In modern electronics manufacturing, particularly in fields like wireless communication, high-speed computing, and radar detection, high-frequency circuit boards play an indispensable role. They are no longer simple connecting components but are the core platform that carries and ensures the complete transmission of high-frequency signals. The primary factor determining the performance limits and applicable scenarios of a high-frequency circuit board is its core material—the substrate. Different high-frequency applications have vastly different requirements for the substrate’s dielectric properties, thermal stability, mechanical processability, and cost, which has led to the development of various high-frequency materials with distinct characteristics. This article will provide an in-depth introduction to the main types of materials covered by high-frequency circuit boards, analyzing their unique properties and application areas.
I. Core Performance Parameters of High-Frequency Materials: The Basis for Selection
Before delving into specific materials, it is essential to first understand several key electrical indicators that measure the performance of high-frequency materials:
Dielectric Constant (Dk): This is an indicator of a material’s ability to store electrical energy. In high-frequency applications, we seek a stable and low Dk value. A low Dk value means signals propagate faster through the dielectric medium with less delay. More critically, the stability of the Dk value is paramount; it should fluctuate minimally with changes in frequency and temperature to ensure stable impedance control and predictable signal accuracy.
Dissipation Factor (Df): Also known as the loss tangent, this measures the material’s ability to dissipate electrical energy (converted into heat). We pursue an extremely low Df value. A low Df means less energy loss (attenuation) during signal transmission, resulting in higher transmission efficiency. This is crucial for high-frequency, long-distance signal transmission and directly determines system performance and power consumption.
Coefficient of Thermal Expansion (CTE): This refers to the degree to which a material expands after heating. The CTE of the substrate needs to match the CTE of the copper foil as closely as possible, especially in the Z-axis (thickness) direction. Excessive CTE mismatch can lead to plated through-hole (PTH) fractures during temperature changes (such as multiple reflow soldering processes), causing interconnection failures.
Additionally, heat resistance (Tg value – glass transition temperature), thermal conductivity, moisture absorption, mechanical strength, and processing difficulty are also important considerations.
II. Main Types of High-Frequency Materials and Their Detailed Description
Based on the different resin systems of the base material, high-frequency materials are mainly divided into the following categories:
- Polytetrafluoroethylene (PTFE) Based Materials: The “Gold Standard” for High-Performance Applications
PTFE, commonly known as “Teflon,” is undoubtedly the king in the field of high-performance high-frequency applications. It inherently possesses an extremely low dielectric constant (Dk typically around 2.1) and an extremely low dissipation factor (Df can be as low as 0.0004), providing an almost lossless transmission path for signals.
Characteristics: Besides excellent electrical properties, PTFE also boasts exceptional chemical stability, high-temperature resistance, and weather resistance. However, its disadvantages are equally notable: Firstly, high cost; Secondly, poor processability. Pure PTFE is soft, has a high coefficient of thermal expansion, and its surface is inert, making it difficult to bond with copper foil. Special surface activation and etching treatments are required to achieve reliable hole metallization, which places extremely high demands on the manufacturing process.
Reinforcement and Composition: To improve its mechanical properties for easier processing, manufacturers incorporate various fillers into PTFE. The most common is ceramic-filled PTFE (e.g., Rogers Corporation’s RT/duroid 6002 series), which significantly enhances the board’s rigidity, stabilizes the Dk value, and reduces CTE. Another type is glass fiber woven fabric reinforced PTFE (e.g., Rogers Corporation’s RO3000 series), which further enhances dimensional stability but slightly increases loss.
Typical Applications: Satellite communication uplink/downlink circuits, military radar and seekers, high-speed RF test probes, and high-frequency channel boards in core parts of 5G base stations—fields with extreme performance requirements.
- Hydrocarbon Ceramic Based Materials: Excellent Balance between Performance and Cost
This type of material has become a mainstream choice in the mid-to-high-end high-frequency market in recent years, achieving an excellent balance between performance, processability, and cost.
Characteristics: It consists of a hydrocarbon resin (e.g., polyolefin) base filled with a large amount of inorganic ceramic powder (e.g., silicon dioxide). By adjusting the ceramic filler ratio, materials with different Dk values (e.g., between 3.0 and 10) can be precisely “formulated.” Their Df values are usually very low (can reach around 0.001). Although slightly inferior to top-tier PTFE materials, they fully meet the vast majority of commercial needs.
Advantages: Its biggest advantage lies in processability. Its rigidity, CTE, and drilling characteristics are closer to those of traditional FR-4 material, allowing PCB manufacturers to use adjusted existing FR-4 process equipment for production, greatly reducing manufacturing difficulty and cost. Furthermore, its moisture absorption is much lower than PTFE, resulting in more stable performance.
Typical Applications: 5G Massive MIMO antenna array boards, automotive collision avoidance radar (77GHz) boards, satellite TV receiving systems (DBS), point-to-point microwave transmission systems, and other high-volume commercial fields.
- Modified Epoxy Resin/PPO Based Materials: A Smooth Transition from FR-4 to the High-Frequency Domain
This type of material can be seen as an upgraded version of standard FR-4 material, aiming to improve the high-frequency performance of FR-4 at a lower cost.
Characteristics: By introducing low-loss thermoplastics, such as polyphenylene oxide (PPO) or polystyrene (PS), into the standard epoxy resin system, the overall Dk and Df values are reduced. Its Dk is typically between 3.5 and 4.0, and Df is around 0.008. While it cannot compete with the first two categories, it represents a significant improvement over standard FR-4 (Df ~0.02).
Advantages: Its core advantages are excellent processing compatibility and low cost. It is almost fully compatible with the entire FR-4 manufacturing process, including drilling, hole metallization, and lamination, requiring no equipment modification or special chemical treatments, resulting in a very low learning curve for manufacturers.
Typical Applications: Mainly used in scenarios operating around 1-3GHz that are cost-sensitive but have certain signal integrity requirements, such as GPS antenna boards for automotive infotainment systems, wireless modules for some IoT devices, and high-speed digital circuit sections of some high-end server motherboards.
- Liquid Crystal Polymer (LCP) Based Materials: The Rising Star for Flexible High-Frequency Applications
LCP is a new type of high-performance thermoplastic material showing great potential in the field of flexible high-frequency circuits.
Characteristics: LCP has a very low Dk (2.9-3.1) and Df (0.002-0.004), but its most outstanding features are extremely low moisture absorption (<0.04%) and excellent hermeticity. This means its electrical performance remains highly stable in humid environments. Additionally, it is inherently suitable for manufacturing ultra-thin, flexible circuit boards.
Advantages: Ultra-thin flexibility and stable performance. Its moisture absorption rate is much lower than PI (polyimide, the traditional flexible board material), avoiding performance drift caused by moisture absorption. It is very suitable for the manufacture of multi-layer flexible boards.
Typical Applications: Currently mainly used in areas requiring extremely high frequencies and miniaturization, such as internal millimeter-wave 5G antenna modules in smartphones, high-speed connectors, micro-sensors, and wearable medical devices.
Conclusion
Selecting the appropriate high-frequency material is a complex systems engineering task that requires careful balancing of electrical performance (Dk/Df), reliability (Tg/CTE), manufacturability (processing difficulty), and total cost. No single material is a perfect universal choice: PTFE offers top-tier performance but comes with the highest cost and processing challenges; hydrocarbon ceramics achieve the best commercial balance; modified epoxy resin is a low-cost upgrade option; and LCP stands out in the flexible millimeter-wave domain. Understanding the essential characteristics of these materials is the first step towards designing successful high-frequency electronic products.