A brief discussion on key materials of high frequency microwave radio frequency pcb circuit board
The circuit material of a printed circuit board (PCB) is a key building block of radio frequency (RF)/microwave circuits – essentially the starting point for these circuits. PCB materials come in many different forms, and material selection depends largely on the requirements of the intended application. For example, materials that reliably support high-frequency circuits in commercial wireless products can quickly fail when exposed to the extreme conditions of a military environment. A basic understanding of PCB material types and their parameters can help match the material to the application.
As with many RF/microwave components, PCB materials are classified and compared by a number of key parameters, including relative dielectric constant (Dk or εr), dissipation factor (Df), coefficient of thermal expansion (CTE), dielectric thermal coefficient constant ( TCDk) and thermal conductivity. When classifying different PCB materials, many circuit designers start with Dk. The Dk value of a PCB material refers to the capacitance or energy available between a pair of very close conductors fabricated on that material compared to the same pair of conductors in a vacuum.
Vacuum production has a reference value of 1.0, other dielectric materials provide higher reference values. For example, commercial PCB materials typically have Dk values in the range of approximately 2 to 10, depending on how they are measured and how often they are tested. A conductor on a material with a higher Dk value can store more energy than a conductor on a material with a lower Dk value.
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Printed circuit board (PCB)
Use PCB materials with lower Dk values
The Dk value of a PCB material affects the size, wavelength and characteristic impedance of transmission lines fabricated on that material. For example, for a given characteristic impedance and wavelength, a transmission line fabricated on a PCB material with a high Dk value will be much smaller in size than a transmission line fabricated on a PCB material with a low Dk value, although other material parameters may be different. Designers of circuits where losses are a critical performance parameter often prefer PCB materials with lower Dk values because these materials have lower losses than materials with higher Dk values.
In fact, PCB materials can lose signal power in four ways: dielectric loss, conductor loss, leakage loss, and radiation loss, although dielectric loss and conductor loss can be better controlled by selecting the PCB material. For example, the Df parameter provides a way to compare the dielectric loss of different materials, where a lower Df value indicates a material with lower dielectric loss.
For a given transmission line impedance (e.g. 50Ω), a transmission line on a low Dk material will be physically wider than a transmission line on a high Dk material, and wider transmission lines will have less conductor losses. These wider transmission lines also translate into higher manufacturing yields (and production cost savings) compared to narrower transmission lines in higher Dk materials. The trade-off, however, is that they take up more real estate on the PCB, which can be an issue for designs where miniaturization is critical. The thickness of the PCB substrate, especially the thickness of its copper conductor layers, also affects the impedance of the transmission line, with thinner dielectric materials and conductors producing narrower conductor widths to maintain the desired characteristic impedance.
Conductors of PCB materials are usually specified in weights of copper, such as 1 ounce. (35 micron thick) copper or 2 oz. (Thickness 70μm) Copper. The quality of these copper conductors also affects conductor losses. A copper conductor with a rough surface will exhibit higher conductor losses than a copper conductor with a smooth surface profile.
Maintaining the impedance of a transmission line is critical to many RF/microwave circuits, so controlling Dk within a narrow range across the PCB and varying with temperature is critical to achieving tight impedance in the design. Most PCB datasheets show the Dk of the material and its Dk tolerance, such as ±0.5.
Another important material parameter, TCDk, provides detailed information on how much the PCB material’s Dk changes over the operating temperature range, as this also affects the impedance of the transmission line. A TCDk value of 150 ppm/°C may be considered high, while a TCDk value of 30 ppm/°C or less is considered low. For circuits that must maintain impedance over a wide operating temperature range, it is best to use PCB materials with lower TCDk values.
In addition to temperature changes affecting Dk and impedance, they can also have a mechanical impact on the PCB. The CTE of a PCB is a paramete r that attempts to show the effect of temperature on the PCB material. Essentially, it is a measure of a material’s expansion/contraction with temperature, with lower values being the goal. For example, materials such as polytetrafluoroethylene (PTFE) have long been used in high-frequency PCBs due to their excellent electrical properties, although pure PTFE has a high CTE (approximately 300 ppm/°C).
Some PCB material manufacturers such as PTFE are used in their materials but add different filler materials to lower the CTE value. It is important to note that the CTE of the PCB dielectric material should closely match the CTE of its conductors and other layers to make Mechanical effects of temperature changes are minimized.
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PTFE composite RT/duroid 6035HTC
This ceramic-filled PTFE composite RT/duroid 6035HTC offers high thermal conductivity for high-power circuit applications
For any commercial PCB material, there are usually separate CTE values listed for all three axes (x, y, and z). CTE provides some evidence on how PCB materials handle extreme temperatures, such as during soldering. For example, mismatched CTE values of materials used in multilayer structures can lead to reliability issues because the dimensions of different circuit layers change with temperature. PCB materials with lower CTE values are generally considered to be more thermally stable than materials with higher CTE values. Circuit materials with a CTE of 70 ppm/°C are considered fairly robust in terms of use over a wide temperature range and should be able to handle the temperature extremes of circuit manufacturing and assembly.
The CTE of the PCB material should closely match the CTE of copper in the x- and y-axes to minimize mechanical stress changes with temperature. Additionally, CTE in the z-axis of the circuit material can provide insight into the expected reliability of plated through holes (PTH) that will be formed through the dielectric material since these drill holes are plated with copper. Ideally, the dielectric material and copper would expand and contract with temperature in a similar manner to achieve high reliability of the PTH.
Heat dissipation in RF/microwave circuits (especially for high-power designs) is an important function characterized by the thermal conductivity of the PCB. Although a standard PCB material may have a thermal conductivity of 0.25 W/m/K, fillers are often added to the PCB material to increase the thermal conductivity to a more favorable value (and better ability to dissipate heat). For example, RO4350B is a hydrocarbon/ceramic PCB material from Rogers Corporation that has long been a reliable building block material for high-frequency applications including automotive and cellular communications systems.
RO4350B is not PTFE based, but has a relatively low z-axis Dk of 3.48±0.05 at 10 GHz, a TCDk of +50 pm/°C, and a dissipation coefficient of 0.0037. It has a reasonably good thermal conductivity of 0.69 W/m/K. In contrast, RT/duroid 6035HTC, also from Rogers Corporation, is a ceramic-filled PTFE composite specifically formulated for high-power, high-frequency circuits with a Dk of 3.50±0.05 and a TCDk of +50 ppm/°C. , and has a low loss factor of 0.0013. It has excellent thermal conductivity, with a typical value of 1.44W/m/K.
RF/Microwave PCB
There is a wide variety of materials used for RF/microwave PCBs, from low-cost FR-4 materials to expensive PTFE-based materials. Circuit boards composed of FR-4 materials are essentially laminates of glass-reinforced epoxy, while PTFE materials are typically reinforced with fiberglass or ceramic filler materials (although pure PTFE-based PCBs are also used). The difference in performance between these two extreme materials points to the trade-offs that must be made between cost and performance of PCB materials and between the ease of processing of FR-4 and the processing difficulty of PTFE materials.
Excellent circuit performance often comes at a high price, although many PCB material suppliers have put significant effort into developing a variety of composites with varying Dk values for use in various RF/microwave applications.
