Speed is all in the modern electronics. With the processors, memory and communication interfaces data rates in the multi-gigahertz range now printed circuit board (PCB) is no longer a simple point to point connector. Rather, it is made a complicated high-frequency space in which each trace is a transmission line. This is the most basic rule to forget, which gives rise to signal integrity problems and results in corrupted data, passed compliance tests, and expensive redesign.
The secret of this management of these high-speed signals resides in what is referred to as controlled impedance. This is not a best practice, but a design requirement that must not be negotiable with devices that require using DDR, PCIe, HDMI, or Ethernet.
What Is Controlled impendence?
Impedance is in essence the resistance to the alternating current (AC). To calculate the characteristic impedance (Z 0 ) of a PCB trace that carries a high-frequency signal, its physical geometry and the materials enclosed by it are used. Imagine that it is water in a pipe. The water flows easily as long as the diameter of the pipe is maintained. When there is sudden change in diameter, there is turbulence and reflections that break the flow.
On the same note, in a high-speed signal, a PCB trace is used with a uniform impedance. Any sudden transition – as a result of differences in trace width or layer changes, or an inappropriate choice of connector – results in some fraction of the signal energy being reflected to the source. These reflections are noise that distorts the original signal and causes bit errors, jitter and source system failure. The engineering art of ensuring controlled impedance is the design of traces in such a way that they have a constant impedance (e.g. 50 ohms single-ended, 100 ohms differential) over their entire length.
Multilayer PCB Design: Crucially Important
It is almost impossible to attain a uniform impedance on a plain single or dual surface board. It is here that an adequate multilayer pcb design will be of necessity. Multilayer board design permits addition of massive, solid copper ground (GND) and power (VCC) planes. The planes act as constant or fixed reference tracks to the high speed PCB Design signals on the neighboring planes.
The structure forms two major types of controlled impedance traces:
- Microstrip: A trace that is routed on an outermost layer and has a solid reference plane immediately below it.
- Stripoline: Trace which has been placed on an internal layer and sandwiched between two reference planes (e.g. GND and VCC). Stripline designs are better shielded and have better EMI performance but are more difficult to make.
Multilayer PCBs offer continuous planes of reference, which offers a predictable electrical environment in which the impedance could be accurately calculated and regulated.
Designing for Controlled Impedance
Controlled impedance is not something that is added into the design and realization of high-speed PCB design services, but rather a core component of the process. It involves proper planning and following certain regulations.
1. Begin with the Stack-up: Specify your PCB stack-up with your fabricator before you lay out the first trace. The essential parameters, which affect impedance, are:
- Trace Width (w) and Thickness (t): The lower the trace width the lower the impedance.
- Dielectric Height (h): The distance in between the trace and its reference plane. The shorter the height, the less is the impedance.
- Dielectric Constant ( ε r): The insulating material (FR-4, Rogers) between the trace and the plane. Dielectric constant increases the lower the impedance.
Their fabricator is able to supply the correct dielectric constant and dielectric layer heights of their materials and you can determine the trace widths needed to achieve desired target impedances such as 50ohms, 90ohms or 100ohms.
2. Apply Impedance Calculators: The majority of the current EDA (Electronic Design Automation) programs have in-built impedance calculators. These tools will give you the precise trace widths required by each of the layers by entering your stack-up parameters. These values can be used to make routing rules in your routing design software.
3. Follow Strict Routing Rules: keep width Consistent: Do not vary the width of a controlled impedance trace in its direction.
- Sharp Bends: Sharp bends should be avoided and 45-degree or (better) curved arches should be used in order to reduce impedance discontinuities.
- Minimise Vias Each via (vertical interconnect access) is an impedance change. Critical high-speed signals should be avoided as much as possible or where necessary, impedance-matched vias should be applied.
- Avoid Splits: It is important to avoid crossing such a split or a gap in the reference plane of a high-speed signal. By doing this the return path is broken causing a massive difference in impedances, which ensures the failure of signal integrity.
Collaborating to achieve First-Pass Success
The concepts of the controlled impedance are simple, but implementation may be complicated particularly in dense boards with many high-speed interfaces. Computing and checking impedance among different layers, and controlling differential pairs and manufacturability are all skills requiring skill. A single inaccuracy in a stack-up calculation can make a full batch of the prototypes of an expensive product useless.
In cases where the project is mission-critical, the best decision to make is to use professional pcb design. Skilled layout engineers possess means and expertise to model impedance with precision, run pre and post layout signal integrity tests and collaborate directly with fabricators to make sure that your design intent is directly and reliably translated into a functional and reliable product.
Conclusion
The foundation of the modern digital design is controlled impedance. Discipline is the factor which makes sure that signals reach their destination clean, clear and on time. With the ability to learn the fundamentals, the ability to thoroughly plan your PCB stack-up, and follow the strictest routing rules, you could create a reliable and stable signal highway to run your high-speed data. In complicated designs that one can not afford a single mistake, collaborating with design professionals is the most guaranteed way of succeeding.
