Rogers: Characterization of PCB board materials at millimeter wave frequencies

Although organizations such as IEEE and IPC have dedicated committees to explore this issue, there is currently no standard industry test method to measure the Dk of 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 single method is ideal, and each method has its advantages and disadvantages, especially in the frequency range of 30 to 300 GHz. AOI Testing supplier china. AOI Testing supplier china.

Circuit test vs raw material test

There are generally two broad categories of test methods used to determine the Dk or Df (loss tangent or tan δ) of the board material: 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 common circuits and extracts material parameters from circuit performance, such as measuring the center frequency or frequency response of a resonator.

Raw material testing methods often introduce uncertainties associated with test fixtures or test fixtures, and circuit test methods involve uncertainty from test circuit design and processing techniques. Because of the difference between the two methods, the measurement results and accuracy levels are usually inconsistent. AXI Testing supplier china.

For example, the X-band clamped stripline test method defined by IPC is a test method for raw materials, and the results cannot be consistent with the Dk results of circuit tests of the same material. Clamped stripline raw material testing method is to build a stripline resonator by clamping two pieces of material to be tested (MUT) in a special 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 material, and the air is not entrained, the measured Dk is different. For high frequency circuit 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 board material is anisotropic and typically has different Dk values ​​on three material axes. The Dk value typically differs very 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, the Dk of the measured z-axis is different from the Dk of the xy plane for the same material to be tested (MUT), although the values ​​of the Dk obtained by the test method and the test 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 transmission/reflection 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.