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 circuit 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. 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.
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 board materials themselves are part of the overall signal integrity equation. 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.
FR-4: Its strengths and its weaknesses
For as long as I’ve been designing printed circuit boards, FR-4 has been the standard material used for the fabrication of a PCB. 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.
Let’s compare board materials
How specific high speed board materials compare with FR4
Dedicated high speed board materials such as Thermoset Hydrocarbon and PTFE laminates, will have better and more reliable properties in higher frequency designs than FR-4. There some trade-offs, which we will discuss shortly, but first let’s look at some of the advantages that high speed board design materials offer:
Decreased Signal Loss. As transmission line frequencies increase, signal loss becomes more of a problem. High speed design board materials have a much lower dissipation factor than FR-4, with some of the materials, such as the nearly pure PTFE laminates, being an order of magnitude better. These lower dissipation factors have are a huge factor in reducing signal loss.
More Tightly Controlled Impedance. Traditional PCB materials such as FR-4 do not offer as precise control over dielectric constants (Dk) as high speed board materials do. FR-4 Dk’s can vary +/- 10% or more while materials such as PTFE hold their Dk tolerance at +/- 2% or better.
Better Thermal Management. Some high speed design board materials such as Thermoset Hydrocarbon laminates have much better thermal conductivity than FR-4. If your design is going to dealing with thermal management issues, then these board materials are the ones to investigate.
Increased Moisture Absorption. Water has dielectric properties and even a small amount of moisture absorbed into a PCB with high frequency circuits can change the electrical performance of those circuits. While FR-4 can have a moisture absorption rating of nearly 50%, some of the PTFE materials are as low as 2% and should be considered to resolve this problem.
Robust Dimensional Stability. For a dense high speed design with tight tolerances, the need for dimensional stability increases. While FR-4 is known for its dimensional stability, it lacks the other advantages that the high speed materials offer. In this case, the Thermoset Hydrocarbon laminates might be a better choice.
As you can see, there are a lot of advantages to using these high speed materials. There are however other factors to consider as well.
Which PCB materials should you choose?
The flip side of high speed board design materials
There is a cost to all the advantages of high speed board materials, and the first one to consider is the actual cost. These materials are expensive, and that needs to be factored into your decision. Another issue to consider is that some of these materials are more difficult to work with during fabrication and assembly than the others. The nearly pure PTFE laminates for example, which offer tremendous electrical performance, require fabrication processes which are not standard.
There are many different high speed board materials to consider, and each of them has their strengths and weaknesses. Part of your decision process will be to weigh the pros and cons of each material. Some may give a better electrical performance while at the same time be more difficult to fabricate. Another may be a better choice to fabricate with, but you may lose some of the thermal advantages of the first choice.
High speed PCB designs require careful selection of board materials for their construction, and FR-4 may not be the best choice. Although FR-4 is being updated to work better in high speed applications, there are many other options to chose from. These guidelines should help you as you consider which materials are best for your design.
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