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.

Share


Suggested Items

AltiumLive Summit—Munich, Germany, Part 2

11/13/2017 | Pete Starkey, I-Connect007
Pete Starkey continues with his review of the AltiumLive PCB Design Summit held recently in Munich, Germany. The second day commenced with a new product launch. “Working together is hard” it read on the screen. Statistics indicated that 33% of new products were late getting to market, of which 28% were late due to insufficient collaboration, and up to 50% of potential revenue could be lost through being late to market. Then the screen read “NEXUS makes it easy!”

AltiumLive Summit—Munich, Germany, Part 1

11/07/2017 | Pete Starkey, I-Connect007
Altium held a very successful AltiumLive PCB Design Summit in San Diego, California at the beginning of October for the benefit of their North American design community, and followed it three weeks later with a counterpart European event in Munich. And what an eye-opener it proved to be—literally hundreds of delegates, a superbly organised and managed programme, billed as a completely immersive two-day interactive design experience on a theme of learning, connecting and getting inspired.

Polar Instruments’ Martyn Gaudion Discusses Tools and Books

10/22/2017 | Andy Shaughnessy, PCBDesign007
Polar Instruments has pretty been busy lately. In the last year, Managing Director Martyn Gaudion has written two books for I-Connect007, and the company has been working to upgrade its tools, especially library functionality. I met with Martyn at PCB West. We discussed Polar’s newest tool updates, Martyn’s new side job as an author of technical books, and the continuing growth of the EDA segment, including among young people.



Copyright © 2017 I-Connect007. All rights reserved.