Electronics manufacturers incorporate dielectric materials in components and devices to provide insulation, electrical storage, or current flow resistance. PCB material companies leverage these characteristics to construct board substrates and laminates. During PCBA development, engineers, board developers, and CMs seek the optimal arrangement of dielectric materials to maximize signal integrity or the fidelity of signal propagation along surface and internal layer traces.
There are several material properties, which are influenced by the type and thickness of the dielectric material, that can affect your RF signals, especially at high speeds. They include the following:
Important PCB Dielectric Material Properties for High Speed Applications
Dielectric constant
The dielectric constant defines the material impedance, or the ability to isolate positive electrical charges from negative ones and resist current flow.
Dielectric loss
Dielectric loss is the amount of electrical energy that is lost during signal propagation.
Dissipation factor
The dissipation factor is also a measurement of the amount of loss for periodic signals.
Resistivity
The surface resistivity defines the board’s ability to resist current flow on the surface. Volume resistivity, which is susceptible to changes in temperature, is the board construction’s overall resistance to current flow.
Electrical strength
A board’s electrical strength, which is measured perpendicular to the external surface, defines its ability to resist electrical breakdown.
Choosing the Best PCB Dielectric Materials for High Speed Applications
The parameters listed above provide a wealth of information to aid you in selecting the best dielectric materials that can meet your reliability and durability requirements for high-speed applications.
The materials that will be selected should be based on the specific high-speed PCB application, their costs, and their manufacturability. The standard material out on the market today that is used for PCBs is flame retardant type-4 (FR-4). This material consists of epoxy laminate that is reinforced with woven glass. It is often used for low- to medium-speed circuit boards as it is cost-effective for high-volume production runs. While it can be chosen for high-speed PCB applications, the standard FR-4 is susceptible to excessive heat, power, and voltage rates. It also does not have a stable Dk for high-speed frequencies as signal loss can become more frequent.
If FR-4 is not an option, thermoset hydrocarbon laminates, PTFE laminates and polyimide may be acceptable alternatives. All these materials are readily available, but at a higher cost.
Thermoset hydrocarbon laminates: Thermoset hydrocarbon PPE and PPO offer good mechanical stability, ages well and can be easily fabricated. It offers the mechanical stability designed for high-speed PCBs. However, there may be an increase in dielectric loss at higher frequencies.
PTFE laminates: Polytetrafluoroethylene (PTFE) materials are becoming standard alternatives to FR-4. This material has excellent dielectric properties for microwave frequencies and millimeter wave frequencies, especially for those now being used for 5G wireless communication systems. It also provides low signal loss, improved impedance control, and good signal performance.
Polyimides: Polyimide materials are usually seen in flexible and rigid-flex PCB layers as they can be bent and folded around components without impacting their performance. This material also offers excellent thermal management, moisture resistance, and chemical resistance.
