Have you ever been outside during a storm and had a thunderclap erupt so close that you feared you were struck by lighting? You are safe, of course, since by the time you hear the thunder, the corresponding lighting strike has already occurred. Thunder travels at the speed of sound, which is about 340 m/s, while lightning travels many times faster. Lightning is not a light beam or ray composed of photons; instead, it is an electrical arc through plasma in the atmosphere. Nevertheless, this arcing can be extremely dangerous if you happen to be in its path.
High voltage arc on PCBA
Arcing can also be a hazard on your circuit board. This is especially true for industrial PCBAs that receive, process, and transmit high voltages in areas that may be subject to high temperatures, moisture, or fluid spills. For these boards, it is imperative that the industrial environment is a major consideration for your high voltage PCB design. Additionally, your board should be built to resist arcing, which depends upon knowing the high voltage arc tracking rate (HVTR). Let’s explore the methods to determine this tracking rate after clearly defining what HVTR is and what information it provides.
What is the High Voltage Arc Tracking Rate?
Under normal operating conditions, circuit boards are designed and built to carry expected current levels and voltages. However, when these parameters exceed predicted values the result can be current paths between conducting elements on the board through the insulation, instead of along traces. This is known as surface tracking. The cause of this tracking may be surface debris (such as ionic contamination), which is a major concern for precision circuit boards), a moisture threat, the result of stray capacitance, or insulation breakdown.
Not only can this tracking damage the insulation and distort circuit operation, but it may cause burning or ignite a fire that could impact nearby PCBAs, devices, or the entire system or product. Due to the severity of potential consequences, a number of standards and certification organizations, such as ASTM, IEC, and UL, have developed methods of determining how susceptible insulation materials are to developing surface tracking.
One of the most significant metrics of how well your PCBA will resist the development of surface tracking is the high voltage arc tracking rate (HVTR). The HVTR provides a measurement of the length of a track over time that will form for a given surface insulation of 3mm thickness when a high voltage is applied and is determined by experimental testing.
High Voltage Arc Tracking Rate Testing
High Voltage Arc Tracking Rate test setup for a PCB
The experimental setup for testing a PCB for high voltage arcing is shown above. This setup is used for a number of standardized tests that can be performed to provide a reasonable estimation of how well or poorly your PCBA will respond to a surface or near-surface high voltage. For example, the testing regimen of ASTM D495 exposes an insulating material to a high voltage, low current source to determine when and why the insulation fails using the same 3mm standard thickness as for HVTR testing. Another test, specified in IEC 61621, is used to compare the resistance of similar materials to high voltage, low current arc discharges close to the surface. Insulating materials are rated in performance level categories (PLCs) that can be used to assess how your board’s insulation would hold up. Similarly, for the HVTR test, there are a series of PLCs, which are shown below.
High Voltage Arc Tracking Rate (HVTR) Performance Level Categories
|High Voltage Arc Tracking Rate Category Ranges|
|Performance Level Category (PLC)||HVTR Range (mm/min)|
|0||0 - 10|
|2||25.5 - 80.0|
|3||80.1 - 150|
The HVTR PLCs, as well as those for the ASTM D495 and IEC 61621 tests, are defined in UL 746-A Polymeric Materials – Short Term Property Evaluations. Collectively, these tests can be used to aid in your high voltage PCB design for arc prevention.
|Tempo‘s Industry 4.0 Custom PCB Manufacturing Services|
The high voltage arc tracking rate is an important parameter that defines if and to what degree your surface insulation will enable unwanted current flow between conductors on your board. If utilized, and in conjunction with other insulation testing the best material and PCB layout decisions can be made for your board’s application. Tempo Automation is the industry leader for building high-quality PCBAs fast and precisely to meet your board’s requirement to support the best tracking resistance based upon your design specifications.
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.