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Probing Materials for Millimeter-Wave PCBs

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by Matthew Lake, Isola

Matthew Lake

Analog frequencies and digital data rates are increasing steadily in electronic circuits and systems, requiring careful choices of circuit materials for printed circuit boards (PCBs). Cellular communications technologies grew by means of RF and microwave signal frequencies, but Fifth Generation (5G) cellular wireless networks are making use of available bandwidth well into the millimeter-wave (mmWave) frequency range. And other commercial applications, such as Advanced Driver Assistance System (ADAS) with collision-avoidance radars to 77 GHz, are extending the requirements of circuit materials to support mmWave analog circuits and high-speed-digital (HSD) circuits, while also attempting to fit them into smaller assemblies. Selecting circuit materials for mmWave and HSD circuits requires an understanding how essential circuit material parameters relate to performance at higher frequencies but also often weighing the tradeoffs that can provide the best possible blend of performance, price, and reliability. Often, it is a combination of materials, especially for mixed-signal, multilayer circuits, which provides an optimum foundation for mmWave/HSD circuits.

Circuit designers select circuit materials according to numerous characteristics, including dielectric constant (Dk) and dissipation factor (Df), depending upon analog operating frequencies, digital data rates, and application conditions. At higher frequencies and faster digital data rates, circuit materials with low Dk and Df values typically provide behavior needed to achieve minimum performance levels. A substrate’s Dk determines a transmission line’s physical dimensions to maintain a specific impedance, such as 50 Ω. Circuit materials with higher Dk values will require much smaller transmission line dimensions for the same characteristic transmission-line impedance. For practical dimensions with transmission lines at mmWave frequencies at 50 Ω, circuit laminates and prepregs typically have Dk values between 2 and 4 when measured in the z-axis or thickness of the material by means of several standardized Dk measurement methods.

As important as the value of the Dk is its consistency across a panel of circuit material, since circuit materials with Dk variations will result in impedance variations for transmission line fabricated on those circuit materials. Similarly, low values of Df make transmission lines with low loss, which can be critical at higher frequencies where signal power levels are limited. But variations in Df value across a panel of circuit material result in variations in the loss performance of circuits fabricated on that circuit material so tight tolerances of Df value are as important as the Df values themselves, especially at mmWave frequencies where small variations can cause significant deviations in amplitude and phase for the small signal wavelengths of those higher frequencies. Mechanical circuit material parameters, such as the surface roughness of the copper film on a circuit laminate or deviations in the z-axis or thickness of a circuit laminate or prepreg, impact circuit performance consistency especially at the smaller wavelengths of mmWave frequencies. In general, the tightest electrical and mechanical circuit material parameters available represent essential characteristics for circuit materials supporting HSD and mmWave analog circuits.

Mix and Match

Selecting circuit materials for HSD and mmWave circuits involves tradeoffs since the highest circuit performance usually requires the most expensive circuit materials. Circuits for military and aerospace mmWave applications traditionally have employed extremely low-loss circuit materials, although fabrication and production of these materials has required tight control at great cost. Commercial mmWave applications tend to be more competitive, requiring more cost-effective circuit materials although while still providing excellent electrical, mechanical, and environmental performance at higher frequencies.

Polytetrafluoroethylene (PTFE) is an example of one of the more expensive circuit materials with low Dk and Df and very good mmWave circuit performance. Its material costs are compounded by expensive, complex processing costs to manufacture PTFE into PCBs. Fortunately, Astra® MT77 ultra low loss laminates and prepreg circuit materials from Isola-Group (www.isola-group.com) offer many of the characteristics needed for HSD and mmWave circuits as a cost-effective alternative. They feature low Dk of 3.00 at 10 GHz with an also low Df of 0.0017 at 10 GHz. With a Dk that is stable across a wide temperature range of -40 to +140°C and low moisture absorption of 0.1%, these materials are well suited for the rugged environmental conditions expected for many mmWave applications, such as in 5G network equipment and in ADAS mmWave radars and even for ADAS Light Detection and Ranging (LiDAR) sensors.

Astra® MT77 circuit materials are less complex to process and manufacture than PTFE. In contrast, they are compatible with processes used for producing circuits from low-cost FR-4 circuit materials. The compatibility serves well for multilayer circuit modules in which lower-frequency FR-4 circuits are combined with higher-frequency Astra® MT77 circuits in a compact module. In addition, Astra® MT77 circuit materials are RoHS- or lead-free compliant in support of environmental safety issues. They can be supplied in a range of thicknesses, with laminates available with copper weights from 0.5 to 2.0 oz. to accommodate the fine circuit structures required for the small wavelengths of mmWave signal frequencies.

When such low Df loss is not critical, I-Tera® MT40 and I-Tera® MT40 (RF/MW) laminates and prepregs, both also from Isola-Group, perform reliably in HSD and mmWave PCBs. I-Tera® MT40 laminates and prepregs feature a typical Dk of 3.45 at 10 GHz for small circuit features than Astra® MT77 materials for a given frequency, with more circuit signal loss with a typical Df of 0.0031 at 10 GHz. I-Tera® MT40 (RF/MW) laminates and prepregs are tailored for higher frequencies, with available Dk values of 3.38, 3.45. 3.60, and 3.75 at 10 GHz that allow a circuit designer to choose a close Dk target. The Dk values remain steady with time and across a wide range of temperatures from -40 to +140°C. The value of Df for each version of I-Tera® circuit material increases with the increasing value of Dk at 10 GHz, for 0.0028 through 0.0035.

When customers are environmentally sensitive, designers and producers of HSD circuits can start with halogen-free TerraGreen® laminates and prepregs from Isola-Group which maintain low Dk of 3.44 at 10 GHz for temperatures from -55 to +125°C. Its moderate Df of 0.0039 at 10 GHz is reduced to typically 0.0032 at the same test frequency for better loss performance at higher frequencies in halogen-free TerraGreen® (RF/MW) laminates, with a slightly higher Dk of 3.45 at 10 GHz. All forms of the material are RoHS compliant and lead free and can be manufactured into PCBs with straightforward processing steps.

These represent a sampling of the substrate materials that can serve as the basis for HSD and mmWave circuits as the applications for them expand in infrastructure and mobile electronic systems. Additional laminates and prepregs with low Dk and Df values suitable for HSD and mmWave circuits include IS680 very low-loss laminate with Dk as low as 0.0028 and Df of 0.0025 at 10 GHz; IS680 AG very low-loss laminate with low Df of 0.0020 for a Dk of 3.00 at 10 GHz; and Tachyon® 100G with Dk of 3.02 and Df of 0.0021 at 10 GHz. Please contact Isola-Group (www.isola-group.com) for any help or guidance in selecting the best circuit materials for a particular application. Sometimes the circuit solution with the best tradeoffs calls for a combination of substrates to optimize cost and performance.

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