Is FR-4 the Best Board Material Choice for High Speed PCB Design? | Forum

Is FR-4 the Best Board Material Choice for High Speed PCB Design?


We’ve all been there at one time or another when we’re tasked with a project that lies outside our comfort zone. For me, that day came when my boss asked me to design a high speed board. While I considered myself to be a somewhat experienced PCB designer, I knew that high speed PCB design has many restrictions that you do not normally encounter when you’re designing an average circuit. Initially I invested time into making my schematics appropriate for high speed designs; however, once that was complete I was completely focused on understanding whether I should use FR-4 or a more specialized material for my high speed PCB layout. Before I get into what I’ve learned, it’s important to know what in this article I refer to “high speed” as anything greater than 50 MHz. These are the material considerations you should take note of when working in this frequency range.To get more news about fr4 circuit board, you can visit pcbmake official website.

High speed designs have tighter specifications on signal integrity than other designs. Although great care is taken in the routing of high speed signals to satisfy these requirements, it must be understood that the circuit board materials themselves are part of the overall signal integrity equation. Circuit board materials for high speed designs, therefore, need properties such as a dielectric constant with tight tolerances to help control impedance. If impedance is allowed to vary across the design, then the high speed signals will begin to reflect energy back as they move through their lines and the signal will be distorted. Also, a low dissipation factor to help maintain signal integrity is desirable. Finally, thermal stability is another important trait to ensure that the dielectric properties will not breakdown.
For as long as I’ve been designing printed circuit boards, FR-4 has been the standard material used for the fabrication of a printed circuit board. Back in the day as junior designers we even had the bad habit of referring to all boards as being “FR-4” whether they were constructed with it or not. FR-4 is a Flame Retardant type 4 woven glass reinforced epoxy laminate. It is a very cost effective material that is both an excellent electrical insulator and very sturdy in dry and humid conditions. It also has good fabrication properties making it an ideal material for constructing a PCB.

The downside of FR-4 is that it has operating limitations when it comes to excessive power, voltage or heat. If you exceed its operating limits, FR-4’s dielectric properties will break down. This means that the insulation of the material will decrease and it will begin to conduct electricity instead. Another problem with FR-4 is maintaining stable impedance for high speed designs. This is because the dielectric constant of FR-4 can vary across the length and width of the board. Also, the signal losses that are acceptable in non-high speed designs will increase to undesirable levels in FR-4 boards as the speed of the design increases.