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56 The PCB Design Magazine • December 2015 column by John Coonrod RoGeRS CoRPoRATIoN LIGHTNING SPEED LAMINATES Why Do Different Test Methods Yield Different Electrical Values? A variety of different test methods may be used for any one electrical concern. This arti- cle will discuss the issues related to determin- ing the dielectric constant (Dk) and dissipation factor (Df or Tan-Delta). On a data sheet, a de- signer 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 dif- ference 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 stan- dard E-glass most often used in PCB laminates has a typical Dk value of about 6 and a dissipa- tion 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 evalu- ating 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 avail- able to evaluate materials for Dk and Df. The methods that are most often used in the PCB laminate industry for making these measure- ments are typically tailored to evaluating mate- rials in very large volume. Because of this issue,