What Challenges Do 5G Antenna PCBs Face? High Frequency, Low Loss, Integration, and Thermal Conductivity

With the continuous advancement of 5G technology, PCB (Printed Circuit Board) antennas are becoming increasingly important. These antennas are critical components in wireless communication, designed to enable wireless reception and transmission functions on PCBs. In this article, we will take an in-depth look at the different types of PCB antennas and their importance in 5G communication.

1. Understanding the Basic Principles of PCB Antennas

Before delving into PCB antennas, let’s first understand their basic principles. A PCB antenna is essentially a transducer capable of converting high-frequency current into electromagnetic waves in space (transmission) or converting electromagnetic waves intercepted from space into high-frequency current and feeding it into a receiver (reception).

From a circuit perspective, an antenna isConvert the guided wave on a transmission line into a free-space wavetransducer. The four key metrics used to evaluate it are:

• Resonance Frequency: The physical dimensions of an antenna are related to wavelength and are typically set to λ/2 or λ/4. The higher the frequency, the shorter the wavelength, and the smaller the antenna—this is the physical basis for 5G’s ability to integrate antennas into PCBs.
• Return Loss S11 / VSWR: Measures the degree of impedance matching. In engineering practice, it is generally required that S11 ≤ −10 dB (VSWR ≤ 2:1) within the operating frequency band, meaning that at least 90% of power is radiated outward rather than reflected back.
• Gain and Directivity Patterns: Gain measures the degree of energy concentration, while a radiation pattern describes the radiation intensity in various directions. Mobile phone antennas are required to be nearly omnidirectional, while base station antennas are required to be directional with high gain.
• Efficiency: The ratio of radiated power to input power. Due to the combined effects of dielectric loss, conductor loss, and matching loss, an efficiency of 50–70% for a 5G antenna board is considered quite good.

2. Requirements for PCB Antennas in 5G Communications

5G communication technology has higher frequency requirements than previous mobile communication technologies, which poses new technical challenges for PCB antennas. The following are some of the main challenges:

2.1 Low Loss at High Frequencies

As 5G frequencies continue to increase, PCB antenna materials must exhibit lower loss characteristics to ensure efficient signal transmission.

There are three mechanisms by which loss increases with frequency; understanding them is essential to addressing the problem effectively:

• Dielectric Loss: It is proportional to the tangent of the loss angle, Df; as the frequency doubles, the dielectric loss increases approximately linearly. This is the primary source of loss at high frequencies.
• Conductor Losses: The skin effect of high-frequency currents causes the current to flow only in a thin layer on the surface of the copper; the higher the frequency, the shallower the skin depth, and the greater the equivalent resistance. At 10 GHz, the skin depth of copper is approximately 0.66 μm; if the surface roughness (Rz) of the copper foil reaches 3–5 μm, the current path is significantly lengthened, resulting in an additional 20–50% increase in losses.
• Radiation Loss and Surface Waves: Thick dielectric materials with high Dk induce more surface waves, causing energy to be ”trapped” within the board rather than radiated outward, which directly reduces antenna efficiency.

2.2 Improvements in Antenna Integration

5G MIMO (Multiple-Input Multiple-Output) systems require a higher degree of antenna integration, which calls for more complex PCB antenna designs to support multi-antenna systems.

The order-of-magnitude change in integration is as follows: 4G devices typically have 2–4 antennas, 5G Sub-6G devices generally have 4–8 antennas, while millimeter-wave devices haveAntenna Array—Integrating 4, 8, 16, or even more antenna elements with the RF front-end into a single package (AiP, Antenna-in-Package). This results in three interdependent requirements: the spacing between antenna elements must be strictly controlled to λ/2; the feeding network must ensure phase coherence among all antenna elements; and there must be sufficient isolation between antenna elements (generally required to be ≥15 dB).

2.3 Thermal Conductivity Requirements

Since 5G communication equipment needs to integrate more functions into a smaller space, the thermal conductivity of PCB antenna materials has also become a key consideration.

Power amplifiers in millimeter-wave front-ends are highly efficient but have high thermal density; combined with the fact that baseband and RF components are on the same board, thermal design becomes a critical constraint. Conventional FR-4 has a thermal conductivity of only 0.3–0.4 W/(m·K), making it virtually non-thermally conductive; There are four approaches to improving this: switching to high-thermal-conductivity materials (1.0–2.0 W/(m·K)), adding an array of thermal vias (0.2–0.3 mm in diameter, 0.6–1.0 mm apart), locally embedding copper coins, or using metal substrates or metal brackets for thermal conduction. Refer to the following for specific selection guidelines:High-Frequency Microwave Board Selection Guide。

3. The frequency band is everything: Sub-6G and millimeter wave are two distinct design approaches

5G is not a single frequency band, but rather two distinct frequency ranges, and the PCB designs for the two are almost entirely different:

For technical details on the millimeter-wave band, seeUnderstanding 5G High-Frequency Boards, Millimeter Wave, and Their Impact on the Communications Industry. Choosing the wrong frequency band is arguably the most common cause of rework in 5G antenna design—applying Sub-6G experience directly to millimeter-wave designs is almost certain to fail.

4. The Four Quadrants of Material Selection: Dk, Df, CTE, Thermal Conductivity

When selecting materials for antenna boards, you shouldn’t just look at Df; you need to consider all four parameters together:

There are also two parameters that are easily overlooked:Dk Tolerance and Temperature Stability(Dk drift directly alters the resonant frequency, which is particularly critical at millimeter-wave frequencies),Z-Axis CTE(Affects the reliability of plated-through holes; low-CTE materials are commonly used in high-frequency boards). For a complete comparison of materials, seePCB Materials。

5. Different Types of PCB Antennas

There are various types of PCB antennas designed for different 5G applications. Here are some common types:

5.1 PCB Loop Antenna

For low-power, short-range RFID systems, PCB loop antennas are often used. They offer reliable communication and are inexpensive, making them ideal for these applications.

5.2 Phased-Array Antenna

In densely populatedBase StationIn wireless networks, phased-array antenna technology is used between base stations and terminal devices. This technology enables higher user density and increases the system's user capacity.

Compile a selection table listing the common types:

6. The Innovative Features of 5G

5G communications bring many exciting new features, some of which include:

6.1 Ultra-High-Speed Transmission

With 5G transmission speeds ranging from 10 to 50 Gbps, we can download ultra-high-definition movies in an instant.

6.2 Low Latency

5G networks feature low latency, which is critical for supporting IoT applications.

6.3 High Reliability

5G communications are highly reliable, which means we can better support critical applications such as telemedicine and autonomous driving.

When applied to PCBs, these three key characteristics translate into three specific requirements: speed requirementsLink Loss BudgetControllable (including board material loss, connectors, and vias); delay requirementsReducing Relays and Retransmissions, placing higher demands on signal integrity; reliability requirementsNo performance degradation under temperature and humidity cycling and vibration, that is, consistency in materials and manufacturing processes. The four characteristics of the RF circuit itself are described inFour Key Characteristics of PCB RF Circuits。

7. Design Implementation: Seven Key Points to Keep in Mind

• Keep-out Zone: Do not place copper, components, or traces beneath or around the antenna radiating element; the clear zone should generally be at least 1.5 times the size of the antenna. This area is the most expensive part of a cell phone design.
• Feeder Point Impedance: 50 Ω single-ended or 100 Ω differential; keep the feedline as short and straight as possible; use rounded corners or 45° angles; avoid sudden changes in impedance.
• Reference Site Integrity: The antenna requires a complete ground plane to act as a mirror; slots or notches in the ground plane will significantly alter the radiation pattern and resonance frequency.
• Array Element Spacing: Millimeter-wave arrays are spaced at λ/2 (approximately 2.5–6 mm in air); if the spacing is too large, sidelobes are generated, and if it is too small, mutual coupling increases.
• Via Processing: Vias on the RF path introduce parasitic inductance; therefore, back drilling or blind vias should be used. Ground vias should be densely arranged to form a ”via wall.” For key points on via design, seeThe Importance of Via Holes in Multilayer PCB Design。
• Layer Symmetry: The laminate structure of high-frequency PCBs must be symmetrical; otherwise, warping after lamination will alter the flatness of the antenna, which has a significant impact at millimeter-wave frequencies.
• The Impact of Enclosures on the Human Body: Millimeter-wave antennas must be placed near the opening in the enclosure, and the "hand effect" must be evaluated; this is typically the most time-consuming part of terminal antenna debugging.

8. Manufacturing Side: Four Technical Challenges in 5G Antenna Board Production

• Media Thickness Control: The antenna's resonant frequency is directly determined by the dielectric thickness and Dk; after lamination, the thickness tolerance must be controlled within ±5%, which is twice as strict as for conventional boards.
• Line Width Accuracy: Millimeter-wave feedlines are typically 50–100 μm wide; a deviation of just 5 μm in line width can cause the impedance to deviate by 2 Ω, requiring LDI exposure and precise etching control.
• Drilling Methods: 5G boards have a large number of micropores; mechanical drilling cannot handle such small diameters, so UV/CO2 laser drilling must be used. The drilling sub-processes vary significantly depending on the material; seeThree Common Drilling Methods for 5G PCBs。
• Copper Plating Uniformity: Microporous copper-filled structures and fine-line electroplating place high demands on the dispersion capabilities of the plating solution; uneven copper plating can alter line width and impedance. Common issues include:Common Issues and Solutions for the 5G PCB Copper Plating Process。

When designing a 5G antenna board, we recommend following these five steps to avoid repeated rework later on:

• Step 1: Select the Frequency Band and Standard... Clarifying whether it is Sub-6G or millimeter wave is a prerequisite for all subsequent decisions.
• Step 2: Select Panels by Frequency Band, and verify the Dk tolerance, Z-axis CTE, and thermal conductivity.
• Step 3: Determine the Stacking Configuration and Antenna Type, Include the substrate thickness tolerance and copper foil type (low profile) in the stack-up diagram.
• Step 4: Simulation...Use 3D electromagnetic simulation software to perform joint simulations of the antenna and feed network, thereby identifying clearance and mutual coupling issues in advance.
• Step 5: Create a Prototype and Conduct Field Testing, First, measure the passive S11; then, go into the anechoic chamber to measure TRP/TIS; finally, perform a reliability retest.

9. Testing and Verification: How to Confirm That the Antenna Meets the Requirements

Antenna boards should not be tested solely for continuity; they must be verified on three levels:

• Passive Testing: Use a vector network analyzer to measure S11 (return loss) and isolation, and verify that the resonance point falls within the target frequency band and that S11 ≤ −10 dB.
• Active Testing: Measure TRP (Total Radiated Power) and TIS (Total Isotropic Sensitivity) in a anechoic chamber; these are mandatory criteria for network acceptance testing by telecom operators.
• Reliability Testing: After temperature and humidity cycling, thermal shock, and vibration testing, retest S11 to confirm there is no drift; for the millimeter-wave board, also measure gain variations at high and low temperatures.

On the production line, consistency must be ensured: Conduct spot checks on the dielectric thickness and line width for each batch, and use a test fixture to perform a quick S11 screening to catch batch deviations before shipment. For the management guidelines for 5G boards, seeManagement Guidelines for 5G PCBs。

Conclusion

In summary, PCB antennas are a key component of 5G communications, and their design and performance are critical to the performance of the entire communication system. As 5G technology continues to evolve, we can expect to see more innovations and improvements to meet the growing demand for communication.

Frequently Asked Questions

1. What is 5G?

5G is the fifth-generation mobile communication technology. It is characterized by high-speed data transmission, low latency, and high reliability, and is a key technology for future communications.

2. What is the role of PCB antennas in 5G?

PCB antennas are used in 5G to enable wireless reception and transmission functions and are a key component of communication systems.

3. Why does 5G require a higher degree of antenna integration?

5G MIMO systems require more antennas to support multi-antenna communication, and therefore require a higher degree of antenna integration.

4. How fast is 5G?

5G transmission speeds can reach up to 10–50 Gbps, which is many times faster than 4G.

5. For which applications is 5G’s low latency particularly important?

The low latency of 5G is crucial for key applications such as telemedicine and autonomous driving, as it enables real-time communication.

6. Do 5G antenna boards have to be made of high-frequency materials?

It depends on the frequency band. For the low-to-mid-frequency Sub-6G bands (such as n1, n3, and n41), low-loss FR-4 or mid-range high-frequency materials are sufficient, keeping costs under control; for n77/n78/n79 (3.3–5 GHz), materials with a Df of ≤ 0.008 are recommended; and for millimeter-wave applications, high-end materials such as PTFE or hydrocarbon ceramics are required, as the loss of standard FR-4 at 28 GHz is completely unacceptable.

7. What are the common causes of low antenna efficiency?

In order of frequency: First, the clear zone is occupied (by copper layers or components placed underneath); second, the Df value of the board is too high; third, excessive copper foil roughness leads to increased conductor loss; fourth, the matching network has not been properly tuned, and S11 has not been reduced. The first three issues must be resolved during the design phase, as they are difficult to remedy later on.

8. Why are millimeter-wave antennas prone to signal blockage?

With wavelengths as short as 5–12 mm, human hands, the device casing, and even components on the circuit board can be comparable in size to the wavelength, causing obstruction and scattering. The solution involves using a multi-module configuration (placing 3–4 millimeter-wave modules at different locations on the device) that the system switches between as needed; this is also one of the reasons for the high cost of millimeter-wave terminals.

9. Why is the tolerance for the dielectric thickness of the antenna board set to ±5%?

The resonant frequency of a microstrip patch is approximately inversely proportional to the dielectric thickness. A thickness deviation of 5% can cause the resonance point to shift by 2–3%. In the millimeter-wave band, this corresponds to several hundred MHz to over 1 GHz—enough to cause the entire frequency band to fall outside the specified specifications. Therefore, the lamination tolerances for antenna boards must be twice as tight as those for conventional boards.

10. Why can't copper be laid beneath the antenna?

Antennas radiate based on current distribution; placing copper beneath them creates a mirror image of the current, altering the current path and near-field distribution, which causes a shift in the resonance frequency, a decrease in gain, and distortion of the radiation pattern. Multilayer boards typically employ the practice of “hollowing out the antenna projection area on all layers,” retaining only the necessary feed layers.

Related Reading：The Importance of Via Holes in Multilayer PCB Design · An In-Depth Look at the Features and Advantages of Protel 99 · High-Frequency PCBs