1. The dielectric constant (Dk), or relative dielectric constant, of PCB circuit board materials is not a fixed value—despite its name suggesting otherwise.
2. For instance, the Dk of a material can change with frequency.
3. Additionally, using different Dk testing methods on the same material can yield varying Dk values, even if the methods themselves are precise.
4. As circuit board materials are increasingly employed in millimeter wave frequencies, such as in 5G and advanced driving assistance systems, comprehending Dk’s frequency variation and identifying the “appropriate” Dk testing method becomes crucial.
5. While organizations like IEEE and IPC have established committees to address this issue, there is currently no standardized industry method for measuring the Dk of circuit board materials at millimeter wave frequencies.
6. This gap is not due to a lack of measurement techniques; in fact, a reference paper by Chen et al.1 outlines over 80 methods for Dk testing.
7. However, no single method is perfect; each has its own strengths and weaknesses, particularly within the frequency range of 30 to 300 GHz.
8. Circuit test vs. raw material test.
1. There are typically two primary types of testing methods employed to determine the Dk or Df (loss tangent or tanδ) of circuit board materials: raw material measurement and measurement within a circuit made of these materials.
2. Raw material testing relies on high-quality, reliable test fixtures and equipment, allowing Dk and Df values to be derived directly from raw materials.
3. Circuit-based testing often utilizes standard circuits to extract material parameters from circuit performance, such as measuring the center frequency or frequency response of a resonator.
4. Raw material testing methods may introduce uncertainties related to test fixtures or devices, while circuit testing methods encompass uncertainties from circuit design and processing technology.
5. Due to the differences in these two methods, measurement results and accuracy levels often diverge.
6. For instance, the X-band clamped stripline test method defined by IPC serves as a raw material testing approach, and its results may not align with the Dk results from a circuit test using the same material.
7. The clamped stripline raw material testing method involves securing two pieces of material under test (MUT) within a specialized fixture to form a stripline resonator.
8. An air gap exists between the MUT and the thin resonator circuit in the fixture, causing the measured Dk to be lower.
9. When a circuit test is conducted on the same circuit board material, the measured Dk will differ if there is no entrained air.
10. For high-frequency circuit board materials with a Dk tolerance of ±0.050 from raw material tests, the circuit test typically yields a tolerance of approximately ±0.075.
11. Circuit board materials are anisotropic and often exhibit varying Dk values across the three material axes.
12. The Dk value generally shows a minor difference between the x-axis and y-axis; therefore, for most high-frequency materials, Dk anisotropy primarily pertains to the comparison of Dk between the z-axis and the x-y plane.
13. Due to material anisotropy, the measured Dk on the z-axis for the same material under test (MUT) differs from the Dk measured on the xy plane, even though both the test method and measured Dk values are deemed “correct.”
14. The type of circuit employed for circuit testing also influences the Dk value under examination.
15. Typically, two test circuit types are utilized: resonant structures and transmission/reflection structures.
16. Resonant structures often yield narrowband results, while transmission/reflection tests generally provide broadband outcomes.
17. Utilizing a resonant structure tends to be more accurate overall.
2. For instance, the Dk of a material can change with frequency.
3. Additionally, using different Dk testing methods on the same material can yield varying Dk values, even if the methods themselves are precise.
4. As circuit board materials are increasingly employed in millimeter wave frequencies, such as in 5G and advanced driving assistance systems, comprehending Dk’s frequency variation and identifying the “appropriate” Dk testing method becomes crucial.
5. While organizations like IEEE and IPC have established committees to address this issue, there is currently no standardized industry method for measuring the Dk of circuit board materials at millimeter wave frequencies.
6. This gap is not due to a lack of measurement techniques; in fact, a reference paper by Chen et al.1 outlines over 80 methods for Dk testing.
7. However, no single method is perfect; each has its own strengths and weaknesses, particularly within the frequency range of 30 to 300 GHz.
8. Circuit test vs. raw material test.
1. There are typically two primary types of testing methods employed to determine the Dk or Df (loss tangent or tanδ) of circuit board materials: raw material measurement and measurement within a circuit made of these materials.
2. Raw material testing relies on high-quality, reliable test fixtures and equipment, allowing Dk and Df values to be derived directly from raw materials.
3. Circuit-based testing often utilizes standard circuits to extract material parameters from circuit performance, such as measuring the center frequency or frequency response of a resonator.
4. Raw material testing methods may introduce uncertainties related to test fixtures or devices, while circuit testing methods encompass uncertainties from circuit design and processing technology.
5. Due to the differences in these two methods, measurement results and accuracy levels often diverge.
6. For instance, the X-band clamped stripline test method defined by IPC serves as a raw material testing approach, and its results may not align with the Dk results from a circuit test using the same material.
7. The clamped stripline raw material testing method involves securing two pieces of material under test (MUT) within a specialized fixture to form a stripline resonator.
8. An air gap exists between the MUT and the thin resonator circuit in the fixture, causing the measured Dk to be lower.
9. When a circuit test is conducted on the same circuit board material, the measured Dk will differ if there is no entrained air.
10. For high-frequency circuit board materials with a Dk tolerance of ±0.050 from raw material tests, the circuit test typically yields a tolerance of approximately ±0.075.
11. Circuit board materials are anisotropic and often exhibit varying Dk values across the three material axes.
12. The Dk value generally shows a minor difference between the x-axis and y-axis; therefore, for most high-frequency materials, Dk anisotropy primarily pertains to the comparison of Dk between the z-axis and the x-y plane.
13. Due to material anisotropy, the measured Dk on the z-axis for the same material under test (MUT) differs from the Dk measured on the xy plane, even though both the test method and measured Dk values are deemed “correct.”
14. The type of circuit employed for circuit testing also influences the Dk value under examination.
15. Typically, two test circuit types are utilized: resonant structures and transmission/reflection structures.
16. Resonant structures often yield narrowband results, while transmission/reflection tests generally provide broadband outcomes.
17. Utilizing a resonant structure tends to be more accurate overall.
