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Rogers: Characterization of PCB board materials at millimeter wave frequencies

2019-09-25 10:54:49
The dielectric constant (Dk) or relative dielectric constant of a PCB board material is not a constant constant, although it is a constant from its designation. For example, the Dk of a material changes with frequency. Similarly, if different Dk values ​​may be measured, even if they are all accurate. As circuit board materials are 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 test method is "appropriate". Free DFM Checks supplier china.

This is an IEEE and IPC directive. This is 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 advantages, 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 two broad categories of tests used to determine the Dk or Df (tangent or tan loss δ) of the board material: raw material measurements, or measurements made in circuits made of materials. Raw material-based testing on high-quality tests, 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 materials.

Raw material testing methods often introduces uncertainties associated with test fixtures or test fixtures, and circuit test methods involve uncertainty from test circuit design and processing techniques. The difference between the two methods is usually inconsistent. AXI Testing supplier china.

For example, the X-band clamped stripline test method defined by the IPC is a test method for raw materials, and the results cannot be consistent with the results of the tests. 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 in the test fixture, The test is performed on the same board, the measured is 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 is anisotropic and typically has different values ​​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, it refers to the comparison between the z-axis and the x-y plane. Two to the anisotropy of the material, the Dk of the measured z-axis is different from the Dk of the xy plane the test are all "correct". .

The type of circuit used for circuit testing 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.