ROG Blog - PCB materials related topics

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Circuit materials are evaluated by a number of different parameters, including dielectric constant (Dk) and dissipation factor (Df). Those two parameters also have temperature-based variants that provide insight into the expected behavior of a circuit material with changes in temperature, notably the thermal coefficient of dielectric constant (TCDk) and the thermal coefficient of dissipation factor (TCDf). 

Specifying the optimum circuit laminate for an electronic design project is often a case of reviewing the numbers. Once all these numbers have been analyzed and reviewed, the field of PCB material choices narrows until, finally, the optimum circuit material for a project can be selected. But what happens next?

Two types of composite materials—thermoplastic and thermoset materials—are commonly used for the dielectric layers in PCBs or as adhesives in manufacturing circuit laminates and they each have their own traits and characteristics. But how do they differ? What are the strengths and weaknesses of each type of material and why choose one over the other for an application?

The dimensions of high-frequency circuit structures, including different types of transmission lines and the spacing between lines for proper isolation and/or coupling, are determined by the circuit material’s dielectric characteristics, and one of the main parameters for understanding those characteristics is Dk. Many designers grow to trust that the Dk value assigned to a given circuit material is truly accurate and consistent from board to board and base their designs on that trust. But how does the material supplier determine a circuit material’s Dk value anyway?

For many circuit designers, plated through holes (PTHs) form pathways, from one circuit plane to another. The key to making PTHs work for the benefit of a circuit design is to understand their effects on electrical performance, especially at higher frequencies. They should be considered as circuit elements, and they can have a great deal to do with a number of analog circuit transmission-line performance parameters, including insertion loss and return loss, and they can also affect high-speed digital circuit performance by degrading signal integrity (SI) and bit-error-rate (BER) performance.

The choice of plated finish can make a real difference in a PCB’s conductive loss, especially for broadband, high-frequency circuits. To better understand the loss performance of different plated finishes, various transmission lines were fabricated on different circuit laminates and different plated finishes applied.