PCB material at millimeter wave frequency

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

Circuit testing and raw material testing

Test methods for determining the Dk or Df (loss tangent or tan δ) of a sheet generally come in two broad categories: raw material measurements or measurements made in circuits made of materials. Raw material-based testing relies on high quality, reliable test fixtures and equipment to directly test raw materials to obtain Dk and Df values. Circuit-based testing typically uses general-purpose circuits and extracts material parameters from circuit performance, such as measuring the center frequency or frequency response of the resonator. Raw material testing methods often introduce uncertainty associated with test fixtures or test fixtures, while circuit test methods involve uncertainty in test circuit design and processing techniques. Because of the differences between the two methods, the measurement results and accuracy levels are often inconsistent.

For example, the X-band clamped stripline test method defined by IPC is a test method for raw materials, and the results are inconsistent with the Dk results of circuit tests of the same material. The clamped strip line raw material test method is to construct a strip line resonator by sandwiching two pieces of material to be tested (MUT) into a dedicated test fixture. There is air between the material to be tested (MUT) and the thin resonator circuit in the test fixture, and the presence of air reduces the measured Dk. If the circuit test is performed on the same board and there is no entrained air, the measured Dk will be different. For high frequency board materials with a Dk tolerance of ±0.050 as determined by raw material testing, the circuit test will achieve a tolerance of approximately ±0.075.

The sheets are anisotropic and typically have different Dk values on three material axes. The Dk value typically differs little between the x-axis and the y-axis, so for most high-frequency materials, Dk anisotropy generally refers to a Dk comparison between the z-axis and the x-y plane. Due to the anisotropy of the material, although the value of Dk obtained by the test method and the Dk of the zk axis of the same test material (MUT) are different from the Dk of the xy plane. The tests are all "correct." .

The type of circuit used for circuit testing also affects the value of the Dk being measured. Typically, two types of test circuits are used: a resonant structure and a transmissive/reflective structure. Resonant structures typically provide narrowband results, while transmission/reflection tests are often broadband results. The method of using a resonant structure is generally more accurate.