many of FR4’s well understood manufacturing methods, which serve to keep cost down.It is commonly used in cellular antennas, automotive radar and sensors, RF ID tags and more. The thermal management of the material is much better than FR4, so it can handle higher stress applications such as automotive environments. However, RO4000 is also typically not a good choice for the extreme-performance demands of space and military tasks. An important thing to note is that RO4000 is a family of materials, and not all of them offer any dissipation factor advantage over FR4. If your application is low stress and power loss at higher frequencies is not a concern for you, then the cost savings of FR4 is a better choice.
High Performance: RT/Duroid
Another family of materials made by Rogers Corporation, RT and Duroid are easily the most expensive choices on the list, but also offer the most elite performance. Both are composite laminates made of PTFE filled with random glass or ceramic and engineered specifically for high performance, high-stress applications such as space, military and millimeter wave applications. The list of performance advantages is extensive and includes low electrical loss, low moisture absorption, a stable dielectric constant over a range of frequencies and low outgassing. Choosing the right material can make the difference between your RF PCB board finishing within budget, or making it out the door and returning with performance issues. Always consider the manufacturing methods and factor them into your cost analysis, especially for materials that don’t use the widely practiced techniques and technologies related to FR4.
This article was written by San Francisco Circuits, a PCB manufacturing and PCB assembly provider in Northern and Southern California. Visit their website for further information on RF PCB material selection.