November 29, 2011

John Coonrod is a Market Development Engineer for Rogers Corporation, Advanced Circuit Materials Division. John has 23 years of experience in the Printed Circuit Board industry. About half of this time was spent in the Flexible Printed Circuit Board industry doing circuit design, applications, processing and materials engineering. The past ten years have been spent supporting circuit fabrication, providing application support and conducting electrical characterization studies of High Frequency Rigid Printed Circuit Board materials made by Rogers. John has a Bachelor of Science, Electrical Engineering degree from Arizona State University. www.rogerscorp.com/acm. This blog is part of Microwave Journal's guest blog series.

Selecting a PCB laminate for a high-frequency application is like picking a foundation for a new building: the strength of the whole project relies on the right choice. The previous blog introduced a strategy to help simplify the PCB laminate selection process, by relating the requirements of an application to laminate specifications. Each RF/microwave application is unique, with its own requirements. But at least one laminate will usually offer the right set of specifications to best meet those requirements.

A glance at the “High Frequency Materials Product Selector Guide” from Rogers Corporation shows the breadth of circuit-board materials available from this single supplier. The materials have been developed over time in response to customers and their different requirements. Picking the optimum laminate for a given application requires understanding how different material capabilities match up with different types of high-frequency applications.

Previous blogs detailed many of these laminate specifications, such as dielectric constant, the consistency of the dielectric constant across the material, coefficient of thermal expansion (CTE), and thermal conductivity. Each parameter describes a specific material characteristic, although each parameter may not be critical for a given application. No one material is ideal for all purposes, so finding the best fit for an application involves pairing a material’s capabilities with the needs of that application. To apply this selection process, and the technique of “filtering” material specifications to focus on the main parameters for an application, it may help to review a few sample applications and types of materials that might work best for them.  

The number of parameters listed for each laminate in the Selector Guide can seem overwhelming. But they all describe the behavior of a material in a different way or under different conditions. To make selecting a laminate a practical process, the different material parameters must be prioritized for a given application. The one parameter not listed in the Selector Guide is cost and, for a fair share of applications, cost is important. But if cost is the only guiding parameter, it is unlikely that all of the other requirements of an application will be met.

To determine the key laminate requirements for an application, the application should be defined in its broadest terms, such as frequency range, broadband or narrowband, small signal or large signal, large circuit or small circuit, controlled or “hostile” operating environment, even the size of a production run and the importance of unit-to-unit repeatability. By defining the needs of an application in terms of broad requirements first, it can be easier to identify key material parameters that should be used to guide the search for an optimum laminate for that application.

For example, describing an intended application circuit as “small signal,” which can include a wide range of active and passive components such as amplifiers, filters, and oscillators, can help determine which material parameters should be considered first. A small-signal circuit will typically operate with no more than a watt or two of RF/microwave power, compared to a “large-signal” circuit which may handle tens or hundreds of watts of RF/microwave power. The thermal requirements for the two types of circuits, and for their laminates, are dramatically different.

Someone designing a small-signal circuit is likely less concerned with a material’s thermal parameters, such as thermal conductivity and CTE, than they are with minimizing loss and in comparing materials for lowest possible dissipation factor. And where frequency stability is critical, the consistency of the dielectric constant is also important. One possible solution is RT/duroid® 5880, which combines low dissipation factor of typically 0.0009 at 10 GHz with a low dielectric constant of 2.20 at 10 GHz, maintained to a tight tolerance of ±0.02.

Of course, the laminate selection process is also about tradeoffs. In spite of its exceptional small-signal performance, RT/duroid 5880 is not engineered for applications where thermal management is critical. It has relatively low thermal conductivity of 0.20 W/m/K, unimpressive z-axis CTE, and lackluster thermal coefficient of dielectric constant (typically -125 ppm/°C from -50°C to +150°C).

In contrast, someone designing circuits with critical thermal-management requirements would give greater weight to a laminate’s thermal characteristics, and perhaps be more willing to tolerate a somewhat higher dissipation factor. RO4350B™ laminate, with outstanding thermal conductivity of 0.69 W/m/K, excellent z-axis CTE, and controlled thermal coefficient of dielectric constant (typically +50 ppm/°C), is well suited for many applications where thermal-management requirements are a concern, such as power amplifiers. There are tradeoffs associated with increased thermal conductivity, including a higher dielectric constant of 3.48 ± 0.05 and higher dissipation factor of 0.0037. The higher dielectric constant will shrink the dimensions of microstrip circuitry somewhat compared to the low 2.20 value of RT/duroid 5880, but RO4350B laminate with enhanced thermal characteristics will help effectively dissipate heat in higher-power circuits.

What if a circuit application is physically large, such as an antenna? This type of application calls for circuit materials with consistent dielectric constant across a large laminate panel, but also with low loss. Rogers RO4730™ laminate, a glass-reinforced, ceramic-filled hydrocarbon-based laminate, features a composition that is relatively low in cost, yields a dielectric constant of 3.00 ± 0.08 and that can translate into consistent impedance for transmission lines across a large panel and predictable, repeatable antenna patterns at RF/microwave frequencies. The material’s low dissipation factor and good thermal conductivity also mean that it can be used in some higher power levels.

Some applications may involve circuits and systems operating in environments with wide temperature ranges, high moisture levels, and other factors that can affect electrical performance. For example, water has a dielectric constant of about 80. Any water absorbed by a laminate will change the dielectric constant of the material. In these cases, laminate selection might be guided by environmentally related material parameters, such as thermal coefficient of dielectric constant and moisture absorption. As noted, RO4730 laminate is excellent for antennas, and it exhibits typical moisture absorption of 0.13%, generally considered quite good. In comparison, RO3730™ laminate with a dielectric constant of 3.00 ± 0.06 is also ideal for antennas, but has somewhat lower loss and improved moisture absorption of 0.04% for those applications where conditions like water vapor may have an impact.

These are just a few examples of how an application’s requirements can help guide the selection of a high-frequency laminate. Obviously, there are many factors to consider in any application, including cost, performance, repeatability, and even the size of the final circuit. Additional factors include the thickness of the dielectric material and the type and weight of the conductive cladding.

The next blog will examine how these different material choices fit into the selection process, and also explore the role of the PCB laminate supplier in terms of how they determine their material parameters. For some material specification, two different types of measurements can yield two different results. Selecting a laminate with the more accurate specification can save a great deal of grief during both design and fabrication stages of a product development.

Do you have a design or fabrication question? John Coonrod and Joe Davis are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.