Effective Impedance Control in mmWave PCBs with PTH Vias

Plated through-hole vias (PTH vias) are fundamental elements in printed circuit boards (PCBs), providing electrical connections between different layers. However, when working with mmWave frequencies (typically above 30 GHz), the design of these vias becomes more complex. As the frequency increases, the behavior of signals changes, and even small imperfections in via design can significantly impact signal integrity, causing issues like reflections and resonances. This article explores the challenges and best practices for designing plated through-hole vias in mmWave PCBs, providing engineers with the necessary insights to optimize their designs.

Understanding the Role of Plated Through-Hole Vias in mmWave PCB Applications

In standard PCB designs, PTH vias are used to route electrical signals between different layers of a board. However, in mmWave PCB applications, these vias interact with signals in ways that can significantly affect signal quality. At mmWave frequencies, the wavelength of signals becomes very small, making the vias behave as impedance discontinuities. This leads to strong signal reflections and degradation. Unlike traditional designs, the tiny physical dimensions and high-frequency behavior of mmWave signals demand more precise management of signal integrity to avoid unwanted effects such as signal loss and distortion.

For mmWave PCB designs, engineers face unique challenges when incorporating PTH vias. These vias, which allow electrical signals to pass between multiple layers of the board, can introduce resonance and distortions when not properly controlled. At mmWave frequencies, even small impedance mismatches can result in significant signal degradation, as the signals experience strong reflections when passing through the vias. This issue becomes especially problematic in high-performance applications like 5G communications, where the integrity of each signal is critical for maintaining reliable data transmission.

To address these challenges, engineers designing mmWave PCBs must carefully consider factors like impedance matching, via length, and the overall board layout. Specialized design techniques are often employed to minimize the negative impact of vias, such as using blind vias or buried vias, or implementing via structures that can reduce signal reflection. Additionally, simulation tools are frequently used to analyze the effects of PTH vias at mmWave frequencies to ensure that the final design will perform optimally. Managing these factors effectively is essential for achieving high-speed, high-performance signal transmission in mmWave PCB applications.

Key Considerations in mmWave Via Design

Impedance Control and Signal Integrity

One of the most critical aspects of mmWave PCB design is ensuring that PTH vias maintain the target impedance, typically 50 Ohms, across the entire signal path. Signal integrity is closely tied to impedance matching; any discontinuity in impedance—such as that caused by a poorly designed via—can lead to signal reflection, loss of power, and increased insertion loss.

For mmWave frequencies, the via must not introduce any significant change in the characteristic impedance. This requires precise calculation of the via size, the pad size, and the distance between the via and the surrounding trace. Any deviation from the ideal impedance can cause signal degradation, which is unacceptable in high-frequency applications.

Insertion Loss and Frequency Bandwidth

Insertion loss is another important consideration when designing PTH vias for mmWave PCBs. At high frequencies, the losses due to the via structure increase, as a portion of the signal is dissipated through the via material. The higher the frequency, the greater the insertion loss, which affects the overall signal quality.

The frequency bandwidth of the signal is closely related to the via design. At mmWave frequencies, the bandwidth is often narrower compared to high-speed digital signals. This narrower bandwidth simplifies some aspects of via design, as engineers only need to consider the behavior near the carrier frequency. For example, at 77 GHz FMCW radar, the signal bandwidth might be around 5 GHz, only a small fraction of the carrier frequency. However, even small misalignments in impedance or frequency can lead to significant losses in performance.

Propagation Delay and Signal Timing

In addition to managing impedance, mmWave PCB designers need to ensure that the propagation delay across the via does not negatively affect the timing of the signals. The delay is the time it takes for a signal to travel from one layer of the PCB to another through the via. In high-speed systems, even small delays can cause misalignment between signals, leading to timing errors and poor signal synchronization.

To optimize propagation delay, the via should be designed so that its length is minimized, and its structure is as compact as possible while maintaining the necessary impedance control. In some cases, reducing the via size and improving the via’s electrical properties can help mitigate excessive delay.

Design Challenges and Solutions for PTH Vias in mmWave Circuits

Parasitic Effects in Via Structures

At mmWave frequencies, parasitic elements within the via structure, such as inductance, capacitance, and resistance, can cause significant deviations in the expected impedance. These parasitic components can alter the behavior of the via, leading to signal loss, reflection, and distortion.

In a PTH via structure, the inductance arises from the loop formed by the via, the PCB traces, and the surrounding conductors. This inductive reactance can increase at higher frequencies, creating impedance mismatches and reducing signal fidelity. Similarly, parasitic capacitance between the via and nearby conductors can affect the signal, especially at mmWave frequencies.

By carefully controlling the design of the via structure, such as adjusting the via’s pad size and antipad dimensions, designers can mitigate these parasitic effects. Additionally, stitching vias can be used to provide additional grounding and reduce parasitic inductance.

Via Stub Considerations in mmWave Interconnects

A via stub is a portion of the via that remains unconnected to the signal path, creating an open-circuit termination. At mmWave frequencies, via stubs can cause strong reflections, especially if the stub length is too long relative to the signal wavelength. These reflections can cause insertion loss, signal distortion, and potentially harmful interference.

To minimize the impact of via stubs, designers should use backdrilling to remove the stub or, if possible, use blind vias that do not create stubs in the first place. Blind vias eliminate the need for drilling all the way through the PCB, offering a cleaner signal path and reducing unwanted reflections.

Advanced Techniques for mmWave Via Design

Using Grounded Coplanar Waveguides (GCPWs)

One of the most effective ways to improve PTH via performance in mmWave PCBs is to use grounded coplanar waveguides (GCPWs). GCPWs are ideal for routing high-frequency signals as they minimize signal loss and maintain controlled impedance. By routing signals on the surface of the PCB using GCPWs, the vias can be placed in a way that minimizes via-related signal degradation.

Using GCPWs also allows the via structures to remain compact while ensuring that the impedance remains stable across the signal path. The GCPW design helps suppress higher-order modes, which can otherwise introduce unwanted resonance and signal distortion.

Stitching Vias and Antipad Adjustments

Stitching vias are additional vias placed around the via structure to ensure proper grounding and to help control the impedance of the signal path. These vias are critical for maintaining a stable impedance spectrum and ensuring signal continuity across the interconnect. The number of stitching vias and their placement should be optimized to ensure that the impedance of the via structure remains consistent across a wide range of frequencies.

Adjusting the antipad size is another technique used to control the via impedance. The antipad is the area surrounding the via where the copper is removed. Proper sizing of the antipad ensures that the impedance is controlled, and it can help reduce the parasitic capacitance and inductive effects that degrade signal integrity.

Choosing the Right PCB Materials for mmWave Radar Circuits in Autonomous Vehicles

In autonomous vehicle technology, mmWave radar circuits play a crucial role in enabling key functionalities, particularly in Advanced Driver-Assistance Systems (ADAS). These systems rely on various electronic components, including radar sensors, to ensure safety and efficient operation. For radar circuits operating at frequencies above 77 GHz, such as those used for detecting obstacles and providing environmental awareness, choosing the right PCB material is essential. The PCB material must support the high-frequency signals typical of mmWave radar, with low loss and stable performance across a wide temperature range. Materials like those from Rogers, designed for mmWave applications, provide the necessary stability and consistency to ensure reliable signal transmission and minimal signal degradation.

Radar systems, an integral part of vehicle safety systems, rely on electromagnetic signals to detect targets and measure their distance, speed, and relative movement. In mmWave radar systems, PCB antennas play a significant role in transmitting and receiving these signals. For instance, radar signals are sent as electromagnetic waves that reflect off objects, allowing the radar system to determine the position, distance, and velocity of targets. With high-frequency operation at 77 GHz or 79 GHz, the PCB material used for the antenna must exhibit minimal signal loss and high precision to support the measurement of these critical parameters. Additionally, the materials used need to maintain their performance consistency despite changes in temperature, humidity, and other environmental factors, ensuring reliable operation in real-world conditions.

As autonomous vehicles evolve, the need for advanced radar systems with higher frequency capabilities becomes more pressing. Systems working in the 77 GHz and 79 GHz bands are becoming increasingly important due to their higher resolution, allowing vehicles to detect objects at greater distances and with higher accuracy. For these systems, the PCB materials must offer low dielectric loss, excellent thermal stability, and precise control over dielectric constants. Materials such as Rogers’ RO3003 and RO4835 are particularly suited for mmWave applications in automotive radar, providing the high-performance capabilities required for precise measurements. These materials ensure that radar systems can perform reliably in diverse driving conditions, contributing to the safety and functionality of autonomous vehicles.

Conclusion: Optimizing Plated Through-Hole Vias for mmWave PCB Performance

In summary, designing plated through-hole vias (PTH vias) for mmWave PCBs requires careful consideration of impedance matching, signal integrity, parasitic effects, and via stub management. At mmWave frequencies, the behavior of signals can change dramatically, and even small imperfections in the via design can have significant consequences on performance. By following best practices such as via sizing, parasitic management, backdrilling, and stitching vias, designers can ensure that their mmWave PCBs deliver optimal performance.

At Highleap Electronic, we specialize in high-frequency PCB design and can guide you through the complexities of mmWave signal routing. Our advanced simulation tools and expertise in via design ensure that your mmWave PCBs meet the stringent performance requirements of high-frequency applications. If you need assistance in optimizing your mmWave designs, contact us today for expert support in PCB manufacturing and assembly.

FAQs

What is the role of plated through-hole vias in mmWave PCB designs?

PTH vias provide electrical connections between different layers of a PCB but can impact signal integrity at high frequencies like mmWave. Proper design and impedance matching are crucial to minimize signal loss and reflections.

Why do mmWave PCBs require specialized via design compared to standard PCBs?

At mmWave frequencies, signal integrity is highly sensitive to impedance mismatches. PTH vias can introduce significant reflections and resonance if not properly designed, making it essential to ensure proper impedance control.

What are the key considerations for managing insertion loss in mmWave PCBs with PTH vias?

Insertion loss can be minimized by selecting appropriate materials, ensuring accurate via sizing, and avoiding resonance. Proper impedance control, especially in the via structure, is crucial for reducing signal degradation.

How do parasitic elements in vias affect mmWave signal transmission?

Parasitic inductance and capacitance in vias can distort signals at mmWave frequencies, leading to impedance mismatches and increased loss. These effects must be carefully managed to maintain signal integrity.

What is the significance of backdrilling in mmWave PCB design?

Backdrilling removes via stubs, which can create unwanted reflections and resonance, improving signal quality. It is particularly useful in mmWave PCBs, where even small imperfections can significantly affect performance.

How do you optimize via impedance for mmWave applications?

Optimizing via impedance requires careful attention to the via size, pad design, antipad size, and the placement of stitching vias. These factors ensure that the impedance remains consistent and matches the system requirements.