Issue link: https://iconnect007.uberflip.com/i/1535183
Designers need to perform impedance calcu- lations—specifying parameters for controlled impedance in the schematic—and include impedance-related notes with their PCB designs. Every PCB manufacturer's process is slightly different, so calling out the impedance requirements in the schematic allows the manu- facturer to confidently tweak trace width, spac- ing, material thickness, and type as needed. If you know your impedance math and do the calculations necessary to create a precise design, the result will be a smoother manufac- turing process. PCB trace impedance is deter- mined by its inductive and capacitive reac- tance, resistance, and conductance (usually ranging from 25 to 125 ohms). Factors dictat- ing impedance include: • Distance from other copper features • Width and thickness of the copper signal trace • e thickness of the material on either side of the copper trace • e dielectric constant of board material You can save time, money, and effort if you are aware of the impedance math when you design your board by using one of the many quality impedance calculators. is allows you to build the right tolerances into your design. Impedance testing becomes a double check of your work instead of the tool you rely on to tell you if your documentation is correct. While documenting impedance require- ments properly may seem simple—state your target impedance, trace requirements, and material tolerances—PCB documentation is a details game that oen leaves knowledge gaps for your manufacturer. For example, picture a design for a four-layer board with two signal layers, two planes, and a seemingly complete set of drawing notes. What if the documenta- tion doesn't specify whether both signal lay- ers and trace widths require impedance con- trol? e board manufacturer must then make assumptions, cross their fingers, and move to production or kick the design back to the designer for clarification. One scenario slows you down; the other risks manufacturing unus- able boards. Proactive design methodology, not reliance on testing, is the best way to control imped- ance and pave the way for efficient production of quality boards. PCB impedance call-outs are helpful, but not as foolproof as craing a design with the right distance to the reference plane, trace widths, and materials tolerances. Incomplete or incorrect impedance-related notes are common and can directly impact both board cost and performance. Delays occur when notes do not match the design, there are two trace widths for the same impedance on the same layer, or each signal layer does not have its own impedance require- ments. Sometimes the adjust- ments required are not possible, because they cause interference with other features. Lack of specificity in the notes can result in extra effort when transitioning from design to manufacture. e documen- tation typically defines the impedance, not the trace size, or gives a trace width that cov- ers the entire board. Determin- 54 DESIGN007 MAGAZINE I MAY 2025