Along with designing electrical hardware and running for stress management, I’m an avid knitter. My grandmother taught me when I was in grade school, and I’ve taught a dozen or so people along the way. Before you can get to the fun part of actually knitting something, you have to select a yarn. Is it for a newborn? Then I want it to be soft, but also machine washable so the mom can actually use it without worrying about handwashing. Does it matter if it’s itchy? What if someone is allergic to wool? Does it need to drape or hold its shape?
I have to sort through a surprising number of parameters before I can start knitting, or I’ll have to scrap everything and start fresh when it doesn’t turn out well. The same is true when you select the materials for a PCB. Early in your PCB design career, you probably didn’t specify much, and unless it was for a specialized project, it probably turned out fine. However, once you level up to high voltage or other niche PCB applications, you need to start taking additional design requirements into consideration.
Select an appropriate PCB material
The foundation of your PCB is the board, so that’s the first material specification you should consider. You want the material to be appropriate for the performance requirements, but also for the operating environment since that will have a large impact on how the PCB material ages.
For a high voltage PCB, you’ll need a board material that’s specifically designed to tolerate an overvoltage event, as well as the regular high V operating conditions. There are a few material options you to consider:
FR4 Laminate: FR4 high a very high dielectric breakdown. However, it is more porous than BT epoxy and polyimide, which makes it easier for the board to become contaminated. It also has a weak edge structure, and as the edge cracks, the dielectric value will decrease. Aging is a likely problem, especially for electronics near the edge. FR4 also has no recovery or protection from carbonization that occurs during overvoltage events.
BT Epoxy: A thermoset resin, BT epoxy has strong sidewalls and is better for applications with planar coils and medium voltage circuits.
Isola, high V laminates: There are several high voltage laminates, Isola is one of the most well known, that actually extinguish arcs and leave a non-conductive base layer. While that is an incredible performance advantage in high V applications, understand the design restrictions before you start. These laminates are usually quite pricey, and you can only produce single sided boards or very simple two sided boards.
When you first start discussing production, get the datasheets for all your options from the manufacturer and make sure the performance matches your requirements. Also, don’t mix and match the insulators on your board. The mismatch in material properties can cause issues in manufacturing and performance down the line.
Don’t use multiple insulating materials within the layers of your PCB.
Check glass and resin contents
If the resin content and glass style used in the PCB aren’t in the datasheets or manufacturing guides you receive, ask for them specifically. In a high voltage PCB, having a high resin content is important for minimizing voids between laminate layers and for changing the effective dielectric of your board. You also want a small glass style to help the resin penetrate.
Voltage gradients are rated for resin systems in units of [VDC/mil]. You should make sure your voltage gradient is less than the aging derated value for the resin system between your insulating sheets in the board. For an AC voltage gradient, use less than half of the DC gradient value.
Choose the right conductors
Once you have the laminate layers and resin of your PCB sorted out, you can move on to the conductors. The spacing between your traces and pads is dictated by safety and standards. For example, MIL-STD-275 recommends spacing at 8 V/mil, however many of these standards are older and haven’t been updated to account for new materials like HVPF or Kapton, which can handle 1000 V/mil.
You will also want the quality of your copper to meet your requirements. Choose a copper weight for your vias that’s adequate for both electrical and mechanical stresses. 1-3 oz copper vias are prone to failure with any over-current, and 1-2 oz vias are susceptible to mechanical stress. Even slight increases in the weight improve the survivability of your board.
Before you send your design to be fabricated, make sure to specify the weight of copper to be used. Low weight won’t hold up to high voltage applications.
Ask about finished surfaces
Lastly, ask your manufacturer about the finishing on your board. Poor surface conditions on the board, like bumps, particles, inconsistencies, or contamination, make arcing more likely and undo all the expensive material selection you’ve done. On the conductor, rough spots or scratches will create areas that concentrate the electric field strength and you will have arcs at lower voltages. Make sure the handling of your boards throughout production protects them from any damage that could decrease their high voltage performance.
Scratches in copper layers increase the risk of arcing.
While choosing dielectrics and conductors is a pricier process than choosing yarn, it doesn’t have to be more difficult. By using design tools that manage your performance requirements and tolerances in one place, it’s easy to identify what you need and save money on what you don’t. Electronic CAD software like Altium Designer and Altium Vault make managing your design and integrating your supply chain simple so you can finish your PCB and get back to knitting, running, or any other project you’d rather be working on.
Have a question about materials for high voltage designs? Contact an expert at Altium.
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