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Another important material parameter, TCDk, provides details on how much a PCB material’s Dk will change over an operating temperature range, as that can also impact the impedance of the transmission lines. A TCDk value of 150 ppm/°C might be considered high, while a value of 30 ppm/°C or less is considered low. For circuits in which impedance must be maintained over a wide range of operating temperatures, PCB materials with lower values of TCDk are to be preferred.

In addition to changes in temperature affecting Dk and impedance, they can also have mechanical effects on a PCB; a PCB’s CTE is a parameter that attempts to show some of the impact of temperature on a PCB material. Essentially, it is a measure of the expansion/contraction of a material with temperature, with lower numbers a target. For example, a material such as polytetrafluroethylene (PTFE) has long been used for high-frequency PCBs because of its excellent electrical characteristics, although pure PTFE has a high CTE (about 300 ppm/°C).

Some PCB material manufacturers, such as Rogers Corp., use PTFE in their materials but add different filler materials to reduce the CTE value (see photo). A concern is that the CTE of a PCB’s dielectric material should be closely matched to the CTE of its conductors and other layers, so that the mechanical effects of changes in temperature are minimized.

Rogers Corps.'s RT/duroid 6035HTC

For any commercial PCB material, separate CTE values are usually listed for all three axes (x, y, and z). CTE provides some evidence as to how PCB materials will handle temperature extremes, such as during soldering operations. For example, mismatches in CTE values for materials used in multilayer constructions can lead to reliability problems as dimensional changes take place with temperature for the different circuit layers. PCB materials with lower CTE values are generally considered more thermally robust than materials with higher CTE values. A circuit material with CTE of 70 ppm/°C is considered fairly robust in terms of use over a wide temperature range and should be capable of handling the temperature extremes of circuit fabrication and assembly.

The CTE of a PCB material should be closely matched to that of copper in the x and y axes to minimize mechanical stress with temperature. In addition, the CTE in a circuit material’s z-axis provides some insight into the expected reliability of plated through holes (PTHs) that will be formed through the dielectric material, since these drilled holes are plated with copper. Ideally, the dielectric material and the copper will expand and contract with temperature in similar fashions for high reliability of the PTHs. 

Dissipating heat from RF/microwave circuits, especially for high-power designs, is an important capability that is characterized by a PCB’s thermal conductivity. While standard PCB materials may have thermal conductivity of 0.25 W/m/K, fillers are often added to PCB materials to boost the thermal conductivity to more favorable values (and better capabilities for dissipating heat). As an example, RO4350B is a hydrocarbon/ceramic PCB material from Rogers Corp. that has long been a reliable building-block material for high-frequency applications, including in automotive and cellular-communications systems.

RO4350B is not based on PTFE, but exhibits a relatively low Dk of 3.48 ± 0.05 in the z-axis at 10 GHz with a TCDk of +50 pm/°C and dissipation factor of 0.0037. It features reasonably good thermal conductivity of 0.69 W/m/K. In contrast, RT/duroid 6035HTC, also from Rogers Corp., is a ceramic-filled PTFE composite material formulated especially for high-power, high-frequency circuits, with a Dk of 3.50 ± 0.05, TCDk of +50 ppm/°C, and low dissipation factor of 0.0013. It offers outstanding thermal conductivity, with typical value of 1.44W/m/K.

Numerous types of materials are used for RF/microwave PCBs, from low-cost FR-4 materials to pricey PTFE-based materials. Circuit boards comprised of FR-4 material are essentially laminated sheets of glass-reinforced epoxy, while PTFE materials are often reinforced with woven-glass or ceramic filler materials (although pure PTFE-based PCBs are also used). The differences in performance between these two material extremes point out the essential tradeoffs in PCB materials, between cost and performance, and also between the ease of processing for FR-4 versus the difficulties in processing for PTFE materials.

Excellent circuit performance usually comes at a high price, although numerous suppliers of PCB materials have put great effort into developing a variety of composite materials at different Dk values for a wide range of RF/microwave applications.

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