Detecting The Dk Of “Raw” Circuit Boards
May 6, 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.
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.
In general, options for measuring the dielectric properties of a material such as a printed-circuit substrate are based on transmission-reflection or resonance methods. A number of the dielectric measurement methods developed for “raw” laminate materials rely on determining an energy level produced at a specific resonant frequency. The clamped stripline resonator test, performed according to IPC standard IPC-TM-6126.96.36.199.5 (Revision C), and the split-post-dielectric-resonator (SPDR) test are two such resonance-based approaches. These two measurement methods, and the waveguide perturbation test, are suitable for determining the Dk of printed-circuit-board (PCB) materials without copper cladding. The other technique for measuring the Dk of “raw” circuit boards, the full sheet resonator (FSR) test, works with copper-clad materials.
As the name suggests, the clamped stripline resonator test involves the use of clamps and stripline resonators. This method can measure dissipation factor (loss) as well as Dk at X-band frequencies (8.0 to 12.4 GHz). The standard lists an extensive lineup of test equipment and components required for either manual or automatic measurements.
One of the keys to the successful application of this measurement method is the fabrication of the stripline resonator pattern card. Ideally, it should be formed of a laminate that closely matches the nominal Dk value of the sample to be tested. The IPC standard provides clear details on the construction of the text fixture, which helps clamp a set of sample dielectric panels to the stripline resonator pattern card. The standard also includes tables of machined thickness values for the samples; thicknesses for the samples vary according to the expected Dk value to be measured. All metal cladding is removed from a sample to be tested (by standard etching process). The test sample is defined by the standard as a set of two sheets of at least 51 x 69 mm which are clamped on both sides of the resonator card. The outer plates are the ground planes and clamp the package together.
It is a measurement approach fraught with conditions. For example, some materials, such as PTFE with ceramic filler, may require additional processing steps for proper preparation of the resonator pattern cards. Force must be evenly distributed over the resonator area, otherwise biases can result that will affect the value of Dk that is measured. In addition, air trapped between the boards (with Dk = 1) can degrade the accuracy of the Dk measurements, so care must be taken when using this Dk test approach.
The SPDR test employs a circular or cylindrical cavity formed by two halves of a waveguide structure. A material sample to be measured is placed in the gap between the two shorted cylindrical cavity sections. A coupling loop in each cavity is used to excite a resonance in the gap between the cavities, which will be affected by the material sample. By making measurements of resonant frequency and quality factor (Q) on this circular/cylindrical cavity fixture, the permittivity and loss tangent of the sample can be determined. The requirements for this Dk test approach are considerably simpler than those of the clamped stripline resonator test, since a sample need only be planar and extend beyond the diameter of the two cylindrical cavity sections, with no other machining required.
Similarly, the waveguide perturbation test is based on the same principle: that the electromagnetic (EM) fields within a resonant (waveguide) cavity will be changed by the introduction of a material, according to the dielectric properties of the material. By applying Maxwell’s equations to the resonant cavity before and after the introduction of the test sample, analysis of the resulting changes in resonant frequency can be used to calculate the material’s Dk and dissipation factor (although these calculations are not trivial). This type of test can test the anisotropy of a sample. It can be performed with a small sample in a fixture formed from a rectangular waveguide cavity, where the dominant resonant mode is well known. It can be performed at different frequencies, if multiple waveguide fixtures are used but the size of the sample must be precisely controlled (to dimensions of about 0.001 in.) to ensure the accuracy of the results.
The FSR test, performed according to IPC standard IPC-TM-6188.8.131.52.6, is aimed at evaluating the Dk of copper-clad laminates without fabricating circuits on them. This nondestructive test approach is suitable for rectangular substrates of dielectric material clad with metal foil on both sides or clad with thick metal on one side and metal foil on the other side. In essence, an entire metal-clad laminate is excited with a test signal, becoming a dielectric-filled waveguide. The resonant frequency from the excitation is measured and, by knowing the dimensions of the sample under test, the Dk can be determined.
The IPC standard includes a total of seven drawings detailing the construction of a FSR test fixture. The test is simple and fast, but is sensitive to sample Dk only in the z-axis. It does not take into account any effects from fringing capacitance at the open edges of the dielectric waveguide (the laminate under test), and because radiation losses are high from this approach, it can not be used to measure dissipation factor.
From this brief description of these test techniques, it is easy to see the obstacles to achieving good repeatability unless test fixtures are precisely machined, samples properly machined, and test equipment regularly calibrated. These nondestructive measurement approaches are typically used by suppliers of circuit-board materials to determine the Dk and other parameters of their products. The next blog will explore some of the “destructive” test methods that are used by laminate users to determine practical values of Dk for design purposes, typically by fabricating circuits and structures with well-known behavior on a board to be tested, and then measuring the transmission and reflection characteristics of the test circuit.