Electronics Guide

Coaxial Connectors for mm-Wave

Coaxial connectors remain one of the most common interconnect solutions for mm-wave test equipment, instrumentation, and modular systems due to their balanced performance, mechanical robustness, and ability to provide shielded signal paths with controlled impedance. Standard connector families have been developed for progressively higher frequency ranges, with dimensions scaling down as frequency increases to maintain proper mode suppression and minimize losses.

The most widely used mm-wave coaxial connector families include the 2.92 mm (K connector) operating to 40 GHz, the 2.4 mm connector extending to 50 GHz, the 1.85 mm (V connector) reaching 67 GHz, the 1.0 mm (W connector) supporting operation to 110 GHz, and specialized connectors extending even higher. Each connector family features precisely controlled dimensions where the outer conductor inner diameter and center conductor diameter are designed to maintain 50-ohm characteristic impedance while keeping the cutoff frequency of higher-order modes well above the operating range.

Critical design considerations for mm-wave coaxial connectors include the precision of the center conductor alignment, the quality and uniformity of the dielectric support, the mechanical repeatability of the mating interface, and the minimization of impedance discontinuities at the transition points. The connector-to-PCB or connector-to-device interface is particularly challenging, requiring careful design of the launch structure to transform from the coaxial mode to the planar transmission line mode with minimal reflection and loss.

Connector losses become increasingly significant at mm-wave frequencies due to conductor losses in the small-diameter center pin, dielectric losses in the support structure, and radiation losses from any impedance mismatches. High-quality connectors use precision machining, gold or silver plating, low-loss dielectric materials, and air-dielectric designs where possible. Proper torque application during mating is essential to ensure consistent electrical performance and avoid damage to the delicate precision structures.

Despite their advantages, coaxial connectors face practical limitations at the highest mm-wave frequencies due to manufacturing tolerances, repeatability challenges, cost, and the physical size constraints. Alternatives such as waveguide flanges or probe-based test methods are often preferred above 100 GHz for critical applications.

Waveguide Interfaces

Rectangular metal waveguides provide low-loss, high-power-handling interconnects for mm-wave frequencies, particularly at the upper end of the mm-wave spectrum where coaxial structures become impractical. Waveguides confine electromagnetic energy within a metal enclosure and propagate signals in TE or TM modes, with the dominant TE10 mode being most commonly used. The rectangular cross-section dimensions determine the cutoff frequency and the operating frequency range.

Standard waveguide bands have been defined for mm-wave frequencies, including WR-28 (26.5-40 GHz), WR-22 (33-50 GHz), WR-15 (50-75 GHz), WR-12 (60-90 GHz), WR-10 (75-110 GHz), WR-8 (90-140 GHz), and smaller sizes for higher frequencies. Each designation indicates the nominal width of the broad wall in thousandths of an inch. These standardized dimensions ensure compatibility across different manufacturers and systems.

Waveguide flanges provide the mechanical and electrical interface between waveguide sections and are critical to achieving low-loss, low-reflection connections. Common flange types include UG flanges (older standard), WR flanges with alignment dowel pins for precise mating, and compact flanges for space-constrained applications. The flange interface must provide excellent electrical contact around the entire waveguide aperture to prevent leakage and ensure proper mode propagation.

A major design challenge in mm-wave systems is the transition between waveguide and planar transmission line structures such as microstrip, coplanar waveguide, or grounded coplanar waveguide on PCBs or integrated circuits. These transitions must convert the waveguide TE10 mode to the quasi-TEM mode of the planar line with minimal reflection and loss over the desired bandwidth. Common transition techniques include probe coupling, where a conductor extends into the waveguide; ridge or fin-line transitions that gradually transform the field distribution; and antipodal transitions using oppositely-oriented conductors.

Waveguide interfaces excel in low-loss, high-power applications but are relatively bulky, mechanically rigid, and expensive to manufacture at mm-wave frequencies. The need for precision machining and careful assembly makes them most suitable for high-performance applications where their advantages justify the cost and complexity.

Probe Interfaces

Radio-frequency probes provide flexible, non-destructive contact to mm-wave circuits for testing, characterization, and on-wafer measurements. These specialized probes, often called GSG (ground-signal-ground) probes, feature precisely positioned contact tips that land on coplanar pads with controlled pitch and geometry. Probe-based testing enables rapid characterization without requiring permanent wire bonding or device packaging.

The most common probe configuration uses a coplanar ground-signal-ground arrangement with pitch dimensions scaled to the frequency range. Standard pitches include 150 μm, 100 μm, and 50 μm, with smaller pitches required for higher frequencies and denser integrated circuits. The probe tips must make consistent, low-resistance contact with the device pads while minimizing inductance, capacitance, and radiation losses.

Probe design for mm-wave applications requires careful attention to the transmission line characteristics of the probe body and tips. The probe must present a well-controlled impedance, typically 50 ohms, from the connector or cable interface through the probe body and tips to the point of contact. This requires precision manufacturing of the internal transmission line structure and proper design of the tip geometry to match the coplanar waveguide or grounded coplanar waveguide on the device under test.

Calibration is essential for accurate mm-wave probe measurements, as the probe itself introduces losses, reflections, and phase shifts that must be removed through de-embedding. Standard calibration methods include Short-Open-Load-Thru (SOLT), Thru-Reflect-Line (TRL), and Line-Reflect-Match (LRM) techniques performed on calibration substrates with known standards. Advanced calibration can account for the probe's frequency-dependent characteristics and enable accurate S-parameter extraction to well over 100 GHz.

Probe interface challenges include maintaining precise tip positioning and planarity, managing contact force to avoid damage while ensuring good electrical connection, dealing with probe wear over repeated contacts, and minimizing parasitic coupling between adjacent probes in multi-port measurements. High-quality probes use wear-resistant materials such as tungsten or beryllium-copper alloys and incorporate mechanical compliance to accommodate slight variations in wafer or substrate planarity.

Ball Grid Arrays for mm-Wave

Ball grid array (BGA) technology can be adapted for mm-wave applications, providing a compact, high-density interconnect solution for attaching mm-wave integrated circuits to substrates or PCBs. While traditional BGA structures face challenges at mm-wave frequencies due to parasitic inductance, capacitance, and signal integrity concerns, careful design and specialized processes can extend BGA viability into the lower mm-wave bands.

The primary challenge in using BGAs for mm-wave is that the solder balls introduce a vertical interconnect with significant inductance and the ball diameter and pitch create impedance discontinuities that cause reflections. To minimize these effects, mm-wave BGAs typically use very small ball diameters, fine pitch, and carefully controlled ball height. The signal ball arrangement often includes dedicated ground balls immediately adjacent to signal balls to provide a low-inductance return path and maintain controlled impedance through the BGA transition.

Ground-signal-ground (GSG) or ground-signal-signal-ground (GSSG) ball patterns are common for differential or single-ended mm-wave signals, with the ground balls providing shielding and impedance control. The substrate or PCB landing pattern must be designed to match the ball pattern and provide proper transmission line characteristics on both sides of the BGA interface. This often requires via-in-pad designs where through-hole vias pass directly through the BGA pads to provide short, low-inductance connections to inner layer signal traces or ground planes.

Underfill material, commonly used in BGAs for mechanical reliability, becomes an important electrical consideration at mm-wave frequencies. The dielectric constant and loss tangent of the underfill affect signal propagation, and the underfill geometry influences the effective transmission line characteristics in the BGA region. Low-loss underfill materials are preferred for mm-wave applications.

While BGA interconnects are practical for lower mm-wave frequencies (30-50 GHz) with careful design, flip-chip bumping approaches are generally preferred for higher frequencies where the shorter interconnect length and more controlled geometry provide better electrical performance.

Ribbon Bonds

Ribbon bonding uses flat metal ribbons, typically gold or aluminum, instead of round wire to connect mm-wave die to package leads or substrate pads. The ribbon geometry offers advantages over conventional wire bonds, including lower inductance due to the wider cross-section, better mechanical stability, and more predictable electromagnetic behavior that can be modeled and optimized for mm-wave frequencies.

A typical ribbon bond might be 25-100 μm wide and 5-25 μm thick, providing a substantially different aspect ratio compared to a round wire of equivalent cross-sectional area. This geometry reduces the loop inductance per unit length and provides a more well-defined current distribution. Multiple ribbons can be bonded in parallel to further reduce inductance and increase current-carrying capacity for power delivery or ground connections.

For mm-wave applications, ribbon bonds are often designed as short, low-profile connections with carefully controlled bond loop height and trajectory. The goal is to minimize the electrical length of the interconnect and create a geometry that approximates a transmission line structure with controlled impedance. Some advanced implementations use ground ribbons placed adjacent to signal ribbons to create a ribbon-based coplanar waveguide or stripline structure with defined characteristic impedance.

The design of ribbon bond interconnects requires electromagnetic simulation to optimize the ribbon width, thickness, length, loop height, and spacing to adjacent ribbons or ground structures. The transitions from the die pad through the ribbon to the substrate pad must be carefully managed to avoid impedance discontinuities. In some designs, the ribbon is modeled as a segment of transmission line and deliberately designed to be a specific electrical length for matching or filtering purposes.

Ribbon bonds are particularly useful in power amplifier modules and transmit chains where handling high RF current is required, as the broader cross-section reduces resistive losses and improves power-handling capability. However, they require specialized bonding equipment and careful process control to achieve consistent geometry. At the highest mm-wave frequencies (above 100 GHz), even ribbon bonds become electrically long, and alternative approaches such as flip-chip bumps or direct die attach are preferred.

Flip-Chip Bumps

Flip-chip bumping provides the shortest possible interconnect length for mm-wave die attachment, directly bonding the active circuit side of the die face-down onto a substrate or carrier using small solder or gold bumps. This face-down orientation places the bump interconnects immediately adjacent to the active circuitry, minimizing parasitic inductance and enabling operation well into the mm-wave range and beyond.

The extremely short vertical height of flip-chip bumps, typically 15-50 μm depending on the bump technology, results in very low inductance interconnects on the order of a few tens of picohenries. This low inductance is critical for maintaining signal integrity at mm-wave frequencies where even small parasitics cause significant impedance discontinuities. The bump diameter and pitch are also small, commonly 50-150 μm, allowing dense arrays of signal, ground, and power bumps.

Common flip-chip bump technologies for mm-wave applications include solder bumps (typically high-lead or lead-free compositions), gold stud bumps, copper pillar bumps, and indium bumps. Each technology has trade-offs in terms of bump height, mechanical compliance, electrical conductivity, manufacturing cost, and process compatibility. Gold and copper bumps offer excellent electrical conductivity and well-controlled geometry, making them popular for high-frequency applications.

A key advantage of flip-chip is the ability to create ground-signal-ground or ground-signal-signal-ground bump patterns that maintain controlled impedance through the flip-chip interface. By surrounding signal bumps with adjacent ground bumps, a low-inductance return path is established that approximates a coplanar waveguide or grounded coplanar waveguide structure. The substrate must have complementary landing pads with appropriate transmission line geometry to maintain impedance control.

Underfill is typically applied around and beneath flip-chip bumps to provide mechanical support and reliability, but the underfill dielectric properties affect the electrical characteristics. Low-loss underfill materials with controlled dielectric constant are essential for mm-wave applications. Some designs use selective underfill patterns that avoid the immediate vicinity of mm-wave signal paths to minimize dielectric loading.

Flip-chip interconnects require precise die placement and alignment, as well as specialized reflow or thermocompression bonding processes. The very short interconnect length makes flip-chip the preferred choice for the highest mm-wave frequencies and for millimeter-wave integrated circuits where minimizing parasitic reactance is paramount. Flip-chip bumping is routinely used in circuits operating to 100 GHz and beyond.

Air Bridges

Air bridges are suspended metal structures that cross over other conductors or transmission lines without making electrical contact, providing a low-capacitance, low-loss interconnect or crossover structure in mm-wave planar circuits. Fabricated using micromachining processes on semiconductor or ceramic substrates, air bridges eliminate the high capacitance and losses associated with dielectric-supported crossovers, making them essential for high-performance mm-wave integrated circuits.

The fundamental structure of an air bridge consists of a metal conductor, typically gold, that is supported only at its endpoints and spans across an air gap above the substrate surface. The air gap, typically 1-5 μm, significantly reduces the parasitic capacitance compared to a conductor directly on the substrate. Multiple air bridge posts can be used to support longer spans or to provide mechanical stability.

Air bridges serve several functions in mm-wave circuits. They are commonly used to connect the two ground planes in coplanar waveguide (CPW) transmission lines, suppressing unwanted coupled-slotline modes and ensuring that both ground planes remain at the same RF potential. This ground strap application is critical for maintaining proper CPW propagation characteristics and preventing mode conversion.

Another important application is providing low-capacitance crossovers where one signal line must cross over another without electrical contact. Traditional crossovers using dielectric layers suffer from high capacitance that loads both signal paths and causes reflections at mm-wave frequencies. Air bridge crossovers minimize this capacitance, though some residual coupling remains. Careful design of the air bridge width, height, and orientation relative to the underlying conductor can minimize parasitic effects.

Air bridges are also used to connect inductors, capacitors, and other circuit elements in mm-wave monolithic microwave integrated circuits (MMICs) where minimizing parasitic capacitance is critical to achieving the desired frequency response. The three-dimensional geometry allows more compact circuit layouts and better control over electromagnetic coupling.

Fabrication of air bridges requires specialized processing, typically involving the deposition of a sacrificial photoresist or polyimide layer that supports the metal bridge during electroplating or deposition, followed by removal of the sacrificial layer to leave the suspended bridge. Process control is critical to achieving consistent bridge height and profile. Mechanical reliability must be considered, as the suspended structures can be fragile and susceptible to damage from mechanical shock or thermal stress.

At the highest mm-wave frequencies, the physical dimensions of air bridges become comparable to the wavelength, and their behavior must be analyzed using full-wave electromagnetic simulation rather than simple lumped-element models. Despite the fabrication complexity, air bridges remain an essential interconnect technology for high-performance mm-wave circuits due to their superior electrical characteristics.

Membrane Crossovers

Membrane crossovers, also known as bridge crossovers or suspended membrane interconnects, are thin dielectric membranes with metalized conductors that are suspended above the substrate surface to create low-capacitance crossovers between signal lines in mm-wave planar circuits. This technology combines some advantages of air bridges with easier fabrication processes, providing a practical solution for signal crossovers in hybrid and monolithic mm-wave circuits.

The membrane structure typically consists of a thin dielectric film, such as silicon nitride, silicon dioxide, or polyimide, with a thickness of 0.5-2 μm, suspended a few micrometers above the substrate. Metal conductors are patterned on top of the membrane to form the signal traces. The membrane is supported at its edges or at discrete posts, with the central region suspended in air. The air gap beneath the membrane significantly reduces the effective dielectric constant and the capacitive loading compared to conductors directly on the substrate.

Membrane crossovers are particularly useful in situations where two transmission lines must cross at different angles without electrical connection. The lower line is fabricated on the substrate using conventional planar processing, while the upper line is formed on the suspended membrane that bridges over the lower line. The air gap and thin dielectric membrane result in much lower parasitic capacitance than would occur with a conventional dielectric layer crossover, enabling the crossover to be modeled as a simple discontinuity rather than a distributed capacitive load.

Compared to pure air bridges, membrane crossovers offer better mechanical stability because the thin dielectric film provides structural support for the conductor. This makes them less susceptible to damage from mechanical shock and thermal cycling. The membrane also provides a well-defined surface for metal deposition, potentially resulting in better reproducibility than air bridge processes.

However, the presence of the dielectric membrane introduces some additional capacitance and dielectric loss compared to pure air bridges. The membrane material must be selected for low dielectric loss and appropriate mechanical properties. Silicon nitride and low-loss polymers are common choices for mm-wave applications due to their combination of mechanical strength and acceptable electrical properties.

Fabrication of membrane crossovers typically involves depositing and patterning the membrane dielectric layer, creating support posts or edge anchors, patterning the metal conductor on the membrane, and then removing a sacrificial layer beneath the membrane to create the air gap. The process is compatible with standard microfabrication techniques and can be integrated into MMIC or hybrid circuit manufacturing flows.

Membrane crossovers find application in complex mm-wave circuits where multiple signal paths must be routed in limited space without excessive mutual coupling. They enable more flexible circuit topologies and help minimize circuit size. The crossover capacitance must still be accounted for in circuit design and can be characterized through electromagnetic simulation or measurement. Properly designed membrane crossovers can provide serviceable performance well into the mm-wave range, typically up to 100 GHz or beyond, depending on the specific geometry and materials.

Design Considerations and Best Practices

Successful mm-wave interconnect design requires attention to several critical factors that differ from lower-frequency practice. Impedance matching at every transition and interface is paramount, as even small reflections accumulate quickly at mm-wave frequencies and can significantly degrade system performance. Every discontinuity, including connector launches, bond wires, flip-chip bumps, and air bridges, must be carefully designed and simulated to minimize return loss.

Loss budgeting becomes critical in mm-wave systems because interconnect losses can easily dominate the overall system loss. Each connector, transition, bond wire, or trace segment contributes loss that must be carefully accounted for in the link budget. Low-loss materials, short interconnect lengths, and optimized geometries are essential. In some cases, the choice of interconnect technology is driven primarily by loss considerations.

Manufacturing tolerances have much greater impact at mm-wave frequencies due to the short wavelengths. Variations in dimensions that would be negligible at microwave frequencies can cause significant impedance variations and resonances at mm-wave. Design rules must account for expected manufacturing variations, and robust designs should include tolerance analysis using Monte Carlo simulation or worst-case analysis.

Electromagnetic simulation is indispensable for mm-wave interconnect design. Full-wave 3D electromagnetic solvers are typically required to accurately capture the field distributions, radiation effects, and higher-order mode behavior. Lumped-element or simple transmission line models are often insufficient. Simulation results should be validated with measurements whenever possible to build confidence in the models.

Grounding and shielding considerations become more stringent as frequency increases. Return current paths must be well-defined and kept close to signal paths to minimize loop inductance and prevent radiation. Ground stitching vias, dense ground plane connections, and shielding structures help contain electromagnetic fields and reduce crosstalk and EMI.

The transition between different interconnect types is often the most challenging aspect of mm-wave design. Coaxial-to-waveguide, waveguide-to-planar, and planar-to-flip-chip transitions all require careful optimization. Stepped impedance transformers, tapered transitions, and mode-suppression structures can improve transition performance. Testing these transitions in isolation helps validate the design before system integration.

Finally, measurement and characterization infrastructure must be considered. High-quality mm-wave connectors, calibration standards, and test fixtures are expensive and require careful handling. Probe-based testing can reduce cost for prototyping but requires investment in calibration substrates and procedures. Planning the test and characterization approach during the design phase helps ensure that performance can be verified once hardware is fabricated.

Conclusion

mm-Wave interconnects represent a diverse and sophisticated set of technologies, each with specific advantages and limitations. From the robustness and repeatability of precision coaxial connectors to the ultra-low parasitic performance of flip-chip bumps and air bridges, the choice of interconnect technology profoundly influences mm-wave system performance. Understanding the electromagnetic behavior, loss mechanisms, fabrication constraints, and design trade-offs of each interconnect type is essential for successful mm-wave circuit and system development.

As mm-wave applications continue to expand into 5G communications, automotive radar, imaging, and sensing systems, interconnect technology will remain a critical enabling factor. Advances in materials, fabrication processes, and design methodologies continue to push the performance boundaries, enabling reliable, low-loss interconnections at ever-higher frequencies. Successful mm-wave engineers must combine strong electromagnetic fundamentals with practical knowledge of manufacturing processes and a thorough understanding of how interconnects behave in real-world systems.