When was your first warning to be careful around high-power electricity? Was it playing near powerlines in your childhood, or perhaps attempting to insert something besides insulated prongs into an outlet? Electrical safety shapes most people’s youth experiences, but you’d be surprised how much of this goes out the window in a second-year electromagnetic course at a community college. A tip for anyone curious: probing an outlet with a multimeter on the current setting will blow a fuse on the tool and shut off the power. Thankfully with automatic backup on documents, I didn’t lose more than a few minutes of word processing.
With that said, high-power electronics do pose significant safety concerns, and there’s a reason why so many fail-safes are implemented to prevent accidental or careless contact. An active main line in a residential setting carries a potential of hundreds of volts, but industrial uses, and their associated risk, can be even greater. Therefore, PCB design for high-power applications needs to consider the significant hazard posed by those nearby and the effects such high power has on the board’s integrity.
Constituents of Power: Current and Voltage
As a brief refresher, power is the product of either instantaneous or average current and voltage within the system. A high-power application may invoke extreme values for voltage, current, or both, and the board’s construction must change considerably relative to standard power conditions to protect the board:
- Current: The two main concerns for high current capacity on the board are the ability of the copper features to withstand the current being transported and properly mitigate heat due to the high levels of Joule heating. Attempting to drive high currents through thin copper foils, layers, and features is like trying to direct rush hour traffic through side streets: they simply cannot support the high volume demands. High-current board fabrications utilize inlays, which have copper span the thickness of the board. Combining power and control electronics on the same board may encounter difficulties due to the massive disparity between the ideal copper feature parameters for the two electronic modes. A more elegant solution would be an additional PCB that isolates the power, drastically reducing the complexity of the formerly combined assembly.
- Voltage: The central issue arising from high voltage design is the increased potential for arcing and corona discharge events. Without adjusting for the distance between conductors, arc current can jump, increasing smaller gaps and leading to failure and a potential safety hazard. Corona is a bit more subtle, though it can be tied to the occurrence of arcing: corona causes ionization from the speed of electrons accelerated by the strong electric field colliding with the insulating material particles, which causes a domino effect. Rounded pads and traces are a priority for layout designers as they drop the necessary arcing voltage by a third, with larger radii providing the greatest benefit. Other factors such as temperature, humidity, and particulate can aid in arcing, necessitating an enclosure or other environmental controls.
While there isn’t a standard cutoff value for high voltage or current, they’re both appreciably high at 1000 volts or amps, respectively, though some establish high currents in the 100s of amps and high voltage up to 100 kV.
How PCB Design for High-Power Assemblies Is Affected
Due to the more taxing electrical requirements of a high-power board, common features must be adapted and entirely new ones adopted. Below is a list of some of these changes one can expect to encounter
- PCB terminals: High currents are often brought onto the board via low gauge wires, requiring an interfacing component. Due to their size, terminals need to be rugged with excellent grip strength to prevent wires from slipping and preventing contact.
- Corona/grading ring: Corona and grading rings are identical in function but diverge in their application. To prevent corona formation, a toroidal ring is attached to the conductor at any point to maximally distribute the potential in the manner least conducive to a corona event. Grading rings are placed around insulators instead, once again ensuring that the potential is equally distributed over a larger area of smooth curvature to prevent dielectric breakdown.
- Solder mask/cover coat: Solder mask typically provides excellent dielectric coverage for voltages up to the low end of the high power spectrum, but the coverage is inexact and discontinuous. Multiple treatments can be applied, and oven curing for certain solder mask formulations. However, a cover coat provides a more robust solution at the cost of additional processing. As the cover coat is added as a single solid layer, drilling/milling must be performed for any outer layer copper features.
- Router slotting: A router slot is a failsafe that prevents arc current from traveling any further than the edge of the conductor placed between two high-voltage conductors.
- Press-fit insulation: A further method of isolation between high voltage conductors that places an insulative material into a routed slot that greatly inhibits the conduction pathway.
- Thermal vias: In addition to normal usage, placing thermal vias on soldering pads improves thermal responsiveness and routing.
- Solder: Whereas the standard shape for solder on a through-hole pin is conical, this sharp point provides another possible arcing and corona point. A solder ball shape minimizes this behavior.
- Metal sleeves: Like solder balls, metal sleeves can be placed on pins normal to the board better to distribute the electric field over a larger area.
Power Up Your PCBs With an Experienced Contract Manufacturer
PCB design for high-power applications requires some additional thought and planning due to the inherent danger raised by elevated electrical properties. Arcing and corona discharges can cause board failure and create conditions significant enough to vaporize the metal core, posing a dire safety hazard due to the rapid expansion within a closed space and resulting fragmentation. VSE can help allay fears related to the operation of high-power electronics with comprehensive assembly analyses and tests. Our team at VSE is a group of engineers that are dedicated to building electronics for our customers, and along with our valued manufacturing partners, we aim to deliver excellent PCBAs that meet or exceed industry standards for safety.