Introduction
Accurate measurement of spacings (creepage distances) on printed circuit boards (PCBs) is a crucial skill in compliance engineering, especially when dealing with electrical product safety involving hazardous voltages and user-accessible areas. It is essential to keep these two aspects separate for obvious reasons. In this article, we will delve into the measurement process, including identifying specific points to measure and understanding the relevant distances.
Creepage Distance
Creepage distance refers to the surface distance between two conductive elements (such as traces or pads) on the PCB. It is measured along the surface of the insulating material.
Creepage ensures that there is enough separation to prevent electrical breakdown or arcing between adjacent conductors.
Factors influencing creepage distance include voltage levels, signal frequencies, and PCB fabrication process manufacturing capabilities.
Clearance Distance
Clearance distance, on the other hand, is the line-of-sight distance between two conductors through the air.
It accounts for the direct path between conductive elements and is crucial for preventing short circuits.
Like creepage, clearance depends on voltage levels and other design considerations. Consult the end-product standard for your device to determine what these other considerations are and how they drive the spacing distances required.
Pro Tip: Parsing regulatory standards to determine proper spacings on the fly can be challenging and time-consuming. I highly recommend creating a custom spacings calculator using a spreadsheet program to streamline this process. Reference 1 provides an example of how to do this. Using a dedicated calculator saves time and reduces errors when providing spacing values.
Design Guidelines
PCB manufacturers often provide specific design guidelines or specifications for trace spacing based on their manufacturing capabilities and industry standards.
These guidelines ensure that the PCB design meets required electrical and safety standards.
Designers must consider trace spacing along with other parameters (such as trace width, impedance control, and signal integrity) to ensure proper functioning and reliability of the PCB.
Use of the PCB Tool
The PCB layout tool you’re using has a measure function. It is imperative that you know how to use it. It will likely have an icon you can click on to enable it. Look for something with an arrowhead on each end like this: 
If you’re measuring from point A to point B, attach one end of the arrow to point A and the other end to point B, then read off the measure value given. The value is usually in millimeters (mm), corresponding to the values in the tables provided in your safety standard. Compare the value obtained using the PCB measurement tool to the requirement. If the value obtained meets or exceeds the requirement, then all is good. If it does not, then you have some work to do. When in doubt, actual measurement results with a set of calipers are the ultimate authoritative answer. I do not know about you, but I do not ever want my spacings to be so close to the requirement that I have to break out the calipers to decide whether they are good.
Pro Tip: Include some margin (up to 10 to 20%) in your layout so there is never any question about meeting the requirement and accounting for manufacturing tolerances with your PCB vendors.
More Clarification on Where to Measure Creepage Distances
The biggest issue I see for new people who get involved in measuring creepage is where to take the measurements. In most instances, creepage is measured from one circuit to another, not within the circuit. For instance, if I have Circuit A positive and negative next to Circuit B positive and negative. I would measure from the points of Circuit A that are closest to Circuit B and ensure these distances are met.
Conclusion
Remember, accurate measurement and adherence to these spacing requirements are essential for ensuring safety in your PCB designs.
References and Further Reading
- The Calculation of Creepage and Clearance Distances Using a Spacings Calculator, In Compliance Magazine, February 2022.
