The need to select PCB materials correctly for high frequency applications
The High Frequency PCB itself is a composite of the core layer of the non-conductive dielectric substrate and the dielectric laminate layer. The copper foil that will be the metal routing and power plane of the board uses the laminate layer as its base. The core and stack are made of various materials, which are designed to meet the specific dielectric constant and other performance requirements. With the increasingly strict requirements of high frequency circuit for signal integrity, the dielectric properties of circuit board materials have become a large part of the whole signal integrity equation.
The standard circuit board material used for many years is FR-4, which is type 4 flame retardant glass fiber reinforced epoxy laminate. This material is cost-effective, easy to process, and very suitable for PCB manufacturers. It’s also a good electrical insulator, a solid material for printed circuit boards.
The problem with the FR-4 is that it does have power, voltage, and heat limits, and when your circuit exceeds these limits, the dielectric properties of the FR-4 begin to degrade. This will create a condition in which the insulation of the FR-4 will begin to collapse and will begin to allow current to pass through.
Another problem with the FR-4 is that at higher circuit speeds, the dielectric constant changes and the signal loss increases. All of these behaviors can cause problems in high-frequency design. If your design is concerned, you should study the available high-speed plate materials.
Benefits of high speed sheet
By using high-speed materials in the construction of circuit boards, you can achieve many benefits:
Controlled impedance: high speed plate materials such as reinforced epoxy or PTFE will maintain a dielectric constant (DK) tolerance of + / – 2% or higher. This is a very important consideration for circuits that require strict control of impedance routing.
Signal performance: another important consideration is the signal performance of the whole board. With the increase of transmission line frequency, signal loss becomes a bigger problem. Reinforced epoxy or PTFE materials can provide a much lower dissipation factor, which in turn helps to reduce signal loss.
Dimensional stability: high frequency design requires strict physical tolerances in both manufacturing and use. Thermosetting hydrocarbon laminate material is an ideal choice to improve mechanical stability.
Reduce moisture: another consideration is the moisture absorption of the materials used on the circuit board. Even a small amount of moisture can change the electrical performance of high frequency circuits. The moisture absorption of traditional FR-4 can be as high as 50%, while that of PTFE and other materials can be as low as 2%.
Thermal management: high speed designs usually handle more heat during operation, and materials such as polyimide are very robust and have excellent thermal properties. This also makes it a good choice for circuit boards to be used in harsh environments.