Thank goodness for the Renaissance. This was the period of awakening (or reawakening) that followed the so-called “Dark Ages” in Europe that followed the fall of the Roman Empire. Although many view the Renaissance as the age of innovations in art, it is probably more significant for the impetus it gave to scientific discovery and an increased understanding of the natural or physical world. One of these catalysts was the formulation of what has come to be known as the scientific method by English statesman and philosopher Sir Francis Bacon in his book Novum Organum Scientiarum, published in 1620.
As engineers, we are no strangers to the scientific method. However, schedule demands can occasionally lead to a quest for a solution even if that means foregoing the steps of the method. Although this may be expedient and in fact work in some cases, the best approach is typically the most thorough one. Just as creating an effective PCB development process depends upon the relationship with your contract manufacturer (CM), building boards that are resilient to high voltage arcing requires that you follow a method that will lead to the best solution. Let’s see how the systematic scientific method can be used to optimize your high voltage PCB design for arc prevention.
Applying the Scientific Method to Arc Prevention
There are electrical systems that make use of the fact that oppositely charged conductors placed physically apart can create an electrical arc through the insulating material that separates them. Motors that use spark plugs, tasers, and telephone high voltage protection circuits are common examples of this where air or gas is the insulation medium. Early circuit boards also used spark-gap transmitters and other similar devices. Today, however, aside from occasional use for inexpensive and unsophisticated overvoltage protection, arcing is an undesirable and dangerous phenomenon on PCBAs.
Arcing on circuit boards occurs when an unexpected high voltage is induced on or generated by a conducting element on your board and a sufficiently close conductor has an opposite or significantly lower charge. Components, pads, traces, or connectors can all serve as the transmission or reception point for the arc. Boards that typically operate at or process high voltages, such as those used in industrial production environments, are most likely to experience arcing. And the results can range from insulation breakdown or surface tracking and greater susceptibility to future arcing, burning of the board, components or traces and even fires.
Preventing high voltage arcing on your industrial PCBAs requires that you design for industrial PCB production and partner with a CM that is capable of meeting the manufacturing quality and reliability levels necessary. This should include establishing a proven method for addressing the potential of high voltage arcing, such as the process listed below.
The Scientific Method for Arc Prevention
1. Defining the problem
In this case, the problem is known. That is unwanted arcing on your board.
2. Define parameters to be studied
The essential parameters to be studied are:
A. High voltage arc tracking rate (HVTR) is the length of a surface arc or track over time that will form in the presence of high voltage.
B. High voltage, low current dry arc resistance is how long a dry uncontaminated surface can resist tracking under increasingly more extreme conditions.
C. Comparative tracking index (CTI), which is the maximum voltage a wet (contaminated water) material surface can withstand without surface tracks appearing.
3. Establish metrics for evaluating the parameters
Fortunately, metrics or performance level categories for each of the parameters above as been established by Underwriter’s Laboratory in UL 746-A.
4. Define a test regimen
Standard tests for the parameters given in 2A, 2B, and 2C are available and defined in the following:
A. HVTR in UL 746A.
B. High voltage, low current arc resistance in ASTM D-495.
C. CTI in ASTM D-3638.
5. Perform testing
Typically, CMs do not perform the tests listed above; however, there are a number of companies that do, including UL.
6. Analyze the results
The testing results for your board should be used to formulate a strategy for preventing arc prevention based upon its environment and risks for exposure to high voltages.
7. Propose a solution
Armed with the test results and analyses, you can devise a solution targeted for your design and application, as discussed below.
PCBA Manufacturing for Extreme Environments - Part 1
The Best High Voltage PCB Design for Arc Prevention
As shown above, following the scientific method will provide you with the PCL for the three important arc tracking parameters: HVTR, high voltage, low current arc resistance, and CTI. This information, along with an understanding of your board’s deployment environment, will enable you to best design your boards to prevent arcing by incorporating the following, as necessary.
- Choose arc-resistant materials
It is important to select board materials that are best equipped to handle high voltages to resist board insulation breakdown.
- Maximize spacing between conducting elements
The closer conductors are together the easier it is for surface tracks between them to be created. However, this has to be balanced against layout restrictions on PCB trace width and spacing.
- Apply conformal coating
Conformal coating is applied to protect your board from external contamination; however, its composition is dielectric which adds insulation to your PCBA surface.
- Utilize shielding
Shielding not only aids in minimizing EMI and is a major aspect of achieving electromagnetic compatibility within your board’s installation environment; it will also help isolate high voltages that could be the source of arcs on your board from other conductors.
|Tempo‘s Industry 4.0 Custom PCB Manufacturing Services
Surface tracking, which is the result of arcing can pose a great risk to your boards, especially in industrial environments. Therefore, it is important that you devise and implement the best high voltage PCB design for arc prevention, which includes the actions and choices listed above. At Tempo Automation, the industry leader for fast, high-quality PCBA prototyping and low-volume production, we will work with you to incorporate the best design to reduce the possibility and impact of arcing on your boards.
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.