by Alexander Ippich, Technical Director, Signal Integrity & Advanced Technology, Product Manager, RF/Microwave Isola
Electronic circuits are rapidly consuming bandwidth, from short-range remote controls and smart phones through satellites and automotive safety systems. Even much heralded Fifth Generation (5G) cellular wireless communications networks are goggling up bandwidth, reaching beyond the frequency range of earlier cellular generations into the millimeter-wave (mmWave) frequency range to provide fast downloadable video and high-speed data in addition to voice services. Orchestrating the “magic” of 5G wireless networks requires circuits capable of operating at mmWave frequencies and doing so with as little loss as possible since signal power becomes scarce as signal frequencies climb higher. The choice of circuit materials for the printed circuit boards (PCBs) that enable not only 5G small cells but many emerging mmWave applications is critical since those materials must provide outstanding, reliable performance and do it at reasonable cost—for the materials and the processes to form the PCBs—to make mmWave systems practical and a part of everyday lives.
Circuit materials for PCBs at lower frequencies are often taken for granted, including for VHF/UHF circuits which once bore the bulk of broadcast signals. However, as the world trends towards personal communications carried by wireless communications systems, PCBs and their circuit materials must handle higher-frequency signals with their smaller wavelength, for communications and many other tasks, including information gathering and command and control. As wires are replaced by wireless links, frequency bands are becoming congested and applications, including 5G networks, are seeking additional bandwidth at mmWave frequencies. But for PCBs to perform effectively above 24 GHz in applications such as indoor and outdoor 5G small cells, they require circuit materials that provide a combination of features that help conserve as much signal power as possible through a mmWave PCB.
Signal losses occur through a PCB because of its materials and how they are treated. What is typically regarded as the insertion loss of a transmission line on a circuit material is actual a combination of losses: conductor loss, dielectric loss, radiation loss, and losses caused by reflection. The amount of copper used to form the thickness and width of the circuit traces as well as the roughness or smoothness of the copper are factors in conductor loss. The dielectric circuit material, along with any fillers such as ceramic or glass fiber, determine the dielectric loss of a circuit material. Radiation loss tends to increase with increasing frequency and can be a factor at mmWave frequencies. Leakage loss is typically higher in circuit materials with higher moisture absorption. Because mmWave transmission lines suffer multiple losses even on the highest-quality circuit materials, selecting circuit materials for minimum loss at mmWave frequencies requires careful consideration of numerous material characteristics including permittivity or dielectric constant (Dk), dissipation factor (Df). and coefficient of thermal expansion (CTE).
Often, the circuit materials supporting mmWave signal transfers are part of a multilayer hybrid circuit assembly with additional materials and circuits providing lower-frequency analog, digital, and power connections that combine to form a system or subsystem on a PCB. For such multilayer hybrid circuit assemblies, the processes applied to each material to form its circuits contribute to total manufacturing cost, with manufacturing/processing compatibility highly favored to simplify manufacturing and lower production costs for multilayer hybrid circuits
Sorting Through Materials
As PCBs climb higher in frequency, two main circuit material properties to consider are Dk and Df and finding materials with the lowest values possible is necessary. Available signal power tends to decrease with increasing frequency and circuit materials with low Dk and Df can help minimize insertion loss and conserve valuable signal strength through a system’s PCBs, whether for active or passive circuitry and for mmWave signals as well as HSD signals. When loss is minimized in the multilayer PCBs of a 5G small cell, network coverage will improve, and multiple users will benefit. 5G small cells are being built in many shapes, sizes, and power levels, with high circuit densities and many functions in a small package. The small cells will be installed indoors and outdoors, filling gaps in coverage left by larger 5G towers and cell sites, typically in metropolitan and heavily populated areas.
Low Dk allows the use of wider copper signal traces for a given thickness of circuit material, where wider conductors contribute to lower insertion loss than narrower conductors. Conductors on circuit materials with high Dk will also exhibit slower propagation speeds than conductors with the same metal and dimensions on circuit materials with lower Dk, encouraging the use of circuit materials with the lowest possible Dk for mmWave and HSD signals. To avoid variations in performance, a circuit material’s Dk should be consistent as possible across the material and across operating frequencies and temperatures. For the small wavelengths at mmWave frequencies, even slight variations in circuit material thickness, copper conductor thickness, and copper conductor surface roughness can result in performance variations in the amplitude and phase of mmWave signals and the timing of HSD signals.
What is a low Dk value for a circuit material containing mmWave circuitry? There is no absolute value, and the requirements of an application should play a part in determining a suitable Dk value for a mmWave circuit’s substrate material. For example, for 5G small cells operating in the 28-GHz range, a circuit material such as I-Tera® MT40 (RF/MW) from Isola Group (www.isola-groupcom) has a relatively low Dk of 3.45 measured at 2 and 10 GHz through the z-axis (thickness) of the material that remains stable with frequency and temperature.
For higher mmWave frequencies, materials with lower Dk enable circuits with wider conductors and lower losses, depending on circuit density requirements. The same company’s TerraGreen® 400G has a Dk of 3.05 through the z-axis at both 2 and 10 GHz. The halogen-free circuit material is well suited for environmentally sensitive installations, such as indoor 5G small cells in shopping centers. Another circuit material well suited for mmWave applications, Isola’s Tachyon® 100G, drops the Dk to 3.02 at 10 GHz. When a circuit material with lower Dk is desired, Isola’s Astra® MT77 features a Dk of 3.00 which maintains excellent consistency with frequency and temperature.
Low Df is an important characteristic for any circuit material supporting PCBs at mmWave frequencies or HSD circuits where circuit losses can mean lost information. A circuit material’s Df, also known as its loss tangent, indicates energy lost by the dielectric material during a reversal in electrical polarization. Circuit materials can be characterized for Df using different approaches based on different transmission-line formats, such as stripline. A zero value of Df would denote no loss, although all circuit materials suffer some amount of loss with the lowest possible values to be preferred, especially for circuits operating at mmWave frequencies.
For example, I-Tera® MT40 (RF/MW) circuit material is widely used for RF and microwave circuits as well as for HSD circuits because of its low-loss properties. Its Df typically ranges from 0.0028 to 0.0035, depending upon which Dk value version of the material is chosen. Its Df remains consistent with frequency and over temperatures from -40 to +140°C. Because it is process compatible with the FR-4 material widely used in multifunction, multilayer electronic circuit assemblies, I-Tera® MT40 (RF/MW) also supports efficient manufacturing approaches for hybrid multilayer PCBs handling a variety of signal types.
While such Df values are considered low and a starting point for low-loss circuits, a circuit designer in quest of low-loss designs at mmWave frequencies would jump at the chance to use a circuit material with a Df value that is one-half that of I-Tera® MT40. For mmWave frequencies, where circuits must conserve as much signal strength as possible, Isola’s Astra® MT77 circuit laminates and prepreg materials (without the copper cladding)
exhibit a Df value of 0.0017 at both 2 and 10 GHz and consistent with frequency and operating temperatures from -40 to +140°C. Like I-Tera® MT40 (RF/MW), Astra® MT77 is process compatible with FR-4 circuit materials to simplify manufacturing of multilayer PCBs that include mmWave circuit layers.
Other parameters are worth considering when selecting a circuit material for mmWave applications such as 5G small cells, with smaller values usually to be preferred. For example, because of the temperature swings that an outdoor 5G small cell must often endure, a circuit material with low CTE ensures good mechanical stability with temperature. Also, materials with as low as possible moisture absorption guard against performance variations that can occur when a circuit material absorbs water from rainfall or high-humidity environments. The one circuit material parameter for which higher values are to be preferred is glass transition temperature (Tg), which denotes the temperature above which a material begins to change mechanical stability. A circuit material with high Tg has better heat resistance and typically lower moisture absorption than a material with lower Tg.
High accuracy should be assumed when comparing values of key parameters for potential mmWave circuit materials. Many circuit material parameters such as Dk and Df require specialized measurements and test equipment to ensure high accuracy. Some circuit material suppliers, such as Isola, perform measurements of circuit material parameters as a service, making it possible to understanding more about a single material or a combination of materials, such as Astra® MT77 and Tachyon® 100G or Astra® MT77 and I-Tera® MT40 (RF/MW) in hybrid DC/RF/mmWave multilayer circuits. (More details on those test services will be available in a separate article.)
At mmWave frequencies and their small wavelengths, smaller is better, when trying to pack more functions into a multilayer PCB assembly or sorting through the parameters of the circuit materials for those multilayer PCBs. Bandwidth is needed and mmWave spectrum provides much-needed frequency range—with the right circuit materials.
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