Several years ago, I had a date take me hiking. Beforehand, he gave me a long list of what to wear and bring for the hike. I had been camping and hiking in the mountains since childhood, and this guy was a southern boy, so I found it a little vexing to have him telling me what to do on my home turf. Fortunately, I had been raised to take environmental hazards seriously and tried hard to appreciate someone else who also had neurotic and overbearing approaches to managing gear and planning.
The same hazards that you pack for on a hike, like heat, weather, and moisture, are also concerns for PCB design. Once you add high voltage into the mix, even things like dust and chemicals can be problems. So, while it may be annoying to have repetitive checklists on how to prepare, it’s certainly better than missing something important in a crisis, whether it’s on the trail or your PCB.
Hiking when it’s hot means lots of water and frequent breaks. On a PCB, heat increases the resistance of copper tracks, decreasing the overall performance of the board. Repeated exposure to heat causes operational lifetimes to drop by degrading the insulation.
On a high voltage board, heat is even more problematic. In addition to degraded insulation lowering the voltage rating over time, arcing can also occur at lower voltages when the temperature is high. That happens when more electrons are “forced” off the surface of heated conductors and increases the magnitude of the surface potential, basically increasing the voltage without any of the performance of actual increased voltage.
You can manage heat with a smart layout, keeping components that will heat up away from each other, and using good cooling practices, like fans and heatsinks.
Moisture is as devastating to a PCB as it is to comfortable socks on a hike.
Moisture is always a concern for electronics and is often “game over” for your PCB. Same thing happens to me if my socks get wet, so always pack extra socks. PCBs don’t recover as easily as changing their socks since moisture can change the breakdown voltage of air, and affect over-arcing. When moisture collects on a board, it can shorten the effective distance between traces, and increases the chances of arcing. Moisture control needs to be managed at the board and the packaging level. Be cautious when applying coatings to high voltage PCBs, some can increase trapped heat or change the voltage ratings across the surface of the board.
Dust and particulates
For hikers, dust is usually more annoying than catastrophic. Still, people don’t attract dust as much as electronics do, as the back of my computer can attest. In high voltage, dust can coat a board in just weeks of operation. A layer of dust reduces the effective dielectric of the board, and is yet another way of increasing your arcing risk. Dust also traps heat on the board, further reducing performance and speeding up the degradation of the board and components.
A fan and filter can be used to clean air before entering a high voltage enclosure, but you need to design your packaging early to ensure you’ll have adequate clearance and power.
Dust and particulates are damaging to high voltage designs, but fans and filters can help you protect your product.
I hike with bug spray, hardcore-wash-your-hands-before-you-eat bugspray. I make people hiking with me not use bug spray, in the hopes that they will lure a few bug bites away from me. It turns out, some bug sprays will degrade plastics, so I’ve learned to be careful putting it on around sunglasses, water bottles, or climbing ropes. I have never had to bug spray a PCB, but plenty of other chemicals are damaging to the board material and component casings. Even oils from skin can accumulate on the board and degrade it over time. Make sure you’re always handling the board carefully, and minimize exposure to any chemicals, both liquid, aerosol, and fume, that might cause damage.
Bug spray and other chemicals can damage plastics and other materials on a PCB.
Aging data is unclear on PCB material, especially under high voltage conditions, which exacerbate damage from heat, moisture, and chemicals. The material can start to break down very rapidly, within months, or even weeks. Operating life may be decreased from insulation degradation, especially due to high temperatures, so you should plan on dielectric ratings that can drop as much as 60-70%. It’s best to be proactive and derate your materials up-front, rather than needing to replace boards in the majority of your products down the road.
While I’m sure you’re an accomplished designer, hopefully, a reminder of good planning is always worthwhile. It’s always better to hike safe and design robust boards than be caught by surprise. And like selecting good gear, you should also select good design tools. When you’re dealing with all the complexities of heat management, moisture resistant packaging, and figuring out where to fit a filter, you need software that makes it easier, not harder. Altium Designer 18 is a great option, with BOM management tools, thermal relief design features, and 3D error checking for that filter. You can get started as soon as you’re ready to go.
Have questions about high voltage board design? Contact an expert at Altium.
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