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Bob and Me
By Dan Beaulieu and Bob Tarzwell
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PCB101: Fabricating High-Voltage Boards
You're sitting at your desk when the phone rings. It's a customer who needs a PCB that can take 20,000 volts. Would you know how to build such a board?
First things first: You cannot just say, "Well, I think FR-4 will work." It won't, and the board will fail. Boom!
For high-voltage boards like this, special attention must be paid to the material and its specifications. FR-4 has an initial dielectric rating of 800-900 volts per mil, but due to aging effects, a more realistic value is only 300 volts. The volts-per-mil rating describes a breakdown condition in volts for each mil, or one thousandth of an inch.
If your design calls for a spacing of 10 mils between layers, then the aged voltage rating would mean the FR-4 could withstand 10 times 300, or 3,000 volts. So your 20,000 volt circuit would arc over and blow up.
There's that boom again!
The manufacturing process used in high-voltage circuits is essentially the same as that for a normal printed circuit. There are, however, differences in the materials used and the properties of those materials. First, you need to pick a material that will have a high enough voltage rating. High-voltage polyimide film (HVPF) has a 3,000 volt-per-mil rating, so a PCB layer 10 mils thick could withstand 30,000 volts. This is more than high enough to successfully build your 20,000 volt circuit.
Medium-voltage boards are typically 600 to 3,000 volts. Great care must be taken when selecting FR-4 types or a BT epoxy base materials, along with the subsequent processing, as this voltage can easily support arcs and corona.
An arc-over occurs when voltage jumps from one conductor to another. Boom! The corona is the blue glow emitting from high-voltage systems; when you walk under a high-voltage power line, the corona is the crackling, hissing sound you hear. Corona is a destructive force that destroys epoxy by ripping apart the bonds between epoxy particles, thus destroying the circuit's insulating value. Boom!
High-voltage boards of 3,000 volts to a maximum limit of about 100 kilovolts are limited to HVPF, Teflon and in some cases BT epoxy. These require a serious effort by the designer to test and finesse the design to eliminate corona, field strengths and arc-overs.
Customers need higher-voltage PCBs for many reasons. When a circuit board is used in outer space or high-altitude airplanes, the low air pressure cause the voltage to arc over at four times the "normal" voltage. So, a PCB design that requires a 2,000 volt rating at sea level would need at least an 8,000 volt rating in outer space. Many lasers use power supplies in the 5,000-10,000 volt range, which requires a very special PCB. And universities and R&D facilities use very high-voltage power supplies for experimenting with particle colliders.
Outer space brings its own issues; PCBs that outgas in this vacuum will contaminate the other critical components of the spacecraft. There are special manufacturing techniques used to make low-outgassing PCBs specifically for use in PCBs in space. We'll cover that in a future column.
Bob Tarzwell is CEO and founder of DMR Ltd. He can be reached at email@example.com. HVPF is a DMR Ltd. technology.
More Columns from Bob and MeControlled Impedance: A Real-World Look at the PCB Side
Bob and Me: The Key to Increasing Quality - Bribe Your Employees
Bob and Me: Tarzwell's First--and Last--Lean Meeting
Bob and Me: A PCB Potpourri
Bob and Me: Spacing is Irrelevant Below 270 Volts
PCB 101: Coefficient of Thermal Expansion
The Bleeding Edge: Serious as a Heart Attack
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