Lightning Speed Laminates: Why Do Different Test Methods Yield Different Electrical Values?

Reading time ( words)

A variety of different test methods may be used for any one electrical concern. This article will discuss the issues related to determining the dielectric constant (Dk) and dissipation factor (Df or Tan-Delta). On a data sheet, a designer may see a Dk value for a material to be 3.5, as an example. Once the designer buys the material and performs necessary evaluations, it may be found that the Dk of the material is 3.8. In some applications this difference in Dk is probably not meaningful; however, for many RF and high-speed digital applications, this difference could be very significant. What is really interesting about this example is that the two Dk values may both be correct, depending on the test methods used.

Most laminates used in the PCB industry are anisotropic and this means that the electrical properties are not the same on all three axes of the material. Typically the thickness (z-axis) of the material will have a different Dk value than the x or y axes of the material. The reasons for this depend on what type of material is being considered.

The laminates used in the PCB industry are typically woven-glass reinforced, however there are notable exceptions. The glass reinforcement layer typically has a different Dk and Df than does the raw substrate of the laminate. The standard E-glass most often used in PCB laminates has a typical Dk value of about 6 and a dissipation factor of around 0.004. The common FR-4 laminates use relatively simple resin systems and the resin itself has a Dk that is around 3 and a Df of about 0.03. Different ratios of resin to glass will cause the laminate to have a Dk that is somewhere between the value of the resin and that of the glass. However, the glass-resin ratio impact on Dk is usually considered when evaluating the material through the thickness axis and if the x- or y-axis is evaluated, the Dk value may be very different than the z-axis result.

A large number of test methods are available to evaluate materials for Dk and Df. The methods that are most often used in the PCB laminate industry for making these measurements are typically tailored to evaluating materials in very large volume. Because of this issue, these test methods need to determine Dk and Df relatively fast, have good repeatability, and be used for quality control. A common test method used is the clamped stripline resonator, where a clamping fixture is used to form a stripline structure; the layer structure of a stripline is ground-signal-ground. This test method determines the Dk and Df of the material in the clamped fixture and more specifically, it is reporting these values related to the thickness axis of the material.

To read this entire article, which appeared in the December 2015 issue of The PCB Design Magazine, click here.



Suggested Items

Advances in Medical Diagnostics Using LoC and LoPCB Technologies

03/31/2020 | Pete Starkey and Happy Holden, I-Connect007
Several techniques for detection and diagnosis of COVID-19 are currently under development, some of which may detect the novel virus exclusively; others may also detect strains that are genetically similar. A detection kit recently announced uses technology based on a portable lab-on-chip (LoC) platform capable of detecting, identifying, and differentiating MERS-CoV, SARS-CoV, and COVID-2019 in a single test.

This Month in Design007 Magazine: Al Neves on Designing for Profitability

03/11/2020 | Andy Shaughnessy, Design007 Magazine
If your company is having trouble designing PCBs for profitability, where does the blame fall? Is it management’s fault for having inefficient processes, or is it the fault of the designers and design engineers for not keeping up to date with their training? During DesignCon, I asked Al Neves of Wild River Technology to weigh in on this question. As he explains, you and your manager might both be to blame for inefficiencies in the design process.

February Design007 Magazine: Eric Bogatin Looks at EMI Root Causes and Solutions

02/12/2020 | I-Connect007 Editorial Team
The I-Connect007 team recently spoke with design instructor and author Eric Bogatin about the EMI challenges facing PCB designers today. Eric is a “signal integrity evangelist” with Teledyne LeCroy, as well as an adjunct professor at the University of Colorado Boulder, and technical editor of the Signal Integrity Journal. In this interview, Eric explains why EMI is so prevalent and what designers and design engineers can do to avoid EMI from the start.

Copyright © 2020 I-Connect007. All rights reserved.