Bleeding Edge: Extreme High-Voltage Printed Circuit Boards

Estimated reading time: 2 minutes

Designers who need high-voltage separation between layers of over 5000 volts find that typical FR4 has too many micro voids within its structure to be effective high-voltage laminate. The micro voids in most laminates allow low-voltage arc-over, and carbon growth within the laminate limits the highest voltage usable--both a bad thing. We use HVPF high-voltage polyimide film, trademarked by Sierra Proto Express, which provides a micro void-free film rated at over 3000 volts per mil of thickness. Copper-coated HVPF film laminates to Fr4 well with a single 106 prepreg as a glue sheet. HVPF accepts electroless plating properly as well as typical solder masks. With a single layer of 5 mils of HVPF, you can obtain a maximum voltage rating of 25,000 volts. At 25,000 volts, you have other problems such as corona to worry about. Corona is a plasma-like field of highly excited molecules that are very corrosive to items such as epoxy laminate. Some laminates and plastics withstand the effects of corona better than others. The HVPFfilm has a very solid void-free matrix, which limits corona's destructive effect.

When a designer asks for high-voltage printed circuit board design help, we usually offer to email our book on high voltage as a starting point. (You can download your copy for free at PCB007.) As with most advanced electronics, high voltage design has special areas of concern. Your traces should be free of 90-degree turns; avoid sharp, pointed corners; round all traces; use fillets to join pads, as this decreases the ability for corona to be formed; and space all traces according to arc-over potential times two.

Spacing on a vertical plane is always tough. Your minimum arc-over rating is 10,000 volts (dc) per .250 inches to arc. This rating is based on sea-level atmospheric pressure. Some extreme designs send the high-voltage board into space or high up in a fighter plane. As you lower the atmospheric pressure, the voltage arc-over drops dramatically, which effect must be calculated and then compensated for.

In really extreme applications, a high-voltage arc guard area can be utilized by creating a lower-voltage shield with a copper guard ring between the high voltage and the ground. The half-voltage guard is divided between the high-voltage source and the ground with a pair of large resistors, such as 100meg ohm.  The shield is at one half the potential of the high voltage, so your arc voltage potential decreases by half. 

With a little R&D, a little imagination and a lot of sweat, most extreme printed circuits can be manufactured successfully.Download your free copy of "High-Voltage PCB Design"

Ken Bahl   CEO, Sierra Proto Express

Robert Tarzwell   Director of Technology, Sierra Proto Express