PCB Tech - Looking for a test method for the dielectric constant of materials suitable for millimeter waves

Looking for a test method for the dielectric constant of materials suitable for millimeter waves

The millimeter wave (mm Wave) frequency was once a section of spectrum reserved for research and development (RD). However, millimeter waves have now been widely used. With the expansion of advanced driver assistance systems (ADAS) and its millimeter-wave radar safety systems, and fifth-generation (5G) cellular communication technology to higher frequencies, millimeter-wave frequencies will be used by billions of people around the world. This means that the demand for PCB materials supporting 28 GHz or higher frequencies will also continue to grow. To characterize such high-frequency circuit board materials, for example, the frequency is around 80 GHz, and it is necessary to measure the dielectric constant (Dk) or relative dielectric constant of the material at millimeter wave frequencies. However, in such a high frequency range, there is no definite industry standard yet.

The dielectric constant is the primary concern for most circuit board materials because it affects the size of the circuit at the operating frequency. Since the wavelength decreases as the frequency increases, especially at millimeter wave frequencies, the circuit size becomes very small, so it is important to know the Dk of the circuit material accurately. Essentially, the Dk or relative permittivity of a material can be defined as the ratio of the amount of charge stored when the material is between two metal plates to the amount of charge stored when the metal plates are in vacuum or air. The Dk of vacuum is "1", and the Dk of any other material is higher than that of vacuum.

Basic knowledge of Dk

The measurement methods used by most circuit board material suppliers are recognized industry standard methods, and are measured at a specific test frequency, such as 10GHz or below. At millimeter wave frequencies, there are also some methods for measuring the Dk of circuit board materials, but these methods are not as well known as the test methods used at low frequencies.

What are the difficulties in accurately measuring Dk at millimeter wave frequencies? To measure the Dk value of a material, either test on the raw material under test (MUT), or process the raw material into some form of reference circuit and test on the circuit. Whether it is radio frequency, microwave or millimeter wave frequency, the Dk characteristics of circuit board materials are usually anisotropic. Therefore, when using the test method to determine the Dk value of the material, it is also necessary to determine the Dk of the test material's Z axis (thickness direction) or X-Y plane (the length and width of the material). For different material directions, these values are usually different and are usually a function of frequency. Therefore, for millimeter wave circuit design engineers, it cannot be assumed that the Dk value at 10 GHz on the Z axis is equal to the Dk value at 60 GHz on the XY plane under the same material. Accurately measuring Dk of circuit materials at millimeter wave frequencies is very important for many upcoming millimeter wave applications and their circuit design engineers.

Screening candidate criteria

There are many methods to determine the Dk value of materials at millimeter wave frequencies. However, none of the methods has been accepted as an industry standard test technique by technical standards organizations such as IEEE or IPC. However, some Dk test methods provide very good measurement accuracy and repeatability, making them candidates for millimeter wave Dk measurement standards.

The microstrip differential phase length method is one of the millimeter wave Dk measurement techniques that can be used as a potential standard. This is a circuit-based test method. In this method, a plurality of 50 ohm microstrip transmission line circuits of different lengths are fabricated on the tested material. In this way, by measuring the phase angle difference of the two circuits, the Dk characteristic of the tested material can be obtained. Since the Dk of the tested material may change, when determining the dk of the material, the two microstrip circuits should be as close as possible to minimize the influence of the material Dk change. You can use a high-precision vector network analyzer (VNA) with a frequency coverage of up to 110GHz to test the S-parameter and phase measurement of two microstrip circuits of different lengths on the MUT.

Another method to determine the Dk value of circuit board materials at millimeter wave frequencies is the ring resonator method, where the ring resonator is a test circuit made on the MUT. The size and design parameters of these resonant circuits can accurately reflect the resonant frequency in detail. When a ring resonator is accurately processed on the MUT, information such as the dielectric constant of the material can be accurately deduced by measuring the resonant frequency. By using VNA to measure the response of gap-coupled ring resonators at millimeter wave frequencies, and comparing these responses with numerical-based results provided by commercial electromagnetic (EM) field calculation software, it can be based on the circuit size and conditions entered into the software Extract the Dk value of the MUT.

Of course, in practical applications, especially for the size of the millimeter wave circuit, the circuit size and tolerances may cause the measured resonant frequency to change, thereby causing errors in the Dk value of the measured material. Changes in conductor width and circuit board(MUT) thickness will also affect the frequency of the ring resonator. In addition, the thickness of the copper foil on the ring resonator circuit may vary across the circuit board. The change of circuit copper thickness will affect the coupling and resonant frequency of the gap-coupled ring resonator. Therefore, when applying the ring resonator test method to determine the Dk of the circuit board material at millimeter wave frequencies, it is necessary to minimize the circuit copper thickness change. Crucial.

The methods mentioned above are two classic methods of many mature circuit board materials Dk value testing technologies, and can be used as industry standard methods for measuring and circuit board materials Dk at millimeter wave frequencies. Both of these are circuit-based test methods, and other test methods based on raw materials can also be used.

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