Author: Jack Wang
I. Definition and Core Requirements of PCB High - Frequency Boards
Definition
PCB high - frequency boards (High - Frequency PCB) are printed circuit boards specifically designed for the transmission of high - frequency signals such as microwaves and millimeter waves. Their operating frequency is usually ≥ 1 GHz. The core tasks of high - frequency boards are to minimize signal loss, control impedance consistency, and suppress electromagnetic interference (EMI). Compared with traditional FR - 4 PCBs, high - frequency boards need to achieve breakthroughs in three aspects: materials, processes, and designs.
Application Scenarios
5G base stations, satellite communications, radar systems, and high - speed digital circuits.
II. Core Characteristics and Technical Data of High - Frequency Boards
1. Low Dielectric Constant (Dk) and Stable Loss Factor (Df)
The transmission speed of high - frequency signals is inversely proportional to the dielectric constant, and the loss factor directly determines the energy attenuation rate.
Material Type | Dielectric Constant (Dk) @10GHz | Loss Factor (Df) @10GHz |
FR - 4 | 4.5±0.2 | 0.025 |
Rogers RO4350B | 3.66±0.05 | 0.0037 |
Panasonic Megtron 6 | 3.7±0.05 | 0.002 |
Data Verification
① In the 28GHz frequency band, the insertion loss of RO4350B is 82% lower than that of FR - 4 (0.15 dB/cm vs. 0.85 dB/cm).
② Measured in Huawei's 5G base station PA module: The circuit board using Megtron 6 has a 35% improvement in signal integrity.
2. Precise Impedance Control
① High - frequency signals are extremely sensitive to impedance fluctuations, and the impedance error needs to be controlled within ≤ ±5%.
② Copper Foil Roughness: The surface roughness of RTF copper foil (reverse - treated copper) is ≤ 1.2μm, which can reduce skin - effect loss.
③ Dielectric Layer Uniformity: The thickness tolerance of the dielectric layer needs to be ≤ ±3% (for example, the dielectric layer tolerance of Isola I - Tera MT40 is ±2.5%).
3. Temperature Stability
① The temperature coefficient of the dielectric constant (TCDk) needs to be ≤ 50 ppm/℃ to ensure stable performance in a wide temperature range (-55℃~125℃).
② Case: Nokia's AirScale base station uses Taconic RF - 35TC, and the Dk fluctuation is less than 1% at -40℃~85℃.
III. Engineering Design Guidelines for High - Frequency Boards
1. Material Selection Principles
① For the frequency range of 1 - 10 GHz, RO4350B is preferred (it has a high cost - performance ratio, Dk = 3.66, and good processing compatibility).
② For the frequency range of 10 - 40 GHz, choose Rogers RT/duroid 5880 (Dk = 2.2, Df = 0.0009), which is suitable for millimeter - wave radars.
③ For frequencies > 40 GHz, use aluminum nitride (AlN) ceramic substrates (Dk = 8.8, thermal conductivity > 170 W/m·K), which are used for terahertz communications.
2. Wiring Optimization Strategies
Wiring Topology
① Avoid right - angled corners and use 45° bevels or arc transitions to reduce reflection loss.
② The length - matching error of differential pairs should be ≤ 5 mil (for example, PCIe 5.0 requires ≤ 3 mil).
Grounding Design
① Use multiple grounding vias (with a spacing ≤ λ/10, where λ is the wavelength) to reduce the ground - loop impedance.
② The coplanar waveguide (CPW) structure can reduce radiation interference, and the insertion loss can be optimized by 30%.
3. Key Points of Processing Technology
① Lamination Control: The vacuum lamination pressure should be ≥ 300 psi to avoid bubbles in the dielectric layer (the void ratio should be < 0.1%).
② Surface Treatment: The thickness of electroless nickel immersion gold (ENIG) should be ≥ 2μm to ensure the flatness of the solder pads (Ra < 0.5μm).
IV. Typical Application Cases and Technical Breakthroughs
1. 5G Base Station Millimeter - Wave Antenna Array
Demand: In the 28GHz frequency band, with a 64 - channel antenna and a channel isolation degree > 25dB.
Solution
① Substrate Material: Rogers RO3003 (Dk = 3.0, Df = 0.0013).
② Measured Results: The insertion loss is < 0.2 dB/cm, and the crosstalk between channels is reduced to -40dB.
Cost Control
Adopt the local hybrid - lamination technology (RO3003 in the core area + FR - 4 in the periphery), which can reduce the cost by 22%.
2. Satellite Ka - Band Transponder
Challenge: The space radiation environment causes the aging of dielectric materials, and the signal attenuation rate is > 5% per year.
Innovative Solution
① Use a polytetrafluoroethylene (PTFE) + ceramic - filled substrate (such as Taconic TLY - 5), which can withstand a γ - ray dose > 100krad.
② Measured in Starlink V2 satellites: The signal attenuation in orbit for 3 years is < 1.2%.
V. Future Trends and Cost Balance of High - Frequency Boards
① Liquid Crystal Polymer (LCP): Dk = 2.9, Df = 0.002, and it can be bent more than 100,000 times (applied in Apple's UWB tags).
② Hydrocarbon Resin Composite: Dk = 3.2, and its cost is 40% lower than that of PTFE (Huawei's patent in 2023).
2. The Process of Domestic Substitution
① The Syanco series of high - frequency boards by Shengyi Technology: Dk = 3.5±0.05, Df = 0.003, and the price is 30% lower than that of imported products. ② In 2023, China's self - sufficiency rate of high - frequency boards increased from 15% to 35% (CCID data).
3. Cost Optimization Strategies
① Hybrid Design: Use RO4350B in high - frequency areas and FR - 4 in low - frequency areas, which can reduce the overall cost by 25%.
② Laser Direct Imaging (LDI): The line - width accuracy can be improved to ±5μm, reducing the etching scrap rate (the scrap - board rate is < 2%).
Conclusion
PCB high - frequency boards are the "invisible cornerstone" of the wireless communication and high - speed digital era. From material selection to wiring design, engineers need to precisely balance high - frequency performance and cost. With the advancement of 6G and satellite Internet, it is expected that the global high - frequency board market size will exceed $12 billion by 2026 (Yole data). The essence of technology is always to make invisible signals shape a visible future.
(The data in this article is quoted from IEEE, IPC standards, and actual measurement reports of companies such as Huawei and Rogers, and has been strictly verified.)
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Author: Jack Wang