How Is PCB Laminate Dk Determined Anyway?
Dielectric constant (Dk) is one of the most important of circuit material parameters and a starting point for circuit designers. The dimensions of high-frequency circuit structures, including different types of transmission lines and the spacing between lines for proper isolation and/or coupling, are determined by the circuit material’s dielectric characteristics, and one of the main parameters for understanding those characteristics is Dk. Circuit designers typically grow familiar with different commercial circuit materials, whether flexible or rigid, and may even gain great understanding of how to work with a material having a certain Dk value, such as 3.0. Many designers grow to trust that the Dk value assigned to a given circuit material is truly accurate and consistent from board to board and base their designs on that trust. But how does the material supplier determine a circuit material’s Dk value anyway?
A circuit laminate’s Dk can be determined by different measurement techniques, generally using a microwave vector network analyzer (VNA) to evaluate the amplitude and phase characteristics of reference circuits with fairly well-known behavior. Such reference circuits include microstrip and stripline transmission lines as well as various types of resonators. By fabricating two microstrip transmission lines of different lengths on a circuit laminate with precisely known thickness, for example, and measuring the phases of the two lines with a VNA, the difference in phase values between the two transmission lines can provide insight into the Dk of the circuit material.
It sounds straightforward, but there is much to consider in this seemingly simple measurement. For one thing, most circuit laminates are anisotropic in nature, with different Dk properties along different axes of a material. A circuit material that has been characterized with a Dk value of 3.0 through the z axis of the material at a certain frequency may not exhibit the same Dk values through its x and y axes (length and width). The use of microstrip transmission lines to determine circuit material Dk via differential-phase-length method is an effective means of discovering a Dk value through the z axis of the material. But it is a measurement that must be performed with precision, with the electrical effects of a test fixture’s connectors removed from the phase values measured for the transmission lines. And this is just one of many test methods used in the RF/microwave industry to determine laminate Dk; some additional test methods provide Dk through the z axis while others help determine Dk through the x and y axes of the material. Some of these test methods are also used to measure a material’s dissipation factor (Df).
It is also important to remember that a circuit laminate’s Dk value depends on frequency, with 10 GHz often used as the test frequency for determining the Dk of a particular circuit material. If a circuit material is characterized for a given Dk value at 10 GHz using a test approach such as the microstrip differential-phase-length method, it will exhibit a different Dk value if tested with the same measurement method at a different frequency. And, unfortunately, two different Dk test methods may not even yield the same values of Dk for the same material under test even at the same test frequency!
A number of circuit material Dk test methods are based on fabricating resonators or resonant cavities on a material and evaluating the performance of the resonator. This use of perturbed resonators yields Dk values that are typically through the x and y axes of the material and can also help determine the material’s Df. One such method, for example, is the use of a split post dielectric resonator (SPDR) to measure both Dk and Df as outlined in application note 5989-5384E from Agilent Technologies (now Keysight Technologies, www.keysight.com), “Agilent Split Post Dielectric Resonators for Dielectric Measurements of Substrates.” The SPDR method, one of the approaches used by Rogers Corp. (along with the differential-phase-length method) to evaluate Dk, is a means of measuring Dk automatically with a VNA and test software at a single frequency. It provides in-plane Dk value through the length and width of a substrate but is not effective beyond a certain thickness and Dk value of material.
The different test methods employed by different circuit material suppliers may lead to some confusion for engineers comparing different circuit materials in search of a laminate with a certain Dk value for a design. It is important to note the test frequency at which the Dk has been characterized as well as which axis or axes for which the Dk value has been determined. Of course, for engineers working on the growing number of millimeter-wave circuit applications, such as for short-haul communications or automotive electronic safety systems (radar), Dk values referenced to a test frequency of 10 GHz offer little insight into how a circuit material will behave at frequencies above 30 GHz and it is at these higher frequencies that work remains to be done in characterizing circuit material Dk.
Fortunately, suppliers of commercial circuit laminates are aware of the differences among the various Dk and Df measurement approaches and are working together to try to eliminate confusion for engineers comparing laminate data sheets, especially in terms of Dk. The IPC D-24C Task Group of the noted global trade association, Association Connecting Electronics Industries, and its Institute of Printed Circuits (IPC, www.ipc.org), is attempting to better understand the impact of different test methods on determining the Dk values of high-frequency circuit laminates and is focused on broadband VNA measurements above 10 GHz for determining Dk and Df values for circuit laminates.
The task group, which includes leading materials test companies and suppliers of RF/microwave circuit laminates, is developing precise methods for testing the same circuit materials from the same production lots, not only with different test methods but at different locations along a board, to better understand all of the variables involved in determining precise, repeatable Dk and Df values for a circuit material. One of the goals of the task group is to establish reliable test methods for Dk and Df above 10 GHz as well as repeatable measurement techniques for determining precise values of material thickness, another important material parameter for circuit designers. Hopefully, this industry teamwork and the efforts of the task group members will yield circuit laminate data sheets that can be compared easily and with confidence.
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