Microwave Journal

Dielectric constant (Dk) is one of the most essential of printed-circuit-board (PCB) material parameters. Circuit designers rely on it for determining such things as impedances and the physical dimensions of microstrip circuits. Yet, it is not unusual to see a laminate data sheet with different values of Dk for the same material, such as a process Dk and a specification Dk. A material supplier may even recommend an additional value of Dk, to be used in computer-aided-engineering (CAE) software simulators. Why all the different numbers and is there one value of Dk that is the one to trust when designing a circuit?

Circuit designers select laminates for printed-circuit boards (PCBs) by merit of relative dielectric constant (Dk), among other parameters. Suppliers of laminates furnish Dk values on their data sheets and web sites, but designers often prefer the reassurance of knowing the Dk value as it relates to their specific application. The last blog explored the way that materials manufacturers typically use four techniques to evaluate the Dk of a dielectric material in its “raw” form, meaning without circuits. This blog will explore some common methods that materials users employ for determining a laminate’s Dk value and focus on a practical method.

Okay, here we go, blog number 3; but first allow me to do a quick review of what we’ve covered so far: 1.) Not everyone who says they can make RF/MW PCBs really can. 2.) High performance Substrates act NOTHING like FR-4 in the fabrication process, and a qualified supplier must be a Material Guru.

When discussing RF/MW PCBs, starting with base materials seems like a logical place to start. However, the topic of Advanced Circuit Materials is...well...complicated, especially for a single blog post. I’m sure this is obvious to you, but it took me the better part of this week to come to this conclusion with the help of Dale Doyle of Rogers Corp. and Denis Boulanger of Ventec. (Thank you both for your help, and graciousness!) In the end, I have resolved to leave the “heavy lifting” to the experts. Rogers, Taconic, Arlon, and Isola all have information-rich websites and employ amazing professionals like Dale and Denis who are invaluable resources. The “Rog Blog”, here on the Microwave Journal website, is an outstanding resource as well.

RF/microwave designers have a wealth of circuit-board materials from which to choose, which can be good and bad. Having so many options can make the selection process difficult, so that many designers start with relative dielectric constant--Dk for short--as a key sorting parameter. As was pointed out in the last blog, the value of Dk can depend as much on the material composition as the type of test used to measure it. Tests for Dk can be performed either on “raw” laminate material, without circuits formed on it, or by making use of test circuits that have been fabricated on the laminate and measuring the electrical responses of those circuits. This blog will address the four most widely used Dk tests in the first group; the next blog will examine four popular Dk tests in the second group.

A few weeks back, I was having a chat with a national PCB broker who has a unique and broad perspective of the Printed Circuit Board and Electronics industry. We were waxing nostalgic about the “old days” and philosophizing about the days to come. When she asked about our company, Transline Technology, I told her that we manufacture a wide variety of boards, but that our strength and focus lay in RF and Microwave products, which account for about 60% of our business. I was bragging about our work and our customer base when she abruptly interrupted: “You know, Judy, not all board suppliers who say they can make RF boards, really can.”

Circuit-board material parameters are printed on every laminate data sheet. They describe the electrical and mechanical characteristics of a PCB material, including such parameters as relative dielectric constant, dissipation factor, coefficient of thermal expansion (CTE), and thermal conductivity. Design engineers count on these values to be accurate, since their circuits depend on them. But the accuracy often depends on the test method used to measure a material parameter. Even when different laboratories perform the same test on the same material, they can obtain different results. This blog will provide a brief overview of the different tests used to evaluate a printed-circuit material’s characteristics; the next several blogs will go into greater details on specific tests, and will explain how various test results impact the way PCB materials are modeled with modern computer-aided-engineering (CAE) software tools.

Performance requirements typically guide the selection of a PCB material. Some applications may also be cost-sensitive, and require evaluation of the total costs of choosing a circuit material. This includes the cost of the material as well as costs associated with processing the material. For example, FR-4 is a low-cost material with minimal processing costs. However, its performance is also low relative to some higher-costing materials, such as PTFE- or hydrocarbon-based circuit materials, although these materials can have considerably different processing requirements and associated costs. By considering the costs of the material as well as its processing requirements, it’s possible to determine if “you get what you pay for” truly applies to circuit materials.

Microwave circuit designers often have to choose, not only among different layouts and substrate materials, but among transmission-line technologies. Stripline has its advantages for certain components and circuits, and microstrip is popular for both active and passive microwave circuits. But when does a coplanar transmission-line technology make sense?

Digital circuit design once had less demands. When clock speeds were 100 MHz or less, signal loss wasn’t an issue. Digital circuits, in fact, have long been designed to be more tolerant of signal level variations than analog circuits. But with digital circuits continuing to increase in speed, they are assuming more of the characteristics of analog microwave signals, and requiring more attention to design detail and even choice of PCB material as in the case of high-frequency analog circuits.