RF PCB Materials

For Microwave Electronics

Selecting RF PCB Materials

Microwave electronics and PCBs operate at signal frequencies ranging from low MHz to very high GHz (mmWave range). These innovative products are built using various synthetic and naturally-occurring materials to ensure consistent operation and signal integrity at high frequencies. Material selection is, in fact, the most critical factor in designing a high-performance microwave PCB.

RF PCB materials have certain properties that define their usage for both printed circuits, in interconnect boards, or as standalone assemblies. These include their dielectric constant, thermal expansion coefficient (CTE), and loss tangent or dissipation. In addition to the dielectric material, the copper material used in an RF PCB stackup is important as it will have some roughness, and thus it will determine the skin effect experienced by signals as they propagate along an interconnect.

Most of today’s RF PCB materials are polytetrafluoroethylene (PTFE)-based materials. PTFE is an engineered thermoplastic fluoropolymer with low dielectric losses at microwave frequencies. The dielectric constant can range from very small values to very large values, which allows the required size of RF printed circuits to be adjusted simply by selecting an alternative material. Three of the most popular RF materials manufacturers include Rogers, Taconic, and Arlon.

One strategy for using PTFE materials alongside standard epoxy resin/fiberglass FR4 materials is to build a hybrid PCB stackup. In this type of design, one or more outer layers can be designed with a low-loss PTFE material to support RF routing. The other layers can then be chosen from standard FR4 materials with higher dielectric constant. This is a useful approach in mixed-signal systems that also have some digital interfaces, but they do not have the low-loss requirements of a typical RF interconnect.

Influence of Copper on RF Signal Integrity

The other major material that influences RF systems is copper foil, which is used to form conductors in RF PCBs. All conductors have finite conductivity, so there is some DC loss in RF transmission lines. In a PCB, copper foils have roughness which will create greater losses than the skin effect would predict for a smooth conductor. In other words, copper foil roughness increases losses from the skin effect and has a greater modification on impedance.

The SEM images shown above illustrate the roughness created by the standard copper foil manufacturing process. This foil will then be roughened intentionally in order to ensure adhesion to the PCB substrate material. Copper foil must be selected alongside the required laminate material in order to keep losses within budget.
Image credit: Coonrod, J. "Advancements in Prepreg Enabling New Applications for Millimeter-Wave (mmWave) and High Speed Digital (HSD)," PCB West 2021.

When an advanced PTFE laminate is used in an RF PCB, the low loss tangent causes the dominant loss mechanism to switch from dielectric-dominated to copper-dominated. The copper-dominated losses can overwhelm dielectric losses up to extremely high frequencies. This demands balancing conductor sizing and predicting lost budgets up to ~100 GHz frequencies, as well as selecting appropriate plating materials that do not create additional loss in RF transmission lines.