Measuring Performance Of Microwave Substrates
Apr 25, 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
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.
Relative dielectric constant is typically the first parameter consulted when selecting a PCB laminate for a design. It is measured by many different techniques, using dielectric materials with and without copper cladding. Relative dielectric constant, or Dk for short, is a complex parameter which typically has a different value for all three axes of a circuit-board material. It normally varies with frequency, but the results of Dk measurements can also vary according to the type of test used and how each measurement is performed.
Unfortunately, there is no “ideal” method for measuring PCB material Dk, so there is not even a “standard” test technique that laminate suppliers and PCB material users can agree upon when comparing the results of their measurements. In fact, the industry trade association for interconnecting electronics, IPC (www.ipc.org), has 13 different test methods to determine Dk. The American Society for Testing and Materials, or ASTM (www.astm.org) also has a set of measurements for Dk, and many laminate suppliers as well as users have their own test methods.
Because of the differences in the Dk test methods, it is often difficult to achieve good agreement between measured values from different groups, such as materials suppliers and their customers. This can be critical when a Dk value for a particular material must be used in a commercial CAE software design tool when creating high-frequency circuit designs. The impedance of a high-frequency microstrip or stripline circuit depends on the Dk value of the PCB material. If designing a resonant circuit for a filter, for example, the Dk value that is used by the CAE program is what will be used in calculating the impedance of the filter’s resonant circuit, and thus its center frequency. If the actual Dk value of the material is different than what has been measured for that material and entered into the CAE program, the software will deliver predictions of circuit performance that are at different frequencies than those achieved when the filter is finally fabricated on that material. In understanding why there are differences in the results from different test methods, it may helpful to briefly review some of the main measurement approaches used to determine PCB relative dielectric constant.
Dk measurements have been developed to test the raw PCB material and when a circuit has been fabricated on the material, using the circuit as part of the test setup. For various reasons, as we will see in the next two blogs, these measurements provide somewhat different results, and some companies may perform several of the measurements and use an average value of the results to arrive at a representative Dk value for design and modeling purposes. In general, the most accurate Dk measurements are also the most engineering-intensive of the test methods. But test approaches that require such attention to detail may not be ideally suited for production-line measurements, where speed is important for handling large material volumes.
In brief, the measurements most commonly used for evaluating the Dk of raw PCB materials (without circuits on them) are
- the clamped stripline resonator test, per IPC standard IPC-TM-650 18.104.22.168c, for dielectric material without copper cladding;
- the full sheet resonator (FSR) test, per IPC standard IPC-TM-622.214.171.124.6, for copper-clad laminates without circuits;
- the split-post-dielectric-resonator (SPDR) test, for dielectric material with no copper cladding; and
- the waveguide perturbation test, for raw material without copper cladding.
More details on each of these Dk test techniques will be provided in the next blog.
Tests that involve fabricating specific circuits on a laminate under test make use of microstrip design equations to calculate the Dk of a dielectric material, based on the dielectric thickness, the copper thickness, and the dimensions of the micostrip, when an electrical parameter, such as phase or frequency, is known. These tests include:
- the differential phase length method, in which microstrip transmission lines of known length and impedance are fabricated on a laminate and tested for phase at precise frequencies;
- the microstrip ring resonator method, in which microstrip ring resonator circuits are etched onto a laminate and measurements performed to determine the precise frequency of the circuit;
- the edge-coupled microstrip resonator circuit test, in which an edge-coupled microstrip resonator is fabricated on a laminate and measurements are made of its precise frequency; and
- tests based on fabricating microstrip bandpass filters on a laminate, and measurements performed to find their precise center frequencies.
More details on these Dk measurement measurements will also be provided in a future blog (Blog #15 in this series).
All of these measurement approaches differ in how they prepare a material for testing. Some require full removal of copper cladding, while others require fabrication of a specific test circuit with precise dimensions for evaluation at a specific frequency, typically 10 GHz. Some measurements are designed to determine Dk at one frequency, while others will measure Dk across a swept range of frequencies. The next blog will take a closer look at the first four Dk test methods, with the blog after that exploring the circuit-based measurement methods and some of the potential problems when comparing Dk measurement results determined with different types of circuits. The goal of achieving accurate results is to establish a valid “design Dk” or a value of relative dielectric constant that can be used with confidence in commercial CAE software tools when designing high-frequency circuits on different PCB laminates.