Anyone who has done some home remodeling is probably very familiar of the tradeoffs that come with selecting what materials to use. When updating my kitchen a few years ago I wanted to replace the old flooring with hardwood, but I didn’t have the budget for that so I settled on a less-expensive option. The problem is that with three teenage boys in the house along with hundreds of their friends as well as a dog, the cheap substitute flooring did not hold up at all. Now I have floor panels that are separating and beginning to peel up all because I made the wrong choice.
When selecting materials for high speed PCB and system designs, we face a similar scenario. Due to tighter demands for maintaining signal integrity, high speed designs require a lot more from their board materials then standard designs. In these designs the board materials themselves become part of the overall signal integrity equations, and must be held to tighter tolerances for their dielectric constant in order to help control impedance. Without this control, the high speed signals could reflect off of each other leading to crosstalk and undesirable signal performance. The question then is, what should we consider when selecting materials for high speed boards?
Before you begin the decision-making process on what kinds of PCB materials to choose from for your board, you must first have a clear definition of what the board will be doing and what it will be used for. For instance, the signal performance may be affected by the materials you choose, and those choices will also have an influence on the durability and cost of the final board. It can be helpful to make yourself a list of what you should be considering, so here are a few ideas to get you started:
Once you’ve established what your board is going to be doing and what it will be used for, you can better decide on what PCB materials to choose from. There are many different types of materials used for PCB laminates, and they can be grouped into general families of products including the following:
Epoxy Resins: Commonly referred to as FR-4, these materials are used for standard PCB manufacturing.
Enhanced Epoxy: These materials have better electrical properties and are well suited to complex multi-layer high speed designs.
Polyimide: These materials are extremely robust and exhibit excellent thermal and mechanical properties making them ideal for advanced applications in harsh environments.
PTFE (Polytetrafluoroethylene): Known also as “Teflon,” these materials have a very low dielectric loss factor making it ideal for high frequency RF applications.
Within each of these families are different classifications, product and brand names. Although FR-4 is considered to be the “standard” material used in PCB construction, there are in fact many different classifications of FR-4 to choose from.
Why FR-4 Isn’t Always the Best Choice for High Speed PCB Materials
FR-4, or one of its variants, have been used traditionally for years in the fabrication of printed circuit boards. It is very cost effective, provides good electrical insulation, and is sturdy in dry and moderately humid conditions. It also is one of the easier materials for fabricators to work with which also adds to its popularity. But when it is pushed past its operating limits, FR-4 can begin to show some problems in a hurry:
Inability to Control Impedance: FR-4 does not allow as tight of control over dielectric constants as high speed board materials do. Using these materials will keep dielectric constant tolerances within 2% or better while FR-4 is only rated for 10%.
Signal Loss: The higher the high speed transmission line frequencies go, the more likely you are to get signal loss. FR-4 typically has a much higher dissipation factor allowing for more signal loss than high speed materials such as PTFE.
Mechanical Stability: Although FR-4 is fairly robust, it does have its limitations if exposed to excessive power, voltage, and heat. If those limitations are exceeded, the insulating qualities of the materials will break down and begin to conduct electricity instead.
Moisture Absorption: FR-4 can have a moisture absorption rating of up to nearly 50%. This makes PTFE material, which can have a rating as low as 2%, the better choice for some environments.
There are considerable advancements being made in the world of PCB materials on a regular basis. Although standard FR-4 materials may not be the best choice for some of your high speed PCB and system designs, there may be a new epoxy resin variant that might be just what you need. The key of course is to find out as much as you can before you commit to one particular material or another
To make the best choice of PCB materials, start first with some research. You can find a lot of useful information on PCB fabricators’ web sites, as well as white papers and other research documents. The absolute best thing to do though is to engage early on with your PCB fabricator of choice and let them help you to make the right decision. Bring them the specifications of your PCB, what it is doing and what it will be used for, and they will be able to show you the best materials for the job. The important thing is to work with your fabricator. This is the business that they are in and they are really good at what they do.
Once you have the board materials selected, you can then move on to defining your PCB layer stackup. Although it may be tempting to get a head start on this part of the job, it is absolutely critical that you not specify the stackup of a high speed design until after you have decided on what board materials will be used. Impedance calculations rely on knowing what kinds of PCB materials you are using in order to be accurate.
Once the materials have been decided, you can plan your PCB stackup with the correct power and ground layer strategy as well as considering layer thickness for impedance controlled routing.
In addition to working with your PCB fabricator to determine exactly how your PCB layer stackup should be composed, another way to help yourself is to use PCB design tools that have been created to best help you with your high speed designs. We use OrCAD PCB Designer, and have found that the high speed rules and constraints are very helpful in the layout of our high speed designs.