What can I do about components with inadequate creepage?

Question

I'm designing a device for use in a non-climate-controlled environment (garage, shed, etc.) If I understand it correctly, this puts me directly into pollution group 3 unless I can also engineer it to be inside a sealed enclosure, which is probably not practical. It is, however, going to be in an enclosed metal box with basic dust protectors over the air intakes.

I would like to use components with standard 2.54 mm pitch, but as I understand it (not helped by the standards being unavailable), PG3 means I need 1.05mm creepage (for 12 V; 1mm for 5V) which is unattainable at this pitch with solder pads of any reasonable size.

I might be able to substitute some components, but not the microcontroller. Never mind the microcontroller board probably isn't built for PG3 either.

"Pot everything" isn't really an option, as a) I'm assembling this myself, and b) I'm dealing with rather large boards, up to 125 mm in some cases.

Is there anything I can do? Do I really need to be following PG3 rules (this isn't entirely clear), or is there something I can feasibly do that would let me get away with only PG2 creepages? Is there a way to increase creepage, keeping in mind I need to put parts on top of things, and I don't have 1mm between pads to add cuts?

p.s. I'm dealing with entirely THT design, no SMD components. (Also, if it makes a difference, with basic two-layer boards.)

Answer 1

If you can't maintain enough creepage distance for a particular component, you really only have three options:

1. Use a different component. (Not necessarily possible)
2. Mechanically increase the creepage distance through the use of slots in the PCB if possible. (Not useful if the component itself doesn't have a high enough creepage distance).
3. Change the pollution degree to a lower level in order to allow for reduced creepage distances.

The first two options are probably not all that useful to you. The third option is your best bet. But how?

If we look at some standards that define creepage distances, such as IEC 62109-1, they will provide information about how the creepage distances and pollution degrees are selected. By changing the environmental factors or enclosures it is possible to move to a different level.

As purely an example, take IEC 62109-1, it provides this table of possible remedies for moving to a lower degree. If you are currently at 3, then the red box shows how to move down to 2 or 1.

Essentially this boils down to either using an IP rated enclosure of suitably high protection, or applying a conformal coating. An IP5X or IPX7 enclosure would reduce the pollution degree to level 2. This would bring down your creepage requirements 0.04mm for PCBs or 0.5mm for other insulators. This is good enough for most components.

Conformal coatings are essentailly like a lacquer applied over the board. These act as water and/or dust barriers which insulate any exposed metal from the outside conditions. With suitable coating you can bring down the pollution degree to level 1, giving creepage of 0.025mm on PCBs or 0.08mm for other insulators.

Note that the above applies specifically to that standard. Other standards may have different requirements, so you should check what standard you are trying to conform to. The online calculator you posted in the comments appears to use UL60950-1 or IEC 62368-1 for example.

Answer 2

I think you may be misunderstanding the point of creepage distance. It doesn't mean that every component must be spaced that much. It only applies to the space between two conductors at a significantly different voltage. I suspect that the pins of your IC are not experiencing a large voltage difference, so creepage distance is not an issue there.

If your product is connected to the AC power, then creepage distance is only an issue in the circuits connected directly to the AC power, and between those circuits and the low voltage circuit.

If your circuit is completely low voltage, creepage distance is not an issue.