High frequency PCBs are the core substrates that support the operation of highend equipment such as communication base stations, automotive radar systems, and satellite communication systems. Their primary function is to carry and transmit High frequency electrical signals at high speed. In the industry, PCBs operating at frequencies above 1 GHz are generally classified as High frequency PCBs.
Compared with conventional lowfrequency circuit boards, High frequency PCBs are subject to much stricter requirements for signal transmission accuracy and operational stability. At high frequencies, electromagnetic waves are highly susceptible to energy loss during transmission, while electromagnetic interference (EMI), signal waveform distortion, and other issues become increasingly prominent. These problems can directly affect equipment operating accuracy and may even shorten the service life of the entire system. Therefore, selecting the appropriate high frequency PCB substrate according to the actual operating conditions is a critical step in maintaining equipment performance and improving overall operational reliability.
Key Material Selection Criteria for High frequency PCBs
1.Low Signal Loss Characteristics (Loss Tangent)
Dissipation factor (Df), also known as loss tangent (tan δ), is a key parameter for evaluating the signal transmission performance of High frequency PCB materials. The higher the Df value, the more electrical energy is converted into heat and lost during signal transmission. This not only reduces transmission efficiency but also degrades signal integrity. To maintain High frequency signal integrity and transmission quality, materials with a dissipation factor below 0.003 are generally preferred for demanding High frequency applications, minimizing the adverse effects of signal attenuation.
The loss characteristics vary significantly among different substrate materials. Rogers RO4350B has a dissipation factor of approximately 0.0037 and is suitable for most commercial mid to High frequency applications, making it a mainstream choice for volume production of communication equipment. PTFE (polytetrafluoroethylene), by comparison, can achieve a dissipation factor below 0.001. Its ultralowloss characteristics make it suitable for demanding ultra High frequency and highprecision signal transmission applications.
2.Stable Dielectric Constant (Dk)
The dielectric constant (Dk) directly affects signal propagation velocity and propagation delay, making it a critical parameter for High frequency transmission performance. When selecting materials, it is important not only to consider a low Dk value but also to evaluate its stability under different environmental conditions. If the dielectric constant changes significantly with variations in temperature and humidity, signal delay and transmission instability may occur, adversely affecting equipment operation.
Among mainstream commercial High frequency materials, the dielectric constant of the Rogers RO4000 series is approximately 3.48, with minimal variation across a broad frequency range, making it suitable for conventional High frequency transmission applications. PTFE has an even lower dielectric constant of approximately 2.1, providing both a lower Dk and excellent stability. This helps reduce signal propagation delay and improve High frequency transmission performance.
3.Excellent HighTemperature Thermal Stability
As the operating frequency of electronic equipment increases, the temperature rise of the PCB during operation can also become more significant. Under prolonged exposure to high temperatures, conventional PCB materials may experience performance degradation, warpage, delamination, or cracking, potentially resulting in equipment failure. High frequency PCB substrates must therefore provide excellent thermal stability and maintain their electrical and mechanical properties under sustained hightemperature conditions.
Different PCB materials have different longterm temperature capabilities and are suited to different applications:
Standard FR4 material (Tg 140°C): maximum longterm operating temperature of approximately 120°C, suitable mainly for lowfrequency, roomtemperature applications;
HighTg FR4 material (Tg 170°C): maximum longterm operating temperature of approximately 150°C, suitable for conventional electronic equipment operating at moderate temperatures;
Hydrocarbonbased High frequency laminates: longterm temperature resistance of approximately 140°C, primarily used in midrange commercial High frequency applications;
Polyimide (PI) laminates, such as Shengyi SH260 and Taconic VT901: temperature resistance of up to approximately 260°C, suitable for demanding hightemperature applications;
Aluminumbased PCBs: maximum longterm operating temperature of approximately 200°C, primarily used in electronics requiring enhanced heat dissipation;
Ceramic substrates: temperature resistance of up to approximately 280°C, with excellent hightemperature resistance and electrical insulation, making them suitable for highend specialized equipment.
Among these materials, RO4350B offers particularly good thermal stability. It can operate continuously and reliably at temperatures of approximately 140°C, making it suitable for the typical temperaturerise conditions of most commercial High frequency equipment.
4.Reliable Overall Mechanical Properties
The manufacturing process for high frequency PCBs is complex and involves multiple processes, including hightemperature lamination, precision drilling, and chemical etching. Throughout production, the substrate is subjected to high temperatures, high pressures, and mechanical stresses. Therefore, the substrate must provide not only excellent electrical properties but also sufficient mechanical strength and rigidity.
Good tensile and flexural strength can effectively reduce problems such as PCB warpage, deformation, and cracking during manufacturing. This helps improve manufacturing yield and process stability while maintaining the structural integrity of the equipment during longterm operation.
Performance and Application Comparison of Mainstream High frequency PCB Materials
1.Rogers RO4000 Series (RO4350B)
The RO4000 series is a glassfiberreinforced composite High frequency laminate developed by Rogers Corporation. It fills the gap between conventional FR4 materials, which have limited High frequency performance, and highend PTFE materials, which generally have higher material costs. It is one of the most widely used and mainstream substrate materials for midrange High frequency applications.
Its key parameters are well balanced. The dielectric constant is approximately 3.48 ± 0.05, with little variation across a broad frequency range from 1 GHz to 10 GHz, providing stable signal transmission. At 10 GHz, its dissipation factor is approximately 0.0037, significantly lower than the typical value of approximately 0.02 for conventional FR4 materials, substantially reducing signal attenuation. Its glass transition temperature (Tg) can reach approximately 280°C, making it fully compatible with standard leadfree soldering processes and providing excellent manufacturingprocess compatibility.
From a manufacturing perspective, its mechanical properties are relatively similar to those of conventional FR4. Dedicated processing equipment is generally not required, allowing it to be manufactured using conventional PCB production lines. Its relatively straightforward processing characteristics and controllable massproduction costs give it a strong overall costperformance advantage.
RO4350B is mainly suitable for mid to High frequency applications in the 1–10 GHz range and is widely used in commercial applications such as short and mediumrange wireless communications, basestation antennas, GPS positioning modules, and conventional RF equipment.
However, this material also has limitations. At frequencies above 10 GHz, its signal loss becomes significantly higher than that of PTFE materials, making it less suitable for millimeterwave and ultra High frequency precision equipment requiring extremely low transmission loss.
2.PTFE (Polytetrafluoroethylene, Teflon)
PTFE is widely recognized as a premium high frequency PCB substrate. With its ultralow loss and highly stable electrical characteristics, it has become a preferred material for microwave, millimeterwave, and ultra High frequency precision circuits. It is commonly used in demanding applications such as aerospace and highend radar systems where high accuracy and reliability are required.
Its High frequency transmission performance is excellent. PTFE can have a dielectric constant as low as approximately 2.1 ± 0.02, with the parameter remaining relatively stable across a broad frequency range. This provides high signal propagation velocity and extremely low propagationdelay variation. At 10 GHz, its dissipation factor can be as low as approximately 0.001–0.002, minimizing signal attenuation and meeting the requirements of highprecision ultra High frequency signal transmission.
PTFE also provides excellent environmental stability and can operate reliably over a wide temperature range of approximately 55°C to 260°C. It also offers excellent chemical resistance and aging resistance.
However, PTFE presents significant manufacturing challenges. Pure PTFE is relatively soft and has insufficient mechanical strength, so glass fiber or other reinforcing structures are generally incorporated to improve rigidity. Even with reinforcement, PTFE PCB fabrication remains relatively complex and requires highprecision manufacturing equipment and tightly controlled process conditions. As a result, its massproduction difficulty and cost are significantly higher than those of conventional High frequency laminates.
PTFE is primarily used for ultra High frequency applications above 10 GHz, including satellite communications, automotive millimeterwave radar, highend microwave and RF equipment, and precision aerospace communication systems. Its ultralow signal loss, exceptional transmission stability, and excellent environmental resistance are its key advantages, while high material cost and demanding fabrication requirements remain the main barriers to wider adoption.
High frequency PCB material selection should not focus solely on achieving the highest possible performance. The key is to match the material to the actual application requirements while balancing electrical performance, operating conditions, manufacturing requirements, and cost.
