The relative permittivity for FR-4 is just that: relative.

Ed.: This is Part 3 of a three-part series on preparing for next-generation loss requirements.

Last month, in Part 2 of this series, I outlined the means by which insertion-loss requirements are determined. Here, I’ll suggest a better method for obtaining accurate Df numbers without having to go to the trouble of building test boards.

A longtime PCB industry technologist asked me recently, “What’s a good Dk (dielectric constant) number for FR-4?” As the interest in signal integrity (SI) was growing roughly 25 years ago, it started to interest me that many SI practitioners considered FR-4 to have monolithic properties. The question reinforced that some still hold that view. One might say the relative permittivity (ϵ_r) of FR-4 is 4.3. Someone else would say 4.1. A third says they always use 4.0. As I read up on it, I realized it varies with frequency, resin content (as a percentage, with the inverse being the glass percentage), and the resin system. At lower frequencies, static numbers for vanilla FR-4 were probably fine for impedance calculations and signal integrity, but those days are far behind us at this point.

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