Designing a PCB for High voltage: PCB Layers
My sister used to decorate cakes as a 4-H project, while I showed chickens and built a computer. She had to create at least six “show quality” cakes a year, plus all the versions that didn’t come out perfect. It turns out you can actually get tired of eating cake, so eventually you just get styrofoam forms and decorate those. It gives you a chance to practice the design before you actually put it on a real cake. Getting the design right might take a dozen tries, and even then, a small mistake while you’re decorating can ruin the whole thing.
For a tasty analogy, you can think of a PCB like a cake. The board is cake, but you need copper traces, solder, and overcoats, like different styles of icing, to bring it all together. You need to decide which materials you’re going to use so you can be sure your design will work. And like icing, a small mistake at any stage turns a fabulous design into a sad mess, especially when you’re working with high voltage.
Designing heavy copper boards
Because lighter copper fails at lower mechanical and thermal stress, you should consider a heavier copper for the traces and pads. With heavy copper PCB designs, you’ll need to use the thickest layers of laminate you can. Unlike in cake, air bubbles in the laminate aren’t fluffy and nice. In this context, they cause the dielectric breakdown, and the voltage rating, of the board to decrease. You can minimize air gaps with a good prepreg, like 2113 or 1080, which have high resin content and small glass particles.
You rarely want to mix laminates at the risk of creating material mismatches or voids during fabrication, but it can occasionally work out to replace a layer of prepreg with HVPF. You should proceed with caution, like mixing layers of chocolate and carrot cake.
Planning for solder masks
Solder mask is like the decoration on top of the base frosting. It is applied over the heavy copper PCB considerations on your traces to prevent oxidation damage, which is especially important for high voltage PCBs.
Like frosting, solder mask is very sensitive to temperature. It doesn’t melt, but the voltage rating for a solder mask drops significantly as the operating temperature increases.
Standard solder masks aren’t coherent, which means they frequently have pinholes, or other small voids along the edge of the tracks after the mask is applied. It’s easy for particles and other contaminants to get caught in these regions of the mask, and also decrease the effective voltage protection.
Solder masks are a good choice if you’re only using voltage up to about 1 kV. You might get away with using a mask up to 5 kV, but you can’t have closely spaced pads or traces anywhere on the board. The over-arc rating for UV solder masks is about 500 V/mil with their usual thickness of 0.7-1.5 mil.
For mid-voltage circuits, you should get two or three applications of the solder mask. The extra applications will mean you’ll need more clearance around pads and holes on the surface. Ask your manufacturer early in the design process so you don’t lose a lot of time rearranging the layout later. You should also ask if the mask can get an additional oven cure (for UV masks only) and/or a UV “bump” (for UV and oven cured masks). The extra curing will make the mask harder and stronger.
You should also plan the surface of the board to account for any routed edges. Routing can chip the solder mask and introduce defects that lower the over-arc voltage. Think of it like writing a message in icing, you don’t want it to run off the edge, so you need a little bit of buffer. Even 0.25 mm is enough to help protect the solder mask from routed edges, but you should increase the distance with heavy copper for high voltage boards. That additional space gives enough room for the solder mask to cover the copper completely and increases the effective dielectric.
You can overcoat solder mask by putting down two layers over the surface of the board, like putting down a crumb layer of icing, then the decorative one. Have the coatings applied at orthogonal angles across the board to minimize the size of voids created during the coating process? This will significantly help decrease arcing and corona damage to the board.
Solder mask is likely to chip if it gets routed, so you should design a buffer zone, rather than having the mask flow over the edges like frosting.
There are other overcoats you can use on the top of your printed circuit board to protect copper traces, and increase the dielectric value over the surface of the board.
The most is a Kapton overcoat, which has a high dielectric value that decreases corona effects and increases the over-arc voltage. Kapton is prerouted and drilled, then pressed onto your board with an acrylic glue. You can get sheets between 1-5 mil with 2-3 kV ratings, and you can use multiple layers.
Your Kapton layer needs to be designed for the appropriate manufacturing method since pad openings are usually drilled. That means the openings need to be round, with sizing and spacing that accounts for the manufacturing tolerances. It’s possible to CNC mill openings with other shapes, but it’s slow and costly.
You can also use Mylar or HVPF as overcoats, depending on what your manufacturer can provide.
If you don’t plan the materials and design for traces and coatings, you’ll probably have to scrap the whole thing.
Communicate with your manufacturer
Nearly every aspect of protecting the surface of your high voltage board depends on manufacturing materials or tolerances. Make sure you’ve clarified all of those specifics before you commit to your board design, or you’ll end up scrapping a tremendous amount of work. If materials and spacings aren’t clearly communicated, the manufacturer may use a cheaper approach that isn’t robust enough to handle the high voltage across the board.
As you work with your manufacturer, make sure they have the capability to handle the materials and manufacturing specifications of your board design. You should specify whether you’ll need vacuum impregnation, pressure curing or additional bakes of your board and coatings. It’s also useful to research their quality control and high voltage product history. Verify their standards for cleanliness and surface preparations to prevent contamination and ensure quality adhesion before coatings and bonding are applied.
Finally, ensure that your design meets the tolerances specified by their manufacturing constraints. If you don’t, you’re likely end up tossing out a design that started out beautiful. While there are no cake decorating rule checks, you can set those up for the coatings on your PCB. Tolerances from your manufacturer can be included in the rules check for your design. PCB CAD software, like Altium Designer®, even include 3D spacing checks in case you have tight requirements around track thickness. It’s easy to get started, and start producing your own show quality masterpieces.
Have a question about 3D design? Contact an expert at Altium.