This post authored by John Coonrod originally appeared on the Rog-Blog hosted by Microwave Journal.

Dielectric constant (Dk) is one of the most essential of printed-circuit-board (PCB) material parameters. Circuit designers rely on it for determining such things as impedances and the physical dimensions of microstrip circuits. Yet, it is not unusual to see a laminate data sheet with different values of Dk for the same material, such as a process Dk and a specification Dk. A material supplier may even recommend an additional value of Dk, to be used in computer-aided-engineering (CAE) software simulators. Why all the different numbers and is there one value of Dk that is the one to trust when designing a circuit?

As detailed in the last several blogs, there are more than a few ways to determine the Dk of a microwave laminate, and these different measurement methods often yield different results for the same material. Some of the measurement techniques are based on the use of “raw” PCB materials—without circuits on them—while some of the methods use a well characterized circuit with predictable performance to then determine the Dk for the material. Materials suppliers may use terms like “process Dk” to refer to the target value for the material when it is being processed, and “specification Dk” to mean a value determined by means of one or more of the measurement methods described in the two previous blogs. Often, the process and specification Dk values are the same for a given laminate.

A more meaningful version of Dk is the “Design Dk” that is currently published in the Rogers’ Product Selector Guide and serves as the values for Dk in the MWI-2010 Impedance Calculator, available for free download (note that it does require sign-up). The Design Dk is a value that provides the most accurate and repeatable results when used for circuit design purposes, notably in commercial CAE circuit and system simulation programs.

For some materials, the process or specification Dk may have the same value as the Design Dk. For Rogers’ popular RT/duroid® 6002 microwave laminate, for example, the process Dk and the Design Dk are both 2.94 in the z-axis. One difference is that the process Dk is specified at 10 GHz on the data sheet, while the Design Dk is given for frequencies from 8 to 40 GHz. The values were determined using two different test methods.

At the same time, the Dk values may differ appreciably. Rogers RO3010™ laminate has a process Dk of 10.2 in the z-axis at 10 GHz, but a Design Dk value of 11.2 is recommended for use with commercial CAE software simulators for more accurate modeling purposes.

If process and specification Dk values are determined by measurements, why should there be a need for a “Design Dk” value? As mentioned in the previous two blogs, there are many test methods for determining the Dk of a laminate. As an example, the global trade organization IPC lists 13 different test methods to determine a material’s Dk. Materials suppliers use any number of these measurement methods for their own determinations of Dk, while laminate users may have their own, and different, methods for determining the Dk of a laminate before using it for design purposes. In the two materials mentioned above as examples, a different measurement was used in each case to find the process/specification Dk and the Design Dk: the clamped stripline method was used for the process/specification Dk and the differential phase length method was used for the Design Dk.

Within Rogers, for example, the X-band clamped stripline resonator test is used for standard quality assurance (QA) testing of specification or process Dk, although the full-sheet-resonator (FSR) measurement method may also be used for QA testing. The split post dielectric resonator (SPDR) method may also be used to characterize materials within Rogers. For determining the Design Dk, the microstrip differential phase-length method will be used for all materials.

While none of the test methods is ideal, the differential phase-length method is elegant in its simplicity. It relies on fabricating two microstrip circuits of significantly different lengths on the same laminate material, using the same connectors or test fixture to determine the phase angle differences between the circuits for a given test frequency. A value of Dk can be determined from simple calculations based on the differences between physical lengths and phase angles. The process is repeated for as many frequencies as is practical. It is not a fast method, but it does provide accurate results for Dk in the z-axis, with anisotropic material effects (Dk values in the x and y axes) having little impact on the measurements.

This test method uses microstrip circuits commonly used in actual applications. It is also performed at the high frequencies often used in applications, to account for “copper effects,” in which a laminate with rougher copper surface can test for a higher apparent Dk value than a laminate with smoother copper surface. Test methods using lower frequencies may not reveal the effects of the copper roughness on measured Dk value.

The Design Dk values have been determined for all of Rogers’ high-frequency laminates and are being reported to all major developers of CAE simulation software tools. In addition, those values are now included in the MWI-2010 Microwave Impedance Calculator, the Product Selector Guide, and in the Slide Rule published in the November 2010 issue of Microwave Journal.

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2 Responses to Learn to Apply Design Dk

  1. george storm says:

    Clearly, it is best to get the “design dk” from measurements that are akin to the physical situation seen inthe application. Probably the transmission-line measurement is most applicable for the majority of components. But the effects of the various factors will be different for printed inductors.
    So I felt it would be good to have a little more explanation of the origins of the differences between the measurements. The author mentioned differences between dk in the x, y and z directions, but I imagine that causes might include dk that varies with depth into the board, or dielectric profile modification due to strain between the conductor and the dielectric.
    Further guidance is always welcome!

    • julianng says:

      George,
      Thanks for your feedback and I mostly certainly agree with your comments / concerns. This is something that would probably be best if we talked on the phone about.

      I know for sure that some material configurations will have different Dk properties at the surface in the x-y plane as compared to the Dk deeper into the material and in the x-y plane. Some microstrip edge coupled features will have coupling fields that do not penetrate deep into the x-y plane of the material and other designs or applications will. So in the case of some high frequency PCB laminates that are glass woven it is
      possible for some edge coupled applications to have the fields use only the resin part of the laminate and other applications go deeper to where the resin and the glass layer is being used. In that case there could be a noticeable difference in x-y plane Dk, as detected by the application. Not sure if that is what you’re implying or not, but I’d be happy to speak with you about the subject.

      I have attached a simple Design Dk presentation that I put together which has some of the basic concepts. I’m not sure if you have seen this yet or not, but maybe this could help explain my previous example.

      Let me know if you want to talk.
      Best regards,
      John
      (posted on behalf of John Coonrod)

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