Bailibo Testing-Technical Analysis of Relative Dielectric Constant Testing of Ceramic Substrates

In the fields of modern electronic packaging and high-frequency communications, ceramic substrates (such as aluminum oxide, aluminum nitride, silicon nitride, etc.) play a core role in mechanical support and electrical interconnection. The relative dielectric constant (εr), as a core physical quantity that characterizes the degree of polarization and energy storage capacity of a material under an electric field, directly determines the transmission speed and characteristic impedance of signals in the substrate. Accurately testing this parameter is of fundamental significance to high-frequency circuit design and material research and development.

The physical basis of the test

The relative dielectric constant is usually defined as: the ratio of the capacitance of a capacitor using the material under test as the medium to the capacitance of a capacitor of the same size using vacuum as the medium. This dimensionless constant reflects the material's ability to bind charges under an alternating electric field. In the actual testing context, the dielectric constant often appears together with the dielectric loss tangent value (tanδ), which characterizes the energy dissipation characteristics of the material in an alternating electric field.

Core test method

For the testing needs of ceramic substrates, the industry mainly adopts the following standardized testing principles based on different frequency ranges and sample shapes:

1. Parallel plate capacitance method

This is the most basic testing method in the low frequency band (usually 1MHz to several MHz), and it is also the national standard GB/T Commonly used technical routes in 5594.4-2015. Its principle is based on the parallel plate capacitance formula: the ceramic substrate to be tested is regarded as a dielectric, and is placed between two metal electrodes to form a plate capacitor. Measure the capacitance (Cx) of the capacitor with a precision LCR meter. Under the premise that the effective area of ​​the electrode (S) and the thickness of the sample (d) are known, the dielectric constant is calculated by substituting it into the formula.

However, this method has the limitation of low measurement accuracy for thin substrates in actual operation, and is susceptible to interference from poor electrode contact or surface air gaps. In order to eliminate the influence of stray capacitance, high-precision measurement usually uses the three-electrode method instead of the simple two-electrode method.

2. Resonant cavity method

When the operating frequency rises to the microwave band (GHz), the parallel plate method is difficult to ensure accuracy due to significant edge effects and increased radiation losses. At this time, the resonant cavity method has become the mainstream choice for high-frequency testing. The principle is to place the ceramic substrate in the electromagnetic field induction area of ​​the resonant cavity. The intervention of ceramics will disturb the resonant frequency and quality factor (Q value) of the cavity. By measuring frequency deviation and Q value changes, the relative dielectric constant of the material can be inferred in conjunction with the electromagnetic model. This method is very sensitive to the anisotropy of the thin substrate, and can simultaneously provide dielectric constant and loss data at high frequencies, with significantly better accuracy than the low-frequency extrapolation method.

3. Transmission line method

This method mainly serves microwave circuit design. By making a specific microstrip line or coplanar waveguide structure, the phase delay or characteristic impedance of the transmission line is measured, thereby calculating the effective dielectric constant of the substrate. Since the test structure is highly consistent with the actual circuit application, the design reference value of this method is often higher than that of simple plate capacitance testing.

The test of the relative dielectric constant of ceramic substrates is not a simple reading of a single value, but a systematic project involving the interaction of multiple physical fields (electricity, heat, force). From low-frequency precision measurement based on national standards in the laboratory to high-temperature wide-spectrum analysis that simulates actual working conditions, each testing method serves a specific R&D and quality control stage. Only by selecting appropriate testing methods and rigorous testing procedures can we obtain dielectric data that truly reflects the intrinsic properties of materials and has engineering design value.