Rogers: select test carriers to evaluate the electrical properties of circuit materials

There are many different types of PCB designs and structures that can be used as test carriers to evaluate electrical performance. If the test vehicle is to be used for an application-specific assessment, it should be as similar as possible to that application. This sounds like common sense, but I see many companies using the same test vehicle for every evaluation. However, if the assessment is made not for a particular application but for general comparisons, such as evaluating the electrical properties of different circuit materials, the test vehicle needs to take into account a number of factors. In order to compare different circuit materials, the appropriate test vehicle should be one that takes into account the design and structure of different material characteristics and has the minimum number of PCB manufacturing variables that can affect the results.

Clearly, a company needs to demonstrate that its electrical evaluation procedures for evaluating different circuit materials have well-defined and very consistent test procedures. This may complicate the evaluation process, but the final test results are more meaningful for the material being evaluated. For example, it might make sense to have a different evaluation procedure for significantly different materials, such as fr-4 materials, which have a different evaluation procedure than high-frequency materials.

To evaluate high-frequency materials, there are many different test carriers available. Some common test carriers are ring resonators, transmission lines, and delay lines. In addition, the circuit structure should also be considered. In general, ring resonators are used as microstrip lines, and transmission lines and delay lines are usually microstrip lines, but sometimes they are banded structures. If the evaluation is solely related to the electrical properties of the material, the simplest construction and PCB manufacturing process should be used. If the assessment involves electrical properties and the way materials are used in the PCB manufacturing process, the structure may need to be more complex and designed to address the PCB board manufacturer's process issues.

The evaluation circuit used for the electrical characterization of materials should be simple in structure and design. It is strongly recommended to use lower frequencies and thicker substrates for material evaluation. This minimizes variables that can reduce the accuracy of electrical assessments. Basically, it's hard to get accurate results at high frequencies. In addition, the effect of etching control has a greater impact on electrical results when thinner materials are used. In general, when a thinner circuit is used, a narrower conductor can maintain a controlled impedance, and changes in conductor width due to etching control will have a greater impact on the electrical performance of the circuit, while a thicker substrate with a wider conductor will not have a greater impact.

In general, I don't favor ring resonators, but ring resonators are a good test vehicle for lower frequencies and thicker substrates. The ring resonator shall be based on a loosely coupled double-sided circuit (microstrip). Low order nodes should be tested at frequencies less than 6 GHz.At the same time, it is also a very good test carrier for double-sided, non-coated through hole (PTH) bare copper circuit, because the electrical change is minimal.

I have seen many companies use strip-line circuits on test carriers. Due to the nature of this structure, these circuits are more susceptible to PCB manufacturing variables than the simple test carrier and structure (microstrip) I recommend. In the case of banded lines, the core material and semi-cured sheet are usually used to make the banded line structure. The electrical properties of these materials are often different and may confuse the electrical results of the assessment. In addition, due to the circuit manufacturing, the thickness control of the semi-cured sheet layer will have a significant impact on the electrical performance of the circuit. The PTH through-hole used to connect the stripline to the ground plane above and below also affects the electrical performance of the circuit. If a stripline is required for the test carrier, a thicker test carrier at a lower frequency can also avoid some inaccuracies in the electrical evaluation.

The simple guidelines I have given for test carriers and structures are used only for material evaluation because they compare only different materials on the same test carrier. Thinner materials, higher frequencies, complex multi-layer PCB structures, etc.In this case, the designer needs to consider the impact of the PCB manufacturing process on the material properties.

The use of PTH technology means that the final conductor thickness will have some normal variations. For gap-coupled ring resonators, changes in copper thickness can affect electrical results and lead to inaccuracies in determining the Dk or Df values associated with the circuit material. Some test carriers are more sensitive to changes in copper thickness than others. As previously mentioned, the gap coupled ring resonator is affected by normal changes in copper plating thickness, as is the grounded coplanar waveguide (GCPW). However, the electrical properties of microstrip transmission line circuits are much less affected by changes in copper thickness. In addition, the surface coating and its normal thickness will also affect the accuracy of the test carrier.

The basic requirement for engineers who define test carriers (circuit design and structure) is to fully and carefully consider how normal PCB manufacturing variables affect the test carriers and thus the electrical evaluation accuracy.