Impedance Management through PCB Stackup Design with Reference Planes
My first PCB was far from a high-speed digital device. It was just an amplifier circuit on a single layer PCB, and controlling the impedance was not even an afterthought. Once I started working on electro-optics systems that required high sampling rates, controlling impedance was always a critical design issue. Controlled impedance on a circuit board is a PCB layout issue that I didn’t feel completely comfortable in handling as a PCB designer for some time.
Once you start working with high-speed or high frequency signals, you’ll find that the impedance of your traces and mismatch at sources and loads can have a major affect on signal integrity. In the RF range and beyond, controlling the impedance encountered by your signals will ensure that your device will operate as designed. There are some simple design strategies that can help ensure your signals are not distorted during operation.
Impedance Control vs. Controlled Dielectric
Impedance control and impedance management are two terms that are loosely interchangeable and refer to different methods for setting the impedance seen by signals in a PCB. Obviously, no fabrication process is perfect, and any PCB that comes off the manufacturing line will have some variations in trace impedance. The basic idea is to set the impedance encountered by signals to a specific value, ideally limiting any impedance mismatches to small values.
This can be done in two ways. First, simply selecting the geometry, arrangement, and material used to form traces will affect their impedance. The surrounding dielectric also affects the impedance. This type of situation corresponds to a single trace routed on a dielectric with infinite thickness. Most simple impedance calculators assume this situation. Obviously, this approximation is only valid in a few special situations.
You might be thinking “wait a minute, why should the dielectric constant of the substrate make a difference?” There are a number of reasons for this. First, the dielectric between adjacent traces and, or between a trace and a ground plane forms a capacitor, and the dielectric constant determines the stray capacitance.
Since the interface between a trace and the substrate is not a perfect reflector, the electric field actually passes into the dielectric and remains coupled with the field in the trace. In short, the signal partially propagates in the dielectric and is not completely confined in the trace.
Both of these facts mean that the layer stackup in multilayer PCBs will affect the impedance seen by signals in a trace. In effect, modifying the layer stackup allows a designer to adjust the overall trace impedance. Adjusting the layer stackup changes the effective dielectric constant seen by signals, allowing impedance control in a number of applications.
PCB with traces between components
Impedance Controlled Design
Most designers are likely familiar with impedance control, where trace arrangement, dimensions, and ground plane arrangement are considered simultaneously. Traces carrying high speed signals should be routed over solid ground planes in order to provide a reliable return path for current that minimizes the loop area, thus minimizing any induced currents due to EMI.
Routing high-speed signals across split planes can cause a delay in signal propagation due to increased series inductance, degraded signal, and interference with other signals. If you must route a high-speed impedance controlled trace across a gap in a ground plane, stitching capacitors can be used to provide a current return path. This also minimizes the loop and any impedance discontinuity created as a trace crosses a gap in a ground plane.
Some manufacturers provide impedance calculators that can help you choose the proper trace dimensions required for a given trace/ground plane arrangement and required impedance value. Alternatively, if your trace dimensions are constrained, you can determine the level of impedance mismatch between a source, trace, and load in your PCB using one of these calculators.
During manufacturing of multilayer boards, your fabricator can help you by varying one of the two cross-sectional dimensions in your PCB traces in order to achieve the desired impedance value. They typically build a test board (called a “coupon”) and modify the trace dimensions and arrangement in order to reach a desired impedance value within a certain tolerance level (usually +/- 10%). When working with differential pairs, the trace spacing is another parameter that can be used to adjust the impedance.
If the designer specifies that the height of a trace must be fixed, then they will vary the width, and vice versa in order to obtain just the right impedance value. This also gives the manufacturer the opportunity to tune their process just so and ensures that you will have higher yield from a production run.
High density traces on a motherboard
Controlled Dielectric Design
Compared to impedance controlled design, where the layer stackup is generally left constant, controlled dielectric design reaches a specific trace impedance value by modifying the layer stackup. Rearranging the layer stack arrangement, layer thickness, and even swapping the dielectric for a different material are all measures that designers can take to manage impedance in a multilayer PCB.
Impedance controlled design generally also uses a controlled dielectric board, but the converse is not necessarily true. Modifying the layer stackup arrangement, dielectric thickness, prepreg thickness, and laminate thickness all change the impedance seen by signals on the board. For a given trace geometry, modifying these board parameters can allow you to fine-tune the impedance of your board.
The best way to determine the behavior of your board is to use a 3D electromagnetic simulation package. Unfortunately, many people don’t have this software, and you will have to resort to using some basic impedance calculators and your intuition to get an idea of how modifying the board will affect the impedance.
Especially when working through signal integrity issues or layout problems with a printed circuit board, the software you choose should be able to keep up with vias, layout, power plane or other plane management and PCB stack-up. Managing PCB layout with trace widths will be a thing of the past when you utilize a circuit board with strong design software.
If you are looking for a PCB design software platform that gives you full control over impedance in your PCB, you need to try Altium Designer. Altium Designer 18.1 provides you with the latest and greatest CAD tools, component libraries, and simulation tools.