What is PCB creepage and clearance?
Factors that determine high voltage creepage and clearance standards.
Design methods to meet PCB creepage and clearance standards.
High voltage parts on a printed circuit board
There are many applications of fire that are productive, warming, and even romantic. However, one place that you never want to see fire is on a printed circuit board. Fire can be a real risk if the PCB hasn’t been designed correctly for high voltages. In some circumstances, incorrectly designed boards can create an electrostatic discharge, which could in turn cause damage to the board and even possibly spark a fire.
To avoid this problem, PCB designers working with high voltage applications should pay particular attention to the separation between exposed metal conductors within their designs. There are two ways to measure this separation—creepage and clearance—and it is important to maintain the required amount of separation for both. In this article, we’ll look at the differences between PCBs with high voltage creepage and clearance standards and the steps you need to take to avoid high voltage problems or unplanned PCB combustion.
What Is Clearance?
To understand clearance from a more introductory position, consider air flight. Traveling by plane is the fastest way to get from point A to point B for numerous reasons, but a large component is due to the pathing. The plane doesn’t have any concern for the contour of the surface below it as it travels through the atmospheric sea above, except for take-off and landing.
Generally, clearance is of little concern in low voltage designs–the arc gap is so small that there are far more issues that will be encountered during operation (and likely even placement) before clearance would have a chance to manifest. As clearance defines the most direct path between conductors, it, in a general sense, means the conductive surfaces under consideration form a capacitor with an atmosphere operating as a dielectric in the space between. Because air is a poor conductor, there is little charge storage action, and when sufficient charge is built up on surfaces at some particular distance, a displacement current may form. Any unintended movement of charge is hazardous to the board’s operations and functionality, and higher voltages are quicker to reach breakdown as well as exhibit increased severity (at the same distance).
At some point, the concern extends beyond the operation of the device to the safety of potential human operators as well as fire and smoke hazards. There are additional environmental considerations that must be accounted for while designing for high voltage clearance; factors such as humidity and particulate concentration will have a direct effect on the breakdown voltage between two charged surfaces.
What Is Creepage?
Creepage is the path-dependent sibling of clearance. In the transportation metaphor, creepage would act equivalent to a train (or car, or boat) in that, by design, it has to travel along the surface of the medium. At a minimum, this means creepage must be as long as clearance, but never shorter. In many cases, following the surface contours greatly increases the creepage distance relative to the clearance for the same charged surfaces.
While clearance is likely to express itself in between the component surfaces of high voltage circuitry, creepage is more insidious. It is typically seen on the board surface between traces. Though component spacing can be a factor in dense designs, it is far more likely to experience issues with trace spacing due to their relative prevalence. Luckily, there are a few novel solutions for adapting a completed design or revision without significant alterations.
The Difference Between PCB Creepage and Clearance
Printed circuit boards that conduct high voltages can be victimized by an electrostatic discharge between exposed metal if that metal is too close together. This discharge can potentially cause damage to the board and its components, and PCB designers need to observe proper spacing between metal conductors on the board. To summarize, conductor spacing on a circuit board is measured in two ways—creepage and clearance:
- Creepage: This is the distance between two conductors on the surface of the board or along the surface of the insulating material.
- Clearance: This is the line-of-sight distance between two conductors through the air.
In high voltage circuits where the voltage is above 30VAC or 60VDC, creepage and clearance spacing rules must be observed to prevent unintentional arcing between two conductors. Often, these two measurements are the same, as the distance between two component pads or other exposed areas of metal on an unobstructed portion of the board. In some cases, however, the two measurements are different, depending on how the components are placed or if there are any unusual contours of the raw board. As shown below, the creepage is the spacing across the surface of the board between the two-component pads. The clearance, on the other hand, is the air gap between the metal heat sink of the component on the left and the component pad on the right.
An example of creepage and clearance spacings on a PCB
Next, we’ll look at where some of these creepage and clearance standards can be found.
What Are the PCB High Voltage Creepage and Clearance Standards?
Different factors go into calculating high voltage creepage and clearance standards, including the working voltage, pollution degree (dust and condensation on the board), and the type of circuit that is being evaluated. Another important factor in the creepage calculation is the layer materials that are being used to build the raw board. The voltage will create a conductive path along the surface of the board that will break down its insulating properties, and some board materials are more resistant to this effect than others. The comparative tracking index (CTI) defines the amount of resistance of each material; the higher the number, the greater its resistance to breaking down. For instance, the default CTI value for FR-4 is 175, while other more specialized materials can have a CTI value as high as 600.
Different documents cover PCB creepage and clearance standards, with the primary one being IPC-2221. This is a general standard that covers many different design rules, including the spacing you need for high voltage circuits. The limits are delineated by DC or AC voltage levels, internal layers, external layers that are coated or uncoated, and substrate materials. In addition to IPC-2221, you can find additional information in these standards:
- IPC-9592: This standard is more specific than IPC-2221 in that it defines the spacing requirements of power conversion devices that run higher than 100 volts.
- UL-61010-1: These standards specify the requirements for safety in the design of electrical testing and laboratory equipment as well as other industrial equipment.
- UL-60950-1: This standard is for high and low-voltage applications across a broad range of equipment.
These standards will provide a solid foundation for setting up the clearances needed on your PCB to protect it from an over-voltage event that can create an arc between two different conductors. It is also important to simulate the power distribution in your design and use those results to help set up the physical attributes of the layout.
Power supply components on a circuit board layout
Incorporating Safety With Creepage and Clearance
As outlined above, UL-60950-1, specifically tables 2K-2N, provides designers with minimum clearance and creepage values dependent on board insulation grade, coating, and other factors. An important facet of safety design with high voltage is the insulation provided to circuits that would have a high likelihood of arcing. When placement necessitates low voltage secondary circuitry near a high voltage power circuit, the standard establishes a multitude of different insulation options depending on the severity of the voltage involved. For example, low voltage circuitry surrounding high voltage power circuitry should have double or reinforced insulation. With the expected peak voltage and distance, designers can use calculators to determine the minimum spacing distance and build out design rules to heed high voltage safety requirements.
How to Design Your PCB Layout With These Standards
Once you have the spacing requirements the board will need for proper creepage and clearance, the next step is to set these values up in your PCB layout system’s design rules. Rules and constraints can be set up for metal to metal clearances, whether it is between traces, pads, copper pours, or any combination of the three. You will also want to set up the correct spacing for your component footprints, especially those larger parts that are conducting a lot of voltage. In some cases, you can even set up unique keep-out clearances around specific footprints to account for heat sinks and other unusual configurations. All of these rules allow you to maintain the necessary spacing between high voltage components.
When you are placing components in a layout, make sure to consider all aspects of the parts as you place them. This will include any metal conductors, such as heat sinks, that may overhang the part, especially if you aren’t able to set up a unique keep-out zone around it. During layout, you will also want to ensure that any edits or changes to the component placement don’t end up violating spacing that you have already carefully set up between the parts. Even rotating a component 90 degrees can cause an unusually shaped part to violate a voltage clearance.
Here are a couple of other techniques to consider during layout for spacing:
- To help maintain the right amount of clearance spacing, try locating high voltage components on one side of the board and low voltage parts on the other. In some cases, high voltage parts may not have as stringent spacing rules between them as they do low voltage parts.
- Creepage will require another solution, as putting parts on the other side of the board won’t give you the separation you need through the surface of the board. Instead, you may want to install insulating barriers or cut grooves or slots into the board. The change in the contour of the board will increase the distance over the surface that the voltage has to travel, giving you the creepage separation you need.
Fortunately, your PCB design tools have many capabilities within them that can help, and we are going to look at those next.
Rely on the Power of Your PCB Layout Tools
The Constraint Manager in Allegro showing the spacing values between component footprints
Setting up design rules and constraints is the best method to ensure your layout meets PCB high voltage creepage and clearance standards. As you can see in the picture above, the Constraint Manager in Cadence’s Allegro PCB Editor can be set up with a specific package to package clearances to maintain the proper creepage spacing of high voltage parts. In addition, you can also create net classes for power and ground nets, and assign them the necessary spacing clearances as well. These settings, working in conjunction with design rules, will help you maintain the spacing that you need.
You should also make good use of the circuit simulators and analysis tools that your PCB design system comes with. These can be used to model the power distribution network of the board to make sure that your high voltage nets are connected the way you need them to be. And lastly, be sure to use the 3D viewing and checking features in your PCB design tools. Parts that are conducting high-voltage may have unusual contours such as heat sinks, and being able to check them in 3D and visually confirm their placement can be a big asset to your design.
For more information on designing the power distribution network in your next PCB design, check out our E-book on the subject.
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