Any PCB carries important industry standards that should be followed and considered during design and assembly. Among the various PCB design standards, one important set governs creepage and clearance distances between neighboring conductors on a PCB. These standards are designed with the goal of designing electrostatically safe products, especially products that run at high voltage.
If high voltage develops between exposed metal in a PCB, electrostatic discharge can occur if the electric exceeds the threshold for dielectric breakdown. This can damage components and, in extreme cases, even cause your board to catch fire. Creepage and clearance limits between exposed metal on your PCB are codified in important industry standards, which should be followed during design and manufacturing. These standards are industry-agnostic, but different standards are broadly application-specific.
If you’re using a CM to produce a batch of PCBs, and your goal is to comply with standards on creepage and clearance, your CM should check these distances in your design data as part of a DFM check. Once you consider the arrangement of traces and components on your PCB, determining creepage and clearance distances can be difficult to do by eye. Creepage and clearance limits also change when considering environmental factors, substrate material, DC or peak AC voltage, and component arrangement.
Creepage vs. Clearance on a PCB
These two terms are closely related and refer to two ways to measure the distance between conductive elements on the surface of a PCB. Note that creepage and clearance have nothing to do with trace width during routing.
There are two ways to measure the distance between two conductors on a board:
- Creepage is the distance along the surface of the board.
- Clearance is the distance in air between two conductors.
If this sounds a bit vague, take a look at the diagram below. This image shows a side view of a PCB with a valley between two copper conductors. Creepage is shown in red and is drawn along the surface of the board.
Creepage and clearance distances in a circuit board.
The diagram above suggests that the creepage distance is always larger than the clearance distance, but this is not always the case. Once you factor in components that have hanging conductive elements or heat sinks, creepage and clearance distances become somewhat less obvious. Consider the case shown below, where two components in decawatt (DPAK) packages have heat sinks that hang over the pads. The clearance distance, in this case, is defined in terms of the overhanging metal on the DPAK components.
Creepage and clearance between two DPAK ICs and their mounting pads (side view).
When laying out components, you’ll need to consider orientation, spacing, and the presence of any overhanging conductors to ensure you do not violate creepage or clearance limits. In some cases, simply rearranging or shift components will place enough clearance between exposed metal so that you can comply with safety standards. In other cases, where creepage is violated while clearance is satisfied, one option is to place a notch that passes through the entire circuit board between components. A knowledgeable CM will be able to recommend the best design changes for your board during a DFM check.
Important Creepage and Clearance Standards
Several important standards define clearance and creepage distances in a PCB. The best-known standards are IPC-2221, IPC-9592B, UL-61010-1, and UL-60950-1 (second edition). These standards list different creepage and clearance design rules that are geared for products designed for various applications and operating environments. While reproducing every rule in these standards is beyond the scope of this article, designers should consult the creepage and clearance limits relevant to their application.
IPC-2221 and IPC-9592B
IPC2221A is a more general standard that specifies creepage and clearance distances. However, the creepage and clearance rules are broken down into different application areas. The limits specified in IPC2221 are delineated by DC or peak AC voltage level, substrate material, and much more. Limits are also defined for conductors on internal layers, and coated or uncoated conductors on external layers. IPC-9592B is more specific in that is defines trace clearance and creepage requirements in power conversion devices running above 100 V.
UL-61010-1
This set of standards specifies safety and design requirements for electrical testing equipment, laboratory equipment, and industrial process control equipment. This standard excludes several products in different application areas. You can find a complete list of excluded application areas and the relevant UL standards in Section 1.1.2 of the UL-61010-1 standards.
UL-60950-1
This particular standard largely replicates the IEC-60950-1 standard for information technology products. This standard specifies safety standards for high and low voltage equipment, including creepage and clearance distances on a PCB. This standard is broad enough that it covers everything from office equipment to telecommunications equipment.
The Value of a Knowledgeable Partner
Given the list of rules related to PCB creepage and clearance and the complexity they can place on DFM checks, it helps to work with a CM that has deep knowledge and experience of different design and quality standards. Your CM can be a major asset when it comes to ensuring high yield and quality, as well as ensuring safe operation of your new product.
