Imagine that you are getting ready to work on a new PCB when the electronics engineer you work with suddenly gives you a controlled impedance requirement for the board. This will be your first experience designing a PCB with controlled impedance traces. Where do you begin? I encourage you to seek out IPC-2141, Design Guide for High-Speed Controlled Impedance Circuit Boards, to help answer that question; in addition, this introduction may also help.
Controlled impedance traces are necessary on some PCBs for high-speed signal transmission. The goal with controlled impedance traces is to design the proper propagation delay into the trace. On boards with an antenna, impedance matching is required for a reliable signal. Other boards may have components that require a specific propagation delay in order to perform calculations.
There are many types of both simple and complex impedance structures. No matter the specification, there is a controlled impedance structure to suit your needs. Fortunately, the simple structures cover many applications. Here is an overview of two of the most popular impedance structures:
The most basic CI structure is a microstrip, otherwise known as the single-ended trace. Most of the time, these traces are specified at 50Ω, but they can have other impedance targets. Most of the engineers who request my assistance with these traces are connecting to antennas. It is generally best practice to reference the next adjacent plane in the stackup. Called the reference plane, it will be the nearest plane to the traces.
Sometimes components and connections require edge-coupled micro strips. These traces are commonly referred to as differential pairs due to the way they operate. This structure has two traces placed side-by-side, with a reference plane. The two traces carry the same signal, but the signals are at different polarities. The device that receives the signal uses the difference between the two lines to perform its function. Some of the common impedance targets for these traces are 90Ω and 100Ω. Like the single-ended traces, the reference plane is typically the closest plane in the stackup.
Both the microstrip and edge-coupled microstrip can be placed on either the external or internal layers. When placed on internal layers, they can be embedded between planes, which will provide some protection from electromagnetic interference (EMI). However, as some components require the external connections from the leads, this is not always an option. Controlled impedance traces are transmission lines just like every other trace on the board. In this respect, they will transmit and receive EMI. In a case where an impedance trace is placed on an external layer, try to keep the trace length as short as possible.
To get the ball rolling on a new design we should start with the stackup. It is much easier to calculate the impedance traces and build them straight into the board; trying to add them in by layer can be a nightmare.
I highly encourage you to talk with your PCB supplier to determine if they can support your planned stackup. Do this well ahead of time. Many times, we plan, we get there—only to have the whole thing fall apart. Why? There are many reasons why the stackup you plan cannot be manufactured by a factory. In addition, many PCB suppliers can provide you with a stackup, complete with dielectric thicknesses and impedance calculations. If you do not have this option, it is not difficult to do yourself. Here are some recommendations:
Learn about the material you plan to use for the PCB. Some applications require the use of an ultra-low loss material, but some do not. Design sustainability into the board by using the appropriate material. Get the datasheet for that material and find the Permittivity value; this will be listed on the datasheet as the Dk, or dielectric constant. The Dk is necessary to calculate the impedances.
Use a good software tool. There are many reliable impedance calculators on the market. Some CAD tools have impedance support already in the environment, meaning only a minimal amount of learning is required to get some work done.
Remember: Changing the dielectric thickness between the controlling plane and the impedance trace will change the impedance value. Assuming all other variables remain the same, increasing the dielectric thickness will increase the impedance value, while—you guessed it—decreasing the dielectric thickness will decrease the impedance value.
There are a few ways the traces can be changed to affect the impedance value, but I will try to keep it simple: Think of the impedance trace as a water pipe. A smaller diameter pipe impedes the water flow more than a larger diameter pipe. Copper traces work just the same in this respect. Decreasing the trace width, or copper weight, will increase the impedance. Again, this is assuming all other variables remain unchanged.
Finally, be very specific in the controlled impedance specifications you provide. Sometimes engineers use impedance requirements from other boards, which is acceptable providing that the stackup remains the same. Sometimes impedance structures that are no longer necessary for a new design get carried over from an old design and cause confusion. This will generate an engineering question, which may cause a stopping point for the production of the PCB.
Controlled impedance traces are not much different from the other traces on a PCB; all the traces on a PCB are transmission lines. Getting the necessary propagation delay in controlled impedance traces just requires a little more planning, software, and sometimes patience. As always, I recommend working with your PCB supplier as early as possible in the design phase to get it right from the start.
Ryan Miller is a Field Application Engineer at the NCAB Group USA.