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Heat can be damaging. Printed circuit board (PCB) materials are formulated to withstand a certain amount of heat, but when the temperatures rise beyond certain limits, circuit performance can suffer, especially at higher frequencies. Heat-tolerant PCB materials and carefully considered circuit designs can tolerate a certain amount of heat, if a circuit designer is aware of the various parameters that best describe a circuit material’s behavior when temperatures rise. 

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?

Not all circuit materials are created equal, and some have more mechanical flexibility than others and can survive a certain amount of bending and flexing without damage. Understanding what makes a circuit material capable of bending and flexing, and what happens to it when it is bent or flexed, helps when specifying circuit materials for such uses. 

Multilayer circuits provide the means for achieving high circuit densities in small volumes. The penalty for the compact circuit sizes can be the challenge of dissipating heat from active devices, such as power transistors. There is a way to manage the heat without adding size, by distributing the heat throughout the multilayer circuit board, using a low-cost circuit material designed for that purpose, 92ML™ thermally conductive epoxy prepreg material.

Solder mask is an often-overlooked component of an RF/microwave printed-circuit board (PCB). It provides protection for a circuit but can also have an effect on final performance, especially at higher frequencies. Solder mask may not always be used in RF/microwave circuits but, when it is part of a circuit, it should be accounted for electrically as well, for the most accurate modeling and simulation. Knowing more about the material properties of solder mask can help boost an understanding of how these circuit layers can impact performance at RF/microwave frequencies.

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?

Thin can be a good thing for high-frequency circuit laminate materials. As this ROG Blog detailed some years ago (see “Thinner Materials Help Target Higher Frequencies,” http://mwexpert.typepad.com/rog_blog/2010/11/thinner-materials-help-target-higher-frequencies.html), thinner printed-circuit-board (PCB) laminates offer many electrical benefits as well as mechanical advantages compared to thicker circuit materials, especially at higher frequencies reaching into millimeter-wave bands.

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.

Microwave circuit dimensions are related to their wavelengths/frequencies and to the dielectric constant (Dk) of their substrates. Quite simply, higher-frequency signals have smaller wavelengths and their electromagnetic (EM) energy of those smaller wavelengths will propagate through circuits with smaller dimensions. Phase velocity is related to wavelength, with slower EM waves having shorter wavelengths which propagate through circuit structures with smaller dimensions than faster waves with their longer wavelengths. 

Millimeter-wave circuits were once considered exotic and only used for specialized applications, typically in the military space. For one thing, frequencies with such small wavelengths, from about 30 to 300 GHz, required special components and circuits scaled to those diminutive wavelengths. But lower-frequency bands are being consumed by a growing number of wireless applications, and millimeter-wave frequency bands are looking more and more attractive for communications systems of the future. Achieving millimeter-wave circuit designs on reliable printed-circuit-board (PCB) materials in a practical manner will be the challenge in making these higher frequencies affordable. Substrate-integrated-waveguide (SIW) circuit technology may just be the solution.