There are many different types of PCB designs and constructions that can be used as test vehicles to evaluate electrical properties. If the test vehicle will be used for an evaluation that is specific to an application, it should be as similar as possible to the application. That may sound like common sense, but I have seen many companies use the same test vehicle for every evaluation they perform. However, if an evaluation is being done that is not specific to an application, but for general comparisons— such as evaluating the electrical performance of different circuit materials—there are multiple things to consider for the test vehicle. A proper test vehicle to compare different circuit materials would be a design and construction that takes into consideration the different material properties and have the least amount of PCB fabrication variables that can impact the results.
It is obviously desirable for a company to show it has well-defined and very consistent testing procedures for its electrical evaluation program used to evaluate different circuit materials. To do this will probably cause the evaluation procedures to be more complicated, but, in the end, the results will be more meaningful for the materials being evaluated. For example, it probably makes sense to have a different set of evaluation procedures for materials which are significantly different, for example, FR-4 materials would have a different evaluation procedure than high-frequency materials.
For evaluating high-frequency materials, there are many different test vehicles used. Some of the more common test vehicles are ring resonators, transmission lines, and delay lines. The circuit construction should also be a consideration; often, the ring resonators are used as a microstrip. The transmission lines and delay lines are also commonly microstrip, but sometimes, they are a stripline construction. If the evaluation is purely electrical performance related to the material only, the simplest construction and PCB fabrication process should be used. If the evaluation is a combination of electrical performance and how the materials are used in the PCB fabrication process, the constructions will probably need to be more complex and designed to be specific to the process concerns of the PCB fabricator.
To read this entire column, which appeared in the December issue of Design007 Magazine, click here.