High voltage PCBs (printed circuit boards) power some of the most innovative applications available. They typically consist of heavy copper material and can withstand thermal fluctuations and high voltages. This post will highlight the design guidelines for building this PCB type so that you can avoid the common mistakes that result in design flaws and unnecessary expenses.
High Voltage PCBs Defined
Any PCB that runs a higher voltage compared to normal rail voltage PCBs is considered high voltage. Power suppliers, inverters and EV boards fall under the high voltage category. There is not a fixed digit that qualifies a PCB for high voltage, but some people unofficially define a PCB with 100V or more as high voltage. Beyond this number, you need to take certain design practices into consideration or else the PCB will fail or crumble.
A designer should be acquainted with the following technical terms before designing a PCB:
- Functional insulation (only required for operating the circuit)
- Basic insulation
- Supplementary insulation
- Double insulation
- Reinforced insulation
- Clearance (the shortest distance in air between 2 conductors)
- Creepage distance (the shortest distance between the conductive layers on the board)
The designer must also decide on the base material of a PCB.
Choosing the Material
The board is the foundation of your PCB design in any case. Therefore, choosing a suitable material is your first concern when designing. The material must tick several boxes in order to become accepted in large-scale production. In case of a high voltage PCB, the material should withstand high voltage as well as overvoltage operations. Some materials you can consider when designing a high voltage board are:
- Laminate FR4: FR4 has a high dielectric profile, but it is more porous than other materials like epoxy. Porous materials can contaminate the board and are also prone to become weak and break. Aging is also an issue.
- BT Epoxy: BT Epoxy is a thermostat resin with strong walls and can handle medium voltage with planar coils.
- High V Laminates: Isola is one of the most well-known manufacturers of high voltage materials that provide you with a non-conductive layer. However, one must understand the design restrictions before locking Isola in. The drawback is that this laminate is very expensive and also not suitable for multi-layered boards. Therefore, you can produce only one-sided boards.
Choosing the Conductor
After sorting out the laminate layers and resin choices of the PCB, it is time to choose the best conductor for your high voltage PCB design. Safety standards safeguard the safe spacing between the traces and pads. However, some of these best practices have become old and redundant. These standards fail to take into account the new materials like Kapton which can handle up to 1000 V/mil.
The quality of your copper should also meet the standard requirements. It is critical to choose a weight that can bear the load of both electrical and mechanical stress. A mere 1-3 oz of copper can fail on any current and is also susceptible to mechanical stress. Any increase in weight contributes significantly to the durability of your PCB.
Best Design Practices
Time and again, manufacturers receive feedback from PCB users on high voltage boards. This information helps in creating best practices that can reduce the chances of errors in production.
Routing best practices are essential to maintain when designing a high voltage PCB. Ideally, the designer maintains clearance between traces that have a high voltage difference in between. Moreover, it would be best to avoid any sharp edges because they can act as areas of high concentration of electric field. In the internal layers of the board, it’s crucial to avoid running high voltage traces as well.
- Internal Layers
It’s also vital to make a multi-layer PCB with a medium voltage on each layer. Filling the spaces between the layers requires caution. The thickness of each separation between layers must be .005” in order to maintain a balance in the overall PCB design. In high voltage PCBs, any voids or useless gaps disturb the dielectric value.
- Polygon Planes
You should also consider increasing the polygon plane clearance until it touches a safe value in all high voltage PCBs. Internal planes in a multi-layer PCB should have the appropriate separation and a high voltage. This enables the smooth passing of the current without disrupting the other elements on the board.
You may have heard about high voltage PCBs emitting a disturbance on a wide spectrum. To minimize this issue, you can shield the high voltage items after potting with the help of a metal sheet. A small loop area in the ground plane can greatly help in minimizing the disturbances.
A Word on Clearance and Creepage Distance
Like other boards, high voltage PCBs also come with tight spacing requirements. An arc could easily develop between the conductive elements of the board, but you can avoid this by correctly spacing components on the PCB. Boards with 80V spacing between conductors should have 0.1mm between internal layers, 0.6mm between external layers and 0.13mm between coated external layers.
Creepage is difficult to get right in a high-density design, but there are a few tricks you can leverage to increase the surface distance in your PCB. For example, adding a vertical barrier of insulation or a slot between tracks will help you increase the creepage space without affecting the trace layout on your printed circuit board.
It’s also crucial to keep the material properties of your board in mind. The CTI or Comparative Tracking Index indicates the electrical insulation of your materials. A total of 6 categories from zero to five are based on the breakdown of the material’s level. Category five has less than 100 V values, while Category zero has the most diverse, and often costly, material properties. When materials with better CTIs are not feasible, the designer can house a slot between traces to increase the board’s creepage.
Need more help with high voltage PCB assembly and design questions? Reach out to our contract electronic manufacturer experts to learn more. We’re happy to help.