The dielectric constant (Dk) or relative dielectric constant of the PCB material is not a constant – although it seems like a constant from its name. For example, the Dk of a material changes with frequency. Similarly, if different Dk test methods are used on the same piece of material, different Dk values may be measured, even if these test methods are accurate. As circuit board materials are increasingly used in millimeter-wave frequencies, such as 5G and advanced driver assistance systems, it is important to understand how Dk changes with frequency and which Dk test method is “appropriate”.

Although organizations such as IEEE and IPC have special committees to discuss this issue, there is currently no standard industry test method to measure the Dk of circuit board materials at millimeter wave frequencies. This is not because of the lack of measurement methods. In fact, a reference paper published by Chen et al.1 et al. describes more than 80 methods for testing Dk. However, no method is ideal, and each method has its advantages and disadvantages, especially in the frequency range of 30 to 300 GHz.

Circuit testing vs raw material testing

There are generally two types of test methods used to determine the Dk or Df (loss tangent or tan δ) of circuit board materials: that is, raw material measurements, or measurements on circuits made of materials. Testing based on raw materials relies on high-quality and reliable test fixtures and equipment, and Dk and Df values can be obtained by directly testing raw materials. Circuit-based tests usually use common circuits and extract material parameters from circuit performance, such as measuring the center frequency or frequency response of a resonator. Raw material testing methods usually introduce uncertainty related to test fixtures or test devices, while circuit testing methods include uncertainty from test circuit design and processing techniques. Because the two methods are different, the measurement results and accuracy levels are often inconsistent.

For example, the X-band clamped stripline test method defined by IPC is a raw material test method, and its results cannot be consistent with the Dk results of circuit tests of the same material. Clamped stripline raw material testing method is to sandwich two pieces of material to be tested (MUT) in a special test fixture to build a stripline resonator. There will be air between the material under test (MUT) and the thin resonator circuit in the test fixture. The presence of air will reduce the measured Dk. If the circuit test is performed on the same circuit board material, the measured Dk is different from that without air entrainment. For high-frequency circuit board materials with a Dk tolerance of ±0.050 determined by raw material testing, circuit testing will result in a tolerance of approximately ±0.075.

Circuit board materials are anisotropic and usually have different Dk values on the three material axes. The Dk value usually differs slightly between the x-axis and the y-axis, so for most high-frequency materials, Dk anisotropy usually refers to the Dk comparison between the z-axis and the x-y plane. Due to the anisotropy of the material, for the same material under test (MUT), the measured Dk on the z-axis is different from the Dk on the xy plane, although the test method and the value of Dk obtained by the test are "correct" .

The type of circuit used for circuit testing also affects the value of Dk being measured. Generally, two types of test circuits are used: resonant structure and transmission/reflection structure. Resonant structures usually provide narrow-band results, while transmission/reflection tests are usually broadband results. The method using a resonant structure is usually more accurate.