Outside of video games, shortcuts and corner-cutting have a more negative connotation. We tend to view shortcuts as cheating, whether the person is performing the action or the recipient. Typically, a shortcut invokes risk, which might momentarily solve a problem but lead to issues in developing knowledge or skills that can later undermine the shortcut user.
An arc is a shortcut path for electric current, but its consequences are immediately more noticeable: high voltage arcing is likely to lead to the destruction of components and board features and, at high enough potential, can even vaporize a board entirely. Any of these outcomes carry a hazard to nearby operators or equipment. High voltage PCB design for arc prevention is, therefore, not only a reliability concern but also contributes to the safety of board end users. It’s necessary to understand the causes of arcing and the solutions designers and manufacturers can implement to prevent its occurrence.
An Overview of Arcing Conditions and Terminology
One of the most basic functions of electronic design is guiding where current flows, but it is just as important that materials can also restrict the flow of current. These material types, broadly grouped as conductors and insulators, are utilized at different points throughout a component or board to control the ability for current to pass. What separates insulators from conductors is the resistivity of the material. This intensive measure determines a material’s opposition to a current, with resistance being the extensive analogous property based on the total amount of the material.
Insulators and conductors are a loose sorting system but can be best differentiated based on the band gap or the maximum energetic difference between the valence band and conduction bands. This is a significant energy gap for inductors, while for conductors, this gap either does not exist or is trivial to overcome. During standard operating conditions, this means that there is not enough energy present for a particular insulator to excite the electrons to move into the next band. This condition occurs for insulators and is known as “electrical breakdown.” This term means the insulative effect of the material is overcome by a sufficiently high electric field, causing an arc to form, and the insulating media (in the case of a solid) experiences a permanent drop in resistance.
Arcing poses survivability issues to the board and affected materials and can cause significant damage to nearby operators. There are three major variables at play when it comes to an electrical breakdown event in an insulator:
- The material’s resistance can be adjusted by changing the density of the media or substituting a different material entirely.
- The voltage developed is great enough that free radicals accelerated into neutral charge atoms can eject the valence electrons, creating more free radicals and an eventual avalanche condition.
- The gap distance between electrodes.
In effect, arc conditions need a strong enough electric field to saturate a space so that an arc can jump the gap between conductors. These two factors are positively correlated – for the same insulator, more voltage is required to jump the gap for a larger space, and vice versa.
Techniques of High Voltage PCB Design for Arc Prevention
This relationship becomes troublesome when coupled with the tendency towards miniaturization in circuit design. With shrinking dies and increased density, space between conductors continues to decrease faster than the voltage levels powering active devices. Take the breakdown of air for two close copper conductors on the surface of PCB: 0% humidity air turns conductive at around 3.3kV/mm, or roughly 83V/mil. While this may seem excessive for common design parameters, it is easily achievable in applications such as satellite design or power grid integration. Designers are faced with one of two choices: either gauge the appropriate minimum distance between conductors accounting for electrical breakdown or look to materials with greater dielectric values for enhanced insulation performance.
Material choice will not be the only effect designers can impose on high voltage designs, however:
- Remove corners from copper features, including traces and pads. Use rounded edges or tapered corners to prevent concentrating the electric field at a sharp point of the conductor.
- Thermal considerations are key. As temperature rises, more energy is available to free radicals. This excitation moves them away from a surface, which can locally increase the potential difference due to a separation in charge, increasing the likelihood of an arcing event.
- Additional solder mask applications can help improve the electrical breakdown performance rating; however, this value is likely far less than the high power rating inherent to high voltage boards.
- A router slot between potential conductors can act as a sacrificial diverter to the direct conductor-conductor jump. This can take up significant layout space on outer levels, however.
- More space between layers to further distribute conductor distance in the z-axis will help reduce internal coupling and prevent arcing.
- An assembly should consider a solder ball implementation instead of the standard heel-toe process to avoid forming sharp points that direct an electric field and increase its density. Additionally, component leads, where applicable, can be placed in solder sleeves, increasing the field’s area and thereby effectively reducing the strength.
Your Contract Manufacturer Realizes the High Potential of Your Boards
High voltage PCB design for arc prevention requires adopting some design practices that are either in general disuse or with deployment rooted more in design superstition than qualitative results. PCB design is a difficult design and manufacturing endeavor, and adding in high voltage considerations on top of that compounds nearly every aspect of the board. VSE is happy to lend our expansive knowledge and experience to help realize your product for all PCB board development, high voltage or otherwise. Quite simply, at VSE, our team of engineers takes pride in building electronics for our customers and coupled with our valued manufacturing partners, we’re eager to bring your next design to life.