December 15, 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 high-frequency PCB laminate from the many commercially available choices may sometimes seem like an impossible task. But it can be simplified by sorting materials by their key parameters, such as dielectric constant, dissipation factor, thermal conductivity, and CTE, and using those parameters to help match a material to an application. Of course, this also assumes confidence in the values of those key parameters as published by different materials suppliers, and such confidence comes from an understanding of the measurement methods used to determine the values of those key parameters.

As detailed in an earlier blog, a number of different measurement methods are used by materials suppliers to characterize one of the most basic of laminate parameters, relative dielectric constant. Commercial high-frequency circuit materials are generally available in roughly three values--3.0, 6.15, and 10.2—and various additional values and, in all cases, designers count on the accuracy and stability of those values when computing such things as circuit dimensions and impedances. The amount of variations in these values also tell a designer how suitable a particular circuit laminate might be for a broadband design, since broadband circuits require relative dielectric constant that is as flat as possible as a function of frequency.

Because the values determined by different measurement techniques can differ, Rogers Corporation provides two values of dielectric constant for their laminates—one that applies during processing and one that can be used for predictions with design equations and computer-aided-engineering (CAE) software tools. More information on the specific dielectric-constant measurement methods, such as the clamped stripline resonator test (IPC test method IPC-TM-650 2.5.5.5c) and the full sheet resonance (FSR) test (IPC-TM-650 2.5.5.6), can be found in the article by John Coonrod and Allen Horn III, “Understanding Dielectric Constant for Microwave PCB Materials,” available for free download from the Rogers Corporation web site.

Test frequency is often as important as test method, especially if using a material characteristic such as dielectric constant as a sort parameter. Accurate dielectric constant is essential for stable, predictable filter performance, for example, and the test method used to determine dielectric constant may or may not be suited to the target application for a laminate. Many of the methods for determining dielectric constant use a test frequency of 10 GHz. Some, such as the split post dielectric resonator (SPDR) test, are run at 1 GHz. The effects of laminate copper roughness, for example, can be easily overlooked at the lower frequency. For a meaningful comparison of different materials based on dielectric constant, their dielectric-constant values should be determined by the same test method and at the same frequency.

A laminate’s thermal characteristics, such as thermal conductivity and coefficient of thermal expansion (CTE), can be useful parameters for choosing a material for circuits that must handle high RF power levels and readily dissipate heat while providing stable performance. As with other material parameters, a number of different test methods are used to determine laminate thermal conductivity, depending upon manufacturer. Rogers Corporation, for example, applies method C518 from ASTM International (www.astm.org) to determine the thermal conductivity of its laminates. The method is based on the use of a heat flow meter to register the steady-state heat flow through a material. It is considered a secondary method since the test equipment must be calibrated by means of a material with known properties, referred to as a primary standard. But this is a method that has been approved by ANSI and adopted by the US Department of Defense (DoD), and has shown to be a fast and reliable measurement method for determining the thermal conductivity of production quantities of materials. Another means of determining high-frequency circuit laminate thermal conductivity is by ASTM method E1461, which is considered a flash measurement method that relies on the use of infrared (IR) sensors for detecting temperature changes.

Various test methods are used by different suppliers to determine a circuit laminate’s CTE across a range of operating temperatures, such as ASTM method D3386-94 and ASTM method E831, which supersedes it. These test methods are based on measuring the linear expansion of solid materials as a function of temperature, using thermomechanical analysis. Such test methods can also be used in many cases to determine a laminate’s glass transition temperature. In comparing CTE values when considering different laminate choices, values in the x and y dimensions should be closely matched, with z-axis values kept as low as possible to ensure plated-through-hole (PTH) integrity. While not always possible, ideally all laminate CTE values in a comparison will have been determined by the same standard test method.

In short, a choice of high-frequency circuit laminate should be steered by the requirements of an application, and different material characteristics used to sort from among the circuit material choices. And when sizing up performance parameters from different materials, the most meaningful comparison will come when those parameters have been determined by similar test methods. The next blog will take one last look at the challenge of making a choice among the many commercially available high-frequency circuit laminates, by considering how different circuit materials are represented within modern RF/microwave computer-aided-design (CAD) software tools, and the role of the

Technical Service Engineer (TSE) in guiding a laminate user through a successful circuit fabrication procedure.

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.