Important Properties of RF PCB Materials
Storm Circuit makes a lot of RF PCB ,it also calls high frequency PCB or microwave PCB . When we choose RF PCB material ,there are some key properties we need to pay more attention.
The Dielectric Constant (Er)
The dielectric constant measures a material’s ability to store electrical energy in an electric field.
It is direction-dependent, so the dielectric constant can change depending on the axis of the material. Unfortunately, it can shift under the high frequency conditions of RF, so it can’t be taken at face-value.
It’s important to understand what frequency range the material was tested under, what testing method was used and if there are values available for frequency ranges and conditions that closely match your target application.
The Coefficient of Thermal Expansion (CTE)
The coefficient of thermal expansion (CTE) explains how an object’s size varies with a change in temperature. It is also a way to measure thermal robustness. It has a huge impact on the drill and assembly stages of PCB manufacturing.
In a multilayer stack, different materials with different CTE’s are going to change shape at different rates. When the layer above grows faster than the layer below, alignment becomes a serious problem during the drilling process.
PTFE, one of the most commonly used RF materials due to its high quality, can smear at drill if it gets too hot and that can’t be removed. At the assembly stage, when components are being soldered on, the CTE affects how well it will handle the thermal stress of soldering. A poor CTE can lead to a broken board during its final stage of production, which is an expensive mistake best avoided.
To mitigate these problems, a material with a lower CTE is going to handle the physical punishment of drill and assembly better.
Loss Tangent (Df)
Loss Tangent, like the dielectric constant, is one of those challenging effects that manifests in RF but isn’t as problematic for lower frequency designs. It’s a consequence of the molecular structure of the material itself.
As the frequency goes up, the signal loss occurs because it gets burned off as heat. In an analog circuit, amplitude loss occurs..
In complex multilayer boards, components can become quite dense and additional heat generated during operation is something that needs to be considered.
Another thing to consider is the environment your device will be operating in. If the board will end up in a lab with an air controlled environment, the material’s ability to absorb moisture may not be a concern for you. But if the board is going to be outside, in the rain, or may spend short, unplanned trips underwater, then moisture ingress becomes a more critical priority.
RF Material Types
RF materials are usually some combination of PTFE, ceramics, hydrocarbon and various forms of glass.
PTFE with microglass fiber or woven glass is the go-to material when quality is more important than price. If budgets are restricted but high quality is still a demand, then ceramic-filled PTFE retains much of the quality but is easier to manufacture, which decreases cost.
Ceramic filled with hydrocarbon is even easier to build, although the reliability of the signal takes a makeable step down.
In addition to price and electrical performance, thermal robustness is important for anyone who will be exposing their devices to soldering stresses at assembly, using multilayer boards in demanding drill scenarios, or deploying the final product in a thermally demanding environment, such as for aerospace.
PTFE with microglass fiber or woven glass has excellent electrical properties, but a high CTE. Ceramic-filled PTFE has great electrical characteristics and a low CTE, making it a thermally tougher choice. Ceramic filled hydrocarbon takes a step down in electrical characteristics, but also has very low CTE.
In regards to moisture, PTFE ceramic will have a lower rate of moisture absorption, but once woven glass is incorporated, that moisture level can become much higher. However, adding hydrocarbon to PTFE ceramic causes a much smaller increase in moisture absorption, making it a good choice to balance cost and resistance to wet environments.
The common RF material we used are RO4350B,RO4003C, RF-35,F4B, AD300,etc. you can click here to see our PCB material .