Detailing Your Desired Control Over Electrical Resistance
In the realm of high-speed PCB design, maintaining controlled impedance is crucial for preserving signal integrity and performance. Here are the best practices for specifying controlled impedance requirements in PCB designs:
- Identify critical signals: High-speed data or clock lines that require controlled impedance should be identified, and the exact impedance values needed according to the signal standards, such as 50 Ω single-ended or 90 Ω/100 Ω differential pairs, should be specified.
- Define trace geometry precisely: Trace widths, spacing for differential pairs, and copper thickness should be specified to achieve the target impedance. Precise trace widths are essential to distinguish controlled impedance traces from non-impedance ones, aiding manufacturers in adjusting for accurate impedance.
- Specify the PCB stack-up and dielectric materials: Details on dielectric thickness, dielectric constant (Dk), and the arrangement of copper and insulating layers should be included, as impedance depends heavily on these parameters.
- Avoid routing controlled impedance traces near PCB edges or reference plane discontinuities: Unintended coupling, reflections, or radiation affecting impedance and signal integrity can result from placing controlled impedance traces near PCB edges or reference plane discontinuities.
- Insist on consistency in differential pair routing: Symmetrical traces with uniform spacing and minimizing discontinuities caused by vias or components should be maintained.
- Include manufacturing tolerances and fabrication processes: Recognize that small variations in etching or substrate can change impedance, and specify the necessary tolerances and fabrication processes to maintain impedance control.
- Use validation and test methods: Time-domain reflectometry (TDR) can be used to verify that the PCB meets the specified impedance.
- Communicate clearly with the PCB fabricator: To ensure controlled impedance traces are accurately translated into fabrication, it is essential to differentiate them from other signals, for example, by slightly adjusting their widths.
Sierra Circuits has recently upgraded their Impedance Calculator, a free 3D field solver, which now uses the 2D numerical solution of Maxwell's equations for PCB transmission lines. The manufacturer receives the controlled dielectric stack-up from the designer and uses tools like Instack and Polar to design the stack-up. The dielectric thickness, trace width, and spacing are used to control the impedance.
To check if the time-domain reflectometry (TDR) coupons can reach the necessary impedance, a test is performed. Test coupons are fabricated at the edges of the panel and inspected to ensure proper layer alignment, electrical connectivity, and internal structures. The report generated by the tools shows the number of layers, dielectric thicknesses, copper thicknesses, and impedance values on different layers.
Characterization is required for each manufacturing process to verify that it is matching the nominal values generated by field solver calculations. Certain modifications may be made based on the findings of the first article inspection reports. Test coupons are used to verify impedance control after the PCB has been manufactured.
When working on documentation for controlled impedance, it is critical for board designers to understand the best approach to express their requirements, ensuring that controlled impedance is accurately translated into PCB fabrication and preserving signal integrity and performance in high-speed designs.
Mentioning the technological advancements, Sierra Circuits has updated their Impedance Calculator, an impedance technology that utilizes a 3D field solver and the 2D numerical solution of Maxwell's equations for PCB transmission lines. In the process of testing, it's essential to use time-domain reflectometry (TDR) to ascertain if TDR coupons can reach the necessary impedance. When documenting controlled impedance requirements, it's crucial for board designers to understand the best practices to communicate their needs effectively, thereby ensuring accurate translation of controlled impedance into PCB fabrication and preserving signal integrity and performance in high-speed designs.
