My first experience with electricity came when I was seven years old. I had the brilliant idea of holding some wires onto a flashlight bulb and plugging them into a wall socket because I didn’t have a battery. Afterward, it was also the first time I saw my dad both cry and laugh at the same time. Fortunately, I’ve learned a little more since then and have a healthy respect for power—and how current and voltage should be used in PCB design.
Designers must carefully manage power on a circuit board for power integrity, and incorrectly routed current can burn through traces and cause thermal problems. High voltage can also create problems if not designed correctly, including arcing between exposed areas of metal on the board and creating shorts. To minimize the potential for board failures and personal injury, here are some PCB high voltage clearance guidelines that can help.
High Voltage Details and Concerns
High voltage is necessary for many applications, from large industrial equipment to computers. Designers need to be aware of the voltages in the board they are working with to prevent unexpected arcing between exposed conductors. Component leads, pads, traces, or other areas of metal can all serve as conductors which, if too close to each other, will transmit or receive unexpected high voltage arcs. These arcs can cause the break-down of component insulation or create surface tracking on the board, leading to an increased chance of future arcing. Eventually, the arcing can create direct shorts, and the resulting surge of current can burn the board, damage components, or endanger the user.
What constitutes high voltage varies from design to design. Whereas 100 volts would usually be considered high, that same voltage level would be considered low in a power system. In general, though, voltages that exceed 30VAC or 60VDC should follow spacing rules for performance and safety. These PCB spacing rules are measured as clearances and creepage, and it is important to understand the difference between them:
- Clearance: This is the direct space between two conductors through the air. It is a line-of-sight measurement.
- Creepage: This is the space between two conductors along the surface of the board or insulating material. It follows the surface contours.
To simplify this, think of crossing the Grand Canyon. Clearance is like walking a tightrope from edge to edge, while creepage is climbing down into the canyon, paddling across the Colorado River, and then climbing up the other side of the canyon. To determine the risk for your design, there is standard high voltage arcing determination testing that you may want to institute. Next, we’ll look at some guidelines for PCB design that will help you avoid some of the concerns associated with high voltage.
PCB High Voltage Clearance Guidelines
The first step in designing for high voltages is determining the separation required between components to prevent arcing. Simply separating the components between the front and the back of the board may not provide enough space between them. Also, in some cases, the higher voltage components may be placed closer to each other than the clearance allowed between high and low voltage parts. The key is to calculate the clearance and creepage you need using the working voltage, the expected operating environment (dust or condensation), and the board materials you will use.
Board materials are critical to your calculations. When voltages arc on the board, it will break down the insulating properties of the material, creating a conductive path. Some materials are more susceptible to this effect than others, and PCB laminate manufacturers measure this susceptibility with the Comparative Tracking Index (CTI). The higher the CTI value of a material, the more insulating and better suited for high voltage circuits it is.
Once you have the correct calculations, you can use them to establish your design rules. Creating a component class for your higher voltage parts can help enforce a greater separation between them. Here are some additional helpful layout tips:
- Remember that some components will have metal protrusions, such as overhanging heat sinks, that will affect the clearance measurement between them. These can be accounted for by designing the PCB footprint accordingly or by creating specific keepout zones.
- Cutting grooves or slots into the board surface will also help extend the creepage distance, as will the addition of insulating barriers.
- Circular or elliptical pads, as opposed to rectangular ones, will help increase clearances between them.
- Keep your power traces short, thick, and as direct as possible. Using specific information from calculators or trace width tables is essential here.
- Be sure to specify the best surface finishes and conformal coatings to protect the board from trapped assembly debris. Fragments of solder or other metal on the board’s surface can increase the likelihood of high voltage arcing.
At this point, you will need data to make your clearance and creepage calculations, and we will explore that next.
DFM for High-Speed Digital PCBs
Getting the High Voltage Design Information You Need
Different clearance and creepage calculators are available online to find the correct spacings for your high voltage PCB layout. You will also want to consult the manufacturers of PCB laminates to determine the CTI values for the PCB materials you are considering. It will also be helpful to use the different standards that cover high voltage layout and creepage and clearance standards as given in the prevailing PCBA manufacturing standard, IPC-A-610:
- IPC-2221: The general standard covering design rules, including high voltage spacings.
- IPC-9592: A standard specifically targeting high voltage power conversion devices.
- UL-61010-1: Safety standards for the design of testing and laboratory equipment.
- UL-60950-1: A broad standard for high and low voltage equipment.
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Another powerful resource available to you is the circuit board manufacturer that will be building your boards. They can advise you on design specifics for your power applications and help you choose the best-suited materials for PCB fabrication.
At Tempo Automation, our staff is ready to help you with your high voltage design questions. We’ve been working with designers like you on high voltage projects for years, and we have the expertise you are looking for to build your circuit board at the highest levels of quality.
And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.
If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on CAD files or how to incorporate your design into a CAD format, contact us.