Frequencies and bandwidths once thought “impractical” or at least “for the military,” at millimeter-wave (mmWave) frequencies, are rapidly becoming more commercially commonplace. Two strong examples are In Fifth Generation New Radio (5G NR) wireless networks and 77 GHz automotive radars, where mmWave frequencies are channeled through highly integrated printed circuit boards (PCBs). The PCBs are often in multilayer configurations that handle different types of signals, including analog, digital, RF, and mmWave signals. Modern circuit designers are faced with integration and miniaturization, trying to pack as many functions into the smallest PCB possible. But different circuit functions have different circuit material requirements, and circuit materials that provide optimum performance at mmWave frequencies may not be the most practical solution for power supply circuitry.

Often, the most practical solution for a multilayer PCB with a variety of circuit functions is a multilayer PCB composed of three or more different circuit materials. Circuit materials are chosen for how their characteristics support each circuit function, such as the best circuit material parameters for power, high speed digital, lower frequency RF and microwave, and higher frequency mmWave circuits. The choice often starts with a circuit material’s electrical parameters, such as dielectric constant (Dk) and dissipation factor (Df) or loss tangent. But a circuit material’s mechanical characteristics will also contribute to the function-by-function and layer-by-layer selection process when choosing circuit materials for a multilayer PCB since mechanical specifications such as the thickness of the circuit material can impact the dimensions of transmission lines and spacing between them for high speed digital and high frequency microwave and mmWave circuits. Yet another consideration when designing and producing a multilayer circuit is that eventually those different layers must be combined and interconnected. Any choice of materials, including the circuit materials and the bonding materials, should make the combination of layers as manufacturable as possible.

Microwave circuit designers who have combined high frequency transmission lines such as microstrip, stripline, and grounded coplanar-waveguide (GCPW) transmission lines on low-cost circuit materials such as FR-4 are familiar with how circuit material characteristics affect the performance of the different circuit types. As frequencies increase, circuit material electrical parameters, such as Dk and Df, will have greater impact on transmission-line performance, such as maintaining the synchronization of radar pulses or the integrity of modulated communications signals.

Mechanical properties, such as circuit material thickness and thickness consistency, may not have mattered so much at RF microwave frequencies but, as the signal frequencies climb into the mmWave range, those mechanical properties will impact circuit performance and must be considered when selecting a circuit material for mmWave frequencies. At those small wavelengths, even the surface roughness of a circuit material’s conductors can impact circuit behavior and result in differences in phase response and insertion loss.

Similarly, for high speed digital circuits, circuit materials must be chosen in support of transmission lines with closely matched (typically 50 Ω or 100 Ω differential) and consistent impedance and propagation characteristics, to avoid unwanted timing delays and signal distortion. And the impedance must be maintained at all junctions, including between PCB layers, which requires the capability to form consistent, high-quality microvias. Some circuit materials may have a composition that is better suited for modern micromachining techniques, such as laser drilling, required to form the microvias needed for transmission-line interconnections, especially critical at mmWave frequencies. For high speed digital circuits and mmWave circuits, the shortest possible distance for microvias usually provides the lowest loss and highest reliability and requires precise layer-to-layer alignment in multilayer PCBs, even when interconnections are being made between different circuit materials. Achieving and maintaining good layer-to-layer alignment with high reliability microvias between layers requires circuit materials with excellent mechanical stability

Selecting Solutions

In general, multiple-function, multilayer PCBs that include mmWave circuitry are being designed and manufactured according to several trends supporting several large-scale commercial applications, such as radar systems in autonomous vehicles and 5G wireless communications networks. These trends include more and thinner circuit layers to fit small size and weight requirements or, in the case of military designs, to meet small size, weight, and power (SWaP) requirements. Additional trends include higher circuit density to achieve more functions on smaller circuits, good stability with temperature, and low moisture absorption to survive in difficult operating environments. Circuit materials for high speed digital circuits and mmWave circuits typically exhibit low Dk, low Df, and stable Dk and Df across wide temperatures.

A growing number of multilayer PCBs with mmWave circuits creates increased demand for circuit materials that can support the electrical needs of high frequency/high speed circuits and meet the mechanical requirements of multilayer circuit assemblies. Such assemblies consist of circuit laminates and prepregs or bondply materials to hold the layers together. For higher frequency applications, for example, RO3003™ circuit material from Rogers Corp. provides the characteristics that support low-loss circuits through microwave frequencies. Based on PTFE with low-Dk ceramic filler, the circuit laminate exhibits a low Dk of 3.00 in the z-axis (thickness) at 10 GHz with low Df of 0.0010 at 10 GHz. The circuit material is available in thicknesses ranging from 0.005 in. (0.13 mm) to 0.060 in. (1.52 mm).

It has been used for single-layer circuits through 77 GHz and has the dimensional stability with temperature for reliable multilayer PCBs. The material’s coefficient of thermal expansion (CTE) across the x and y axes is the same as copper, about 17 ppm/°C, for high dimensional stability in multilayer assemblies. The z-axis CTE is 25 ppm/°C for stable and reliable plated through holes (PTHs) when forming microvia interconnections between layers. It features low moisture absorption of 0.04% needed for operating environments that vary in humidity.

Because conductor surface smoothness is so significant at higher frequencies, RO3003G2™ circuit materials were developed by Rogers Corp. as a refined version of RO3003 laminates, with optimized filler with reduced dielectric porosity and very low-profile electrodeposited (ED) copper. With essentially the same Dk and Df values as RO3003, RO3003G2 laminates exhibit slightly less loss at mmWave frequencies due to the optimization. While matching the CTE values of RO3003 laminates in the x-y plane, the optimized filler of RO3003G2 laminates yields a z-axis CTE (18 ppm/°C) close to that of copper, for extremely reliable PTHs. The RO3003G2 laminates are available in thicknesses of 0.005 in. (0.13 mm) and 0.010 in. (0.25 mm).

When mechanical stability is a concern and thinner circuit materials are needed, such as in multilayer PCBs where size and weight must be minimized or resonances between layers reduced, the CLTE-MW™ circuit material provides the characteristics needed for good mmWave performance with spread glass construction and low-Dk ceramic filler for excellent dimensional stability. It is available in thicknesses from 0.003 in. (0.076 mm) to 0.010 in. (0.25 mm) to meet different signal-to-ground spacing requirements while minimizing size and weight in multilayer circuit assemblies. Depending upon thickness, the Dk ranges from 2.94 to 3.02 in the z-axis at 10 GHz. The CTE performance supports excellent dimensional and PTH stability with temperature, while the moisture absorption is low, at 0.03%, for use in challenging operating environments.

Bondply and prepreg materials do more than just hold the layers together in a multilayer PCB, they become part of the PCB. For that reason, they should be selected as much for their electrical and mechanical qualities as for their adhesive capabilities. As an example, 2929 bondply material from Rogers Corp. is compatible with flat press and autoclave bonding methods and is available in different sheet thicknesses; sheets can be stacked when greater thicknesses are needed. It features a Dk value of 2.94 in the z-axis at 10 GHz, with low Df of 0.003 at 10 GHz, low z-axis expansion for reliable plated through holes, and is compatible with PTFE-based circuit materials for secure adhesion of multilayer PCBs.

SpeedWave™ 300P prepreg from Rogers Corp. is compatible with RoHS-compliant, lead-free assembly and fabrication processes for FR-4 and PTFE-based circuit materials, such as CLTE-MW laminates. With low Dk of 3.0 to 3.3 (depending upon thickness) and low Df values of 0.0019 to 0.0022, the prepreg features good flow and fill characteristics with low z-axis expansion, which supports reliable plated through holes. It is available with various spread and open weave glass configurations and different resin content combinations for optimum bonding especially in high-layer-count circuit assemblies.

As mmWave bandwidths become more commonly used, multilayer PCBs containing mmWave circuitry will also become more common, designed with greater numbers of layers for smaller places. With the right choices of circuit materials and prepregs, everything will hold together.

Do you have a design or fabrication question? Rogers Corporation’s experts are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.

Often, the most practical solution for a multilayer PCB with a variety of circuit functions is a multilayer PCB composed of three or more different circuit materials. Circuit materials are chosen for how their characteristics support each circuit function, such as the best circuit material parameters for power, high speed digital, lower frequency RF and microwave, and higher frequency mmWave circuits. The choice often starts with a circuit material’s electrical parameters, such as dielectric constant (Dk) and dissipation factor (Df) or loss tangent. But a circuit material’s mechanical characteristics will also contribute to the function-by-function and layer-by-layer selection process when choosing circuit materials for a multilayer PCB since mechanical specifications such as the thickness of the circuit material can impact the dimensions of transmission lines and spacing between them for high speed digital and high frequency microwave and mmWave circuits. Yet another consideration when designing and producing a multilayer circuit is that eventually those different layers must be combined and interconnected. Any choice of materials, including the circuit materials and the bonding materials, should make the combination of layers as manufacturable as possible.

Microwave circuit designers who have combined high frequency transmission lines such as microstrip, stripline, and grounded coplanar-waveguide (GCPW) transmission lines on low-cost circuit materials such as FR-4 are familiar with how circuit material characteristics affect the performance of the different circuit types. As frequencies increase, circuit material electrical parameters, such as Dk and Df, will have greater impact on transmission-line performance, such as maintaining the synchronization of radar pulses or the integrity of modulated communications signals.

Mechanical properties, such as circuit material thickness and thickness consistency, may not have mattered so much at RF microwave frequencies but, as the signal frequencies climb into the mmWave range, those mechanical properties will impact circuit performance and must be considered when selecting a circuit material for mmWave frequencies. At those small wavelengths, even the surface roughness of a circuit material’s conductors can impact circuit behavior and result in differences in phase response and insertion loss.

Similarly, for high speed digital circuits, circuit materials must be chosen in support of transmission lines with closely matched (typically 50 Ω or 100 Ω differential) and consistent impedance and propagation characteristics, to avoid unwanted timing delays and signal distortion. And the impedance must be maintained at all junctions, including between PCB layers, which requires the capability to form consistent, high-quality microvias. Some circuit materials may have a composition that is better suited for modern micromachining techniques, such as laser drilling, required to form the microvias needed for transmission-line interconnections, especially critical at mmWave frequencies. For high speed digital circuits and mmWave circuits, the shortest possible distance for microvias usually provides the lowest loss and highest reliability and requires precise layer-to-layer alignment in multilayer PCBs, even when interconnections are being made between different circuit materials. Achieving and maintaining good layer-to-layer alignment with high reliability microvias between layers requires circuit materials with excellent mechanical stability

Selecting Solutions
In general, multiple-function, multilayer PCBs that include mmWave circuitry are being designed and manufactured according to several trends supporting several large-scale commercial applications, such as radar systems in autonomous vehicles and 5G wireless communications networks. These trends include more and thinner circuit layers to fit small size and weight requirements or, in the case of military designs, to meet small size, weight, and power (SWaP) requirements. Additional trends include higher circuit density to achieve more functions on smaller circuits, good stability with temperature, and low moisture absorption to survive in difficult operating environments. Circuit materials for high speed digital circuits and mmWave circuits typically exhibit low Dk, low Df, and stable Dk and Df across wide temperatures.

A growing number of multilayer PCBs with mmWave circuits creates increased demand for circuit materials that can support the electrical needs of high frequency/high speed circuits and meet the mechanical requirements of multilayer circuit assemblies. Such assemblies consist of circuit laminates and prepregs or bondply materials to hold the layers together. For higher frequency applications, for example, RO3003™ circuit material from Rogers Corp. provides the characteristics that support low-loss circuits through microwave frequencies. Based on PTFE with low-Dk ceramic filler, the circuit laminate exhibits a low Dk of 3.00 in the z-axis (thickness) at 10 GHz with low Df of 0.0010 at 10 GHz. The circuit material is available in thicknesses ranging from 0.005 in. (0.13 mm) to 0.060 in. (1.52 mm).

It has been used for single-layer circuits through 77 GHz and has the dimensional stability with temperature for reliable multilayer PCBs. The material’s coefficient of thermal expansion (CTE) across the x and y axes is the same as copper, about 17 ppm/°C, for high dimensional stability in multilayer assemblies. The z-axis CTE is 25 ppm/°C for stable and reliable plated through holes (PTHs) when forming microvia interconnections between layers. It features low moisture absorption of 0.04% needed for operating environments that vary in humidity.

Because conductor surface smoothness is so significant at higher frequencies, RO3003G2™ circuit materials were developed by Rogers Corp. as a refined version of RO3003 laminates, with optimized filler with reduced dielectric porosity and very low-profile electrodeposited (ED) copper. With essentially the same Dk and Df values as RO3003, RO3003G2 laminates exhibit slightly less loss at mmWave frequencies due to the optimization. While matching the CTE values of RO3003 laminates in the x-y plane, the optimized filler of RO3003G2 laminates yields a z-axis CTE (18 ppm/°C) close to that of copper, for extremely reliable PTHs. The RO3003G2 laminates are available in thicknesses of 0.005 in. (0.13 mm) and 0.010 in. (0.25 mm).

When mechanical stability is a concern and thinner circuit materials are needed, such as in multilayer PCBs where size and weight must be minimized or resonances between layers reduced, the CLTE-MW™ circuit material provides the characteristics needed for good mmWave performance with spread glass construction and low-Dk ceramic filler for excellent dimensional stability. It is available in thicknesses from 0.003 in. (0.076 mm) to 0.010 in. (0.25 mm) to meet different signal-to-ground spacing requirements while minimizing size and weight in multilayer circuit assemblies. Depending upon thickness, the Dk ranges from 2.94 to 3.02 in the z-axis at 10 GHz. The CTE performance supports excellent dimensional and PTH stability with temperature, while the moisture absorption is low, at 0.03%, for use in challenging operating environments.

Bondply and prepreg materials do more than just hold the layers together in a multilayer PCB, they become part of the PCB. For that reason, they should be selected as much for their electrical and mechanical qualities as for their adhesive capabilities. As an example, 2929 bondply material from Rogers Corp. is compatible with flat press and autoclave bonding methods and is available in different sheet thicknesses; sheets can be stacked when greater thicknesses are needed. It features a Dk value of 2.94 in the z-axis at 10 GHz, with low Df of 0.003 at 10 GHz, low z-axis expansion for reliable plated through holes, and is compatible with PTFE-based circuit materials for secure adhesion of multilayer PCBs.

SpeedWave™ 300P prepreg from Rogers Corp. is compatible with RoHS-compliant, lead-free assembly and fabrication processes for FR-4 and PTFE-based circuit materials, such as CLTE-MW laminates. With low Dk of 3.0 to 3.3 (depending upon thickness) and low Df values of 0.0019 to 0.0022, the prepreg features good flow and fill characteristics with low z-axis expansion, which supports reliable plated through holes. It is available with various spread and open weave glass configurations and different resin content combinations for optimum bonding especially in high-layer-count circuit assemblies.

As mmWave bandwidths become more commonly used, multilayer PCBs containing mmWave circuitry will also become more common, designed with greater numbers of layers for smaller places. With the right choices of circuit materials and prepregs, everything will hold together.

Do you have a design or fabrication question? Rogers Corporation’s experts are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.