The circuit materials of printed circuit boards (PCBs) are key building blocks for radio frequency (RF)/microwave circuits – essentially the starting point of these circuits. PCB materials come in many different forms, and the selection of materials largely depends on the requirements of the intended application. For example, when entering extreme situations in military environments, materials that reliably support high-frequency circuits in commercial wireless products may quickly fail. A basic understanding of PCB material types and their parameters can help match materials with applications.
Like many RF/microwave components, PCB materials are classified and compared through many key parameters, including relative dielectric constant (Dk or ε r) , dissipation factor (Df), coefficient of thermal expansion (CTE), thermal coefficient constant of dielectric (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 manufactured on the material compared to the same pair of conductors in vacuum.
The reference value generated by vacuum is 1.0, while other dielectric materials provide higher reference values. For example, the Dk value of commercial PCB materials typically ranges from approximately 2 to 10, depending on their measurement method and testing frequency. Conductors on materials with higher Dk values can store more energy than conductors on materials with lower Dk values.
Printed Circuit Board (PCB)
The Dk value of PCB material will affect the size, wavelength and characteristic impedance of transmission lines manufactured on the material. For example, for a given characteristic impedance and wavelength, the size of the transmission line manufactured on a PCB material with a high Dk value will be much smaller than the size of the transmission line manufactured on a PCB material with a low Dk value, although other material parameters may be different. Designers of circuits with loss as a key performance parameter usually tend to use PCB materials with lower Dk values, as these materials have lower losses than materials with higher Dk values.
In fact, PCB materials can lose signal power through four methods: dielectric loss, conductor loss, leakage loss, and radiation loss, although selecting PCB materials can better control dielectric loss and conductor loss. For example, the Df parameter provides a method of comparing the dielectric losses of different materials, where lower Df values represent materials with lower dielectric losses.
For a given transmission line impedance (such as 50 Ω), the transmission line on a low Dk material will be physically wider than the transmission line on a high Dk material, and the conductor loss of a wider transmission line will also be smaller. Compared to narrower transmission lines of higher Dk materials, these wider transmission lines can also be converted into higher manufacturing yield (and save production costs). However, the trade-off is that they occupy a larger area on the PCB, which may be a problem for designs that are crucial for miniaturization. The thickness of the PCB substrate, especially the thickness of its copper conductor layer, will also affect the impedance of the transmission line. Thinner dielectric materials and conductors will produce narrower conductor widths to maintain the required characteristic impedance.
The conductor of PCB material is usually specified by the weight of copper, such as 1 ounce. (35 microns thick) copper or 2 ounces. (Thickness is 70 μ m) Copper. The quality of these copper conductors also affects conductor losses. Copper conductors with rough surfaces will exhibit higher conductor losses than copper conductors with smooth surface contours.
Maintaining the impedance of the transmission line is crucial for many RF/microwave circuits, so controlling Dk within a narrow range of the entire PCB and changing with temperature is crucial for achieving strict impedance in design. Most PCB datasheets display the material’s Dk and its Dk tolerance, such as ± 0.5.
Another important material parameter, TCDk, provides detailed information on how much Dk of PCB materials changes within 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 lower is considered very 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 that affect Dk and impedance, they also have mechanical effects on PCBs. The CTE of a PCB is a parameter that attempts to display the effect of temperature on the PCB material. Essentially, it is a measure of the expansion/contraction of a material with temperature, with lower values being the target. For example, although pure PTFE has a high CTE (approximately 300 ppm/° C), materials such as polytetrafluoroethylene (PTFE) have long been used for high-frequency PCBs due to their excellent electrical properties.
Some PCB material manufacturers, for example, use PTFE in their materials but add different filling materials to reduce CTE values. It is worth noting that the CTE of PCB dielectric materials should be closely matched with the CTE of their conductors and other layers to minimize the mechanical effects of temperature changes.
PTFE composite material RT/duroid 6035HTC
This ceramic filled PTFE composite material RT/duroid 6035HTC has high thermal conductivity and can be used for high-power circuit applications
For any commercial PCB material, separate CTE values are typically listed for all three axes (x, y, and z). CTE provides some evidence on how PCB materials handle extreme temperatures, such as during the welding process. For example, mismatched CTE values of materials used in multi-layer structures can lead to reliability issues, as the size of different circuit layers varies with temperature. It is generally believed that PCB materials with lower CTE values have stronger thermal stability than materials with higher CTE values. For use over a wide temperature range, circuit materials with a CTE of 70 ppm/° C are considered quite robust and should be able to withstand the extreme temperatures of circuit manufacturing and assembly.
The CTE of PCB materials should closely match the CTE of copper on the x and y axes to minimize the variation of mechanical stress with temperature. In addition, the CTE on the z-axis of the circuit material can provide insight into the expected reliability of the plated through holes (PTH) to be formed through the dielectric material, as these holes are all copper plated. Ideally, dielectric materials and copper will expand and contract in a similar manner with temperature to achieve high reliability of PTH.
The heat dissipation of RF/microwave circuits (especially for high-power design) is an important function, characterized by the thermal conductivity of PCBs. Although the thermal conductivity of standard PCB materials may be 0.25 W/m/K, fillers are usually added to the PCB material to increase the thermal conductivity to a more favorable value (and better heat dissipation capacity). For example, RO4350B is a hydrocarbon/ceramic PCB material from Rogers Corporation and has long been a reliable building block material for high-frequency applications, including automotive and cellular communication systems.
RO4350B is not based on PTFE, but the Dk of the z-axis is relatively low at 10 GHz, at 3.48 ± 0.05, with a TCDk of+50 pm/° C and a dissipation coefficient of 0.0037. It has a reasonable good thermal conductivity of 0.69 W/m/K. On the contrary, RT/duroid 6035HTC, also from Rogers, is a ceramic filled PTFE composite material specially 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
From low-cost FR-4 materials to expensive PTFE based materials, there are a wide variety of materials used for RF/microwave PCBs. A circuit board composed of FR-4 material is essentially a glass reinforced epoxy resin laminate, while PTFE material is usually reinforced with glass fiber or ceramic filling material (although pure PTFE based PCBs are also used). The difference in performance between these two extreme materials indicates the trade-off that PCB materials must make between cost and performance, as well as between the ease of processing of FR-4 and the difficulty of processing PTFE materials.
Excellent circuit performance usually comes at a high cost, although many PCB material suppliers have invested significant effort in developing various composite materials with different Dk values for various RF/microwave applications.