With the development of wireless communications and broadband networks, high-frequency circuit boards are no longer simply metal traces laid on an insulating substrate to provide interconnections. In many cases, the substrate and metal conductors have become integral parts of functional components. Particularly in RF applications, components interact with the substrate; therefore, the design and manufacture of high-frequency circuit boards have an increasingly significant impact on product functionality.
High-Frequency Printed Circuit Boards
As a manufacturer of high-frequency circuit boards, we are primarily involved in design-related work, particularly in the area of high-frequency, high-speed signal transmission. Similarly, designers must have a deep understanding of the manufacturing processes for high-frequency circuit boards in order to consistently produce high-quality, high-performance circuit boards.
Starting with this issue, we will introduce some parameters that people frequently encounter and explore them from basic to advanced levels, with the hope of fostering better communication and collaboration between design and manufacturing.
Dielectric constant
The dielectric constant (Dk, ε, Er) determines the speed at which electrical signals propagate through a dielectric. The speed of electrical signal propagation is inversely proportional to the square root of the dielectric constant. The lower the dielectric constant, the faster the signal propagates. Let’s use a vivid analogy: imagine you’re running on a beach with water up to your ankles. The viscosity of the water represents the dielectric constant. The more viscous the water—that is, the higher the dielectric constant—the slower you’ll run.
The dielectric constant of high-frequency RF circuit boards is difficult to measure or define. It depends not only on the properties of the dielectric material itself, but also on the test method, test frequency, and the state of the material before and during testing. The dielectric constant also varies with temperature, and temperature factors are taken into account during the development of certain specialized materials. Humidity is another important factor affecting the dielectric constant, as water has a dielectric constant of 70, meaning even a small amount of water can cause a noticeable change.
The following are the dielectric constants of some typical materials (at 1 MHz):
It is evident that for high-speed, high-frequency applications, the most ideal material is an air dielectric wrapped in copper foil, with a thickness tolerance of ±0.00001”. In terms of material development, everyone is working toward this goal. For example, Arlon’s patented Foamclad is very well-suited for base station antenna applications; however, not all high-frequency RF lines are designed with a low dielectric constant, as this is often based on practical design considerations. Volume-sensitive applications often require materials with high dielectric constants; for example, Arlon AR1000 is used in miniaturized circuit designs, with some examples as follows: Power amplifiers commonly use a dielectric constant of 2.55 (such as Arlon DIClad 527, AD255, etc.), or a dielectric constant of 3.5 (such as AD350, 25N/FR, etc.). Some designs also use a dielectric constant of 4.5 (such as AD450). This shift is primarily driven by the transition from FR-4 designs to high-frequency applications, with the goal of further refining previous designs.
In addition to directly affecting signal propagation speed, the dielectric constant also largely determines the characteristic impedance. Characteristic impedance matching is particularly important in microwave communications across different sections. If impedance mismatch occurs, it is referred to as VSWR (Voltage Standing Wave Ratio).
CTEr: Since the dielectric constant varies with temperature, and the materials used in high-frequency printed circuit boards for microwave applications are often exposed to outdoor environments or even space, the CTEr (Coefficient of Thermal Expansion) is also a key parameter. Some ceramic powder-filled polytetrafluoroethylene (PTFE) materials can exhibit excellent properties, such as CLTE.
Loss, tangent of the loss angle, Df, dissipation factor
In addition to the dielectric constant, the loss factor is an important parameter that affects a material’s electrical properties. Dielectric loss—also known as the tangent of the loss angle or the loss factor—refers to the loss of a signal in a dielectric, or energy loss. This occurs because when high-frequency signals (which continuously alternate between positive and negative phases) pass through a dielectric layer, the molecules within the dielectric attempt to align themselves with these electromagnetic signals; however, in reality, because these molecules are cross-linked, they cannot truly align. Nevertheless, the frequency fluctuations cause the molecules to move constantly, generating a significant amount of heat and resulting in energy loss. However, some materials, such as PTFE (polytetrafluoroethylene), are nonpolar and therefore not affected by electromagnetic fields, resulting in low loss. Similarly, the loss factor is also related to frequency and the test method. As a general rule, the higher the frequency, the greater the loss.
The most obvious example is the loss of electrical energy during transmission. If a circuit is designed to minimize losses, battery life can be significantly extended. When receiving a signal, an antenna made of lossy materials is more sensitive to the signal, resulting in a clearer reception.
The commonly used FR4 epoxy resin (Dk 4.5) is relatively polar. At 1 GHz, its loss is approximately 0.025, whereas the loss of PTFE (polytetrafluoroethylene) substrate (Dk 2.17) is 0.0009. Compared to glass-filled polyimide, quartz-filled polyimide not only has a lower dielectric constant but also lower loss, due to its high quartz content.
Thermal conductivity
In many areas of microwave technology, there are numerous high-power applications. The thermal dissipation characteristics of materials can significantly affect the reliability of the entire system. Therefore, thermal conductivity must also be taken into account. For certain specialized applications requiring high reliability and high power dissipation, metal-backed substrates (aluminum-based or copper-based) may also be used.
Manufacturability
We understand that PTFE (polytetrafluoroethylene) is difficult to process, particularly when it comes to via metallization, which requires plasma or sodium naphthalide treatment to enhance its reactivity. Furthermore, since PTFE is a thermoplastic material, the processing temperature for multilayer boards is high. Now, new low-loss thermosetting resin materials have been developed for high-frequency circuits, enabling the fabrication of multilayer circuit boards without the need for plasma activation, such as Arlon 25N/FR. These materials are currently widely used in LNA, PA, and antenna designs. Moisture absorption is also a factor to consider. Materials with low moisture absorption should be selected whenever possible to ensure more stable electrical characteristics.
Coefficient of Thermal Expansion (CTE)
The coefficient of thermal expansion (CTE) of high-frequency circuit boards is one of the material’s key thermodynamic properties. It refers to the expansion of a material when heated. In practice, material expansion refers to a change in volume; however, due to the characteristics of the substrate, we often consider expansion separately in the in-plane (X-, Y-) and out-of-plane (Z-) directions.
Lateral thermal expansion can often be controlled using reinforcing layer materials (such as glass cloth, quartz, or Thermomount), whereas longitudinal expansion is always difficult to control above the glass transition temperature.
The CTE of the substrate is critical for mounting high-density packages. If a chip (typically with a CTE of 6–10 ppm/°C) is mounted on a standard high-frequency PCB (CTE 18 ppm/°C), repeated thermal cycling may cause the solder joints to age prematurely due to stress. The CTE in the Z-axis directly affects the reliability of plated-through holes, especially in multilayer boards.
PTFE generally has a relatively high CTE. Pure PTFE is rarely used to manufacture multilayer boards. PTFE filled with ceramic powder is commonly used instead.
It is used in global communications satellites.
Passive Intermodulation (PIM)
Front-end designs for RF and high-frequency circuit boards—such as antennas and filters—require passive intermodulation (PIM) control, which is also related to the substrate material used in high-frequency circuit boards. Some companies use specific types of copper foil to keep PIM within a certain range. The table below shows the differences in PIM between high-frequency PCB materials with no passive intermodulation requirements and those with specific PIM requirements.
Causes of Passive Intermodulation
Passive intermodulation is primarily caused by passive nonlinearities, of which there are typically two types: nonlinearities caused by metal contacts and nonlinearities inherent to the material itself. For example, coaxial cables and connectors are generally considered linear, but they exhibit nonlinear effects under high-power conditions. Contacts in the cable braid, connector threads, and other metal joints do exhibit slight nonlinearity. Each surface of these metal contacts has a thin insulating layer formed by metal oxidation. It is precisely this contact nonlinearity that generates low-level passive intermodulation interference, which significantly degrades receiver performance.
The main causes of nonlinearity in metal contacts are loose connections and corrosion. Its volt-ampere characteristic is a curve. The specific mechanisms are as follows:
- Nonlinearity caused by poor installation practices;
- Nonlinearity associated with high currents at points of contact with metal;
- Nonlinearities associated with dirt, metal particles, and carbonization on metal surfaces;
- Secondary electron multiplication through pores and microcracks in metal structures;
- The electron tunneling effect and semiconductor behavior at metal contacts through a thin oxide layer (less than 50 Å thick);
- Thermal cycling caused by the relative motion of metal contact surfaces due to high current.
There is no strict distinction between linear and nonlinear behavior. Metal contacts are generally considered linear, but they exhibit nonlinear effects at high power levels.
Nonlinear effects cannot be completely eliminated; they can only be minimized as much as possible.
The main emission reduction measures include:
- Minimize thermal cycling to reduce nonlinear contact caused by the expansion and contraction of metallic materials.
- Minimize the number of metal contacts. For example, use choke connections or other dielectric connections to provide sufficient current paths, ensuring that all mechanical connections remain clean and tight.
- Avoid using adjustment screws, metal parts, or moving metal components in current paths whenever possible. If necessary, place them in areas with low current density.
- Improve the joining processes for materials. Ensure reliable joints and minimize gaps, contamination, and corrosion as much as possible.
- The current density in the conductive path should be kept low. For example, the contact area should be large, and the conductor should be large.
Due to the complexity of passive intermodulation problems, it is difficult to establish circuit models for high-power circuits; therefore, some methods for analyzing nonlinear circuits cannot be applied. However, the nonlinearity of metal contacts can be represented by a simple system. By simulating the entire process of metal-contact nonlinearity generation using a single transfer function and applying the input-output method for analysis, the primary solution methods include the power series method and the Volterra series method. Since the power series method offers advantages such as simplicity, fast computation, and ease of implementation, this method is adopted in this paper.
Summary
The selection of microwave materials for high-frequency circuit boards primarily depends on the dielectric constant, loss, coefficient of thermal expansion, and thermal conductivity. Low-cost, low-loss, thermosetting, high-dielectric-constant ceramic-filled PTFE (polytetrafluoroethylene), PTFE (polytetrafluoroethylene) with low dielectric constant and low loss; CTER-stabilized ceramic-filled PTFE; and commercial-grade PTFE (polytetrafluoroethylene) with low cost.
