The best way to choose high frequency PCB materials

High-frequency PCB design is considered esoteric and some engineers called it “black magic”! This topic also has some confusion, especially when we discuss the frequency value that is known as high. Practically the issues existing in high-speed design configuration also exist in high frequency PCB design, but these challenges are discussed in different manners. You must have some precautions for high frequency PCB materials before making any changing in high-speed or RF boards.

Some engineers that make PCB boards function on the radio frequency use less losses FR4 laminate material or low dielectric constant PTFE laminate material. The less dielectric constant is not needed always but on the value of frequency, and we can work with standard laminate also with high DK laminate material according to our projects. If you have questions about the materials you need for your project, please continue to read the article below, FS Technology will give a detailed explanation of high frequency PCB materials.

High-Frequency PCB Material Features

The material of the printed circuit board is the focus of providing PCB properties. FS Technology divides высокочастотная печатная плата materials into organic materials and inorganic materials, including:

  • Organic materials: phenolic resin, glass fiber/epoxy resin, Polyimide, BT/Epoxy, etc.
  • Inorganic materials: aluminum, Copper-invar-copper, ceramic, etc.
 

When purchasing high-frequency PCB materials, you cannot choose blindly, but pay more attention to the material properties according to the special properties of the project. Different manufacturers make these materials and try to make better products than their competitors according to the electromagnetic features of the materials.

Dk and Df

It is a very common point that you will see when reading the datasheet. These values are mentioned on a certain value of frequency normally 1GHz or 10 Ghz on the basis of the requirements of customers. Many engineers read these features since they are working for systems that need less loss, therefore will make comparisons between dielectric losses before starting work. The significant fact to make high-frequency PCB for the required design are mentioned below:

  • To make small RF PCB, you need a larger portion of dielectric constant
  • If you want to have low losses then go for less dielectric constant
 

If interconnection has a long length and has a chance of high losses, then use a material having a smaller imaginary portion of DK. The real portion of DK will not use for dielectric losses if the impedance is according to your required value. Though, Dk has no effect on the determination of the wavelength of operating signal for HF boards. Small circuits normally required a small wavelength that indicates we need a high DK value.

Wavelength comparison of high DK and low DK for high frequency PCB

There is one point related to High frequency PCB based on the resonance is the electric field direction. The dielectric constant of substrate materials will not same about every axis in the material, so the dielectric constant will find wave propagation speed and resonance will rely on the direction of the electric field in the system. These differences can be about five percent but have an effect in high-Q structures such as short resonators and emitters which are compatible with modulated signals. The dielectric constant value related to different electric polarization directions must be certain in the datasheet of material.

Thickness and Panel Size of High Frequency PCB

It looks somewhat pedestrian but HF board material cannot get in any desired thickness and panel size. These materials are normally copper-clad laminates having a certain thickness, and many Laminates can be stacked together using an adhesive layer. They can be used in hybrid stack-up having another category of FR4 laminates with the same thickness. The laminate thickness will give the total board thickness, with that copper trace linewidth we can use it for RF signal routing and make printed RF circuitry on PCB boards.

The thickness is the main factor since it will find the linewidth that is needed to fulfill our required system impedance. It is a significant portion of any category of High frequency board since components and any printed components will overwhelmingly be created to fifty-ohm system impedance. If you have impedance matching circuits to match printed circuitry impedance with system impedance, substrate thickness will also find impedance mismatch that has to compensate for matching circuits since it adjusts the distance to the ground for the coming year. So if you needed to get small circuit boards and linewidths, consider these two factors.

  • You have to apply a large DK value for smaller circuits
  • Use thinner laminate for smaller circuits
Typical ThicknessesTypical Panel Sizes
InchesmmInchesmm
0.010 0.2512×18305×457
0.020 0.5116×18406×457
0.030 0.7618×24457×610
0.061.5236×48914×1220

If work is with outer turnkey manufacturers and you have to draw with material stocks, then you have to be restricted to materials and normally not have a certain panel size. The Wholesale PCB manufacturer has a good for having huge material stocks. Panel size will give prices for one unit as the panel has the highest number of boards. If you have the facility to get larger or smaller panel sizes than the standard panel area, so it takes as a bonus and avail opportunity to get some additional boards.

High frequency PCB copper foil material

High-frequency material can use a specific group of copper foil materials, which can be rolled-annealed copper or low-profile rough copper which is designed to provide less loss. A good datasheet will define the approximate roughness therefore we can get an idea of approximate losses for operating frequency. Normally smoother cooper is favorable on the basis of loss point of view since it makes a small skin effect enhancement and small impedance deviation. If the interconnection is smaller and we need a smaller PCB, then prefer DK/Df over the type of copper foil.

If you have an evaluation of certain HF board materials and we need to consider the conductor losses in circuitry, we can use basic techniques for propagation constant to find the conductor losses through the use of trace DC resistance, skin effect, and copper roughness factor.

Power loss formula per unit length of high frequency PCBA conductor

This expression gives us per unit length power losses about the conductor. the skin effect and DC resistance can be measured through the use of a calculator, but copper roughness factor K needs a measurement or a standard roughness model to measure.

Smoother copper foil is normally used for RF board structure. With that, there is plating material to take into consideration as this material can make a rough interface in a copper layer which makes high rough losses. There are 2 surface plating that can give virtually zero extra losses as compared to bare copper, OSP, and immersion silver.

Normally if we are working over 2.4Ghz Wi-Fi or connections are long then we need to select a less loss plating material.

Thermal and Mechanical Properties of High Frequency PCB Materials

It is sometimes seen that the project that we are going to make does not have reliable operation. In specific types of systems like high-frequency avionics, thermal and mechanical features are considered first since devices may have high temperatures, repeated thermal cycling, mechanical vibration, or shocks during function. For example, we have made some devices for aerospace customers that use Roger laminates due to tensile modules. Another material feature that is desired from a group of hybrid stack-ups is CTE matching, some PTFE material can match CTE parameters with FR4, therefore accurately used in hybrid board stack up.

Summary

There are numerous types of material existing in the market, and some lesser known suppliers are not the first choice for your project, and they do not have the capability to provide high frequency PCB laminates.  China’s top-quality turnkey PCBA manufacturers like FS Technology can make ordinary high-frequency PCBA boards, but also have no problem with mmWave PCBA. Not all manufacturers can provide knowledge of mmWave PCBA or 5G PCBA, and even if they do, they cannot provide such a service.

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