Millimeter-wave frequencies were once few and far-between, in terms of applications and circuits using frequencies above 30 GHz. But that is about to change quickly, with Fifth Generation (5G) wireless networks and automotive radar systems both incorporating millimeter-wave frequency bands. For many circuit designers, these frequencies may represent uncharted territory and may require some thought not only about a suitable printed-circuit-board (PCB) material, but of the optimum transmission-line technology, board layouts, and connector launches. Many circuit designers face new challenges with the inevitable increase of millimeter-wave applications.
Circuit designers familiar with a particular transmission-line technology may ask: Can’t I stick with microstrip at these higher frequencies, if the PCB material delivers the performance I need? Microstrip is widely used in circuits from about 300 MHz to 30 GHz. Above 30 GHz, at millimeter-wave frequencies (30 to 300 GHz), microstrip suffers increased radiation loss and problems with spurious propagation modes. Designers working on circuits with both microwave and millimeter-wave transmission lines will often make a transition from microstrip to grounded coplanar-waveguide (GCPW) transmission lines which, when designed and fabricated properly, have little or no radiation loss and minimal spurious mode propagation.
For circuits with wideband coverage and without transitions between different transmission-line technologies, stripline is often used from lower microwave frequencies to millimeter-wave frequencies. However, forming a signal launch from a coaxial connector to stripline on a PCB has never been easy at microwave frequencies, and can become more challenging at higher, millimeter-wave frequencies with the shrinking dimensions of transmission-line structures. Ideally, the transition from the coaxial domain of a high-frequency connector to the parallel plane of a stripline PCB should be smooth, with little or no signal loss or reflections and no spurious modes. Assuming well-matched signal launches, stripline can be an excellent choice of transmission line for millimeter-wave PCBs, although circuit fabrication is somewhat more involved than when forming microstrip or GCPW transmission lines.
Easier to Build?
Microstrip and GCPW circuits are attractive for their ease of assembly, each with a single dielectric layer with ground plane on bottom and signal conductors and components on top. Since the circuitry is exposed, components can be attached directly to the transmission lines on the signal plane. Stripline, on the other hand, surrounds its signal conductors with dielectric layers which in turn have ground planes on top and bottom. Because stripline’s signal conductors are buried in a multiple-layer circuit assembly, making connections between components and the signal conductors is never routine. Signal connections in microwave stripline PCBs are typically made by means of conductive viaholes: holes drilled through the dielectric layers and plated with conductive metal. Plated viaholes, or plated through holes (PTHs) as they are known, provide short, electrically conductive signal paths through the dielectric layers but also add their own capacitance and inductance values to the circuitry, impacting performance at higher frequencies. They become part of a circuit diagram (which must be modeled) at millimeter-wave frequencies.
Effective use of stripline transmission-line technology for millimeter-wave PCBs depends on finding the optimum plated viahole structure for low-loss, low-reflection transmission of high-frequency signals to the embedded signal plane. The transition provided by well-formed viaholes through stripline circuitry is essential not only for energy from signal-launch connectors but any electrical connections made to and from external components.
Laser technology can be an effective means of forming the small viaholes, or microvias, needed for stripline PCB interconnections at millimeter-wave frequencies. Precisely controlled laser drilling systems are designed to cut micron-sized microvias by burning through the top copper ground plane of a stripline circuit assembly, through the dielectric material beneath it, and to the signal plane lying between the two dielectric layers. Copper plating is applied and, in this way, a conductive path is formed through the hole from the top copper layer to the signal plane beneath. Such microvias can be formed with extremely small diameters and with the short lengths needed for thin dielectric materials typically used at smaller-wavelength, millimeter-wave frequencies.
By using this commercially available laser-based microvia-forming process, excellent performance can be achieved in stripline interconnections at millimeter-wave frequencies. Larger PTHs formed in stripline circuit assemblies can add unwanted capacitances and inductances at millimeter wavelengths, even in the shortest lengths.
Low-loss, low-reflection signal launches in stripline have been commonly realized in circuits for use to about 40 GHz; it can be difficult to achieve the good match and construction between connector interface and viahole for stripline circuits with coaxial launches at frequencies above 40 GHz. However, the choice of PCB material can play a role in the effectiveness of stripline circuits at millimeter-wave frequencies, based on recent experiments with RO3003™ laminates from Rogers Corp. Using these materials with standard stripline transmission-line structures, low-loss coaxial signal launches were measured to as high as 60 GHz. With several minor modifications, it should be possible to achieve practical coaxial-to-stripline signal launches out to 80 GHz using these same circuit materials.
When considering a PCB material that can support microvias for millimeter-wave circuits, stability at those higher frequencies is a key requirement. RO3003 circuit material has shown excellent mechanical and electrical stability above 30 GHz. It is mechanically stable, with the stability typically realized on other glass-reinforced materials as part of a multilayer construction. However, RO3003 laminates do not use glass reinforcement, so microvias can be laser-formed reliably and consistently without the effects from the lasered glass. RO3003 features coefficient of thermal expansion (CTE) closely matched to that of copper, so that microvias remain structurally and electrically sound even with thermal cycling. Regardless of the choice of transmission line, RO3003 circuit material, with its consistent dielectric constant (Dk) and dissipation factor (Df) over a wide range of frequencies, is a logical starting place for those higher-frequency circuits.
While there may not be one perfect transmission-line technology for millimeter-wave circuits, the choice of a starting point—the PCB material—can make a difference in the final performance possible at those higher frequencies. Microstrip and GCPW technologies support many millimeter-wave circuit applications with ease of fabrication and testing, but it has been shown that stripline is capable of excellent circuit performance at millimeter-wave frequencies when teamed with the right circuit materials.
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