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I have designed the layout of a PCB that will have an unique area with crimpage terminal connectors receiving 6KV by an external wire. I have done the shortest polygon (instead traces) for 9 PCB terminal connectors like these:

terminal farnell link

These 9 points must have the same potential. They (1+8) connect 6KV with 8 wires coming from contact relays (flying lead contacts). So doing this way, coil (low voltage) contacts can be separated from 6kV contacts (and 6 6KV polygon) using bigger distances. Finally, I will have a board with 24 HV relays so, 24 coils pads at 12V by one side of the board (middle), traces will go from them to the left corner of the board. HV polygon will be allocated at the right upper edge, with some slots (air gaps) that I made and adding more than the minimal distance that calculations give me using CTI0, conformal coating, etc.. (i.e.: minimal d + 50%(d))

I have read that using conformal coating distances can be minimized. So I use clearance and creepage at the PCB design that matches with conformal coating usage tables because my idea is to use conformal coating.

But I sincerely have never chosen conformal coatings neither worked with that. After searching I found that there are various types of products for several application cases. I see that acrylic solution has high dielectric. But I can see that there are many type of acrylic comercial solutions too.

The second chance is that there are several ways for applying the coating. From manual way until automatic one, depending on the number of PCB to produce. I think my case could fit with spraying. And it is said that this brings the need of doing kapton masks for the rest of the board.

So the question are two:

1.- Could anyone share with me commercial acryilic conformal coating solution for good isolation (max high voltage 6kV) of this design?

2.- Where exactly may I spray with solution over the board? Maybe over the HV connectors and HV polygon after wire connectors being connected/coupled to the crimpage board terminals? Maybe over the coil contacts, traces and pads? Maybe over the whole board after soldering all the components?

NOTE:

I have seen this one at farnell web that has 40Kv/mm:

acrylic spray

Or this other one with 70Kv/mm and liquid (but says something about spray gun):

liquid conformal coating

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  • \$\begingroup\$ re "... So I use clearance and creepage at the PCB design that matches with conformal coating usage. ..." -> DON'T. 6 kV is astoundingly unforgiving. If you do not utterly HAVE to minimise distances, don't do so. If you HAVE to, still don't do so! :-) :-(. Having your finished actual layout checked by somebody skilled in not carbonising PCBs and circuitry at say 10 kV. || There is seldom any reason apart from cost to not use conformal coating in other locations on the PCB. Components subject to wicking or contact masking may need to be avoided or masked. \$\endgroup\$
    – Russell McMahon ♦
    Commented Dec 3, 2019 at 9:53
  • \$\begingroup\$ @RussellMcMahon No, I don't use less creepage than tables says, I'm saying that I doubled the distance of that minimal one. I know that for having 6KV conductor I need to apply conformal coating but I don' t know how to apply it, what one and where to put (or spray) this coat. I found that acryilic type is used for insulation, but there are a lot of commercial solutions so I don't know which one to choose.. or how to know if one of the acrylic solutions will be enough for my desired isolation. \$\endgroup\$
    – Suvi_Eu
    Commented Dec 3, 2019 at 10:06
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    \$\begingroup\$ Keep in mind that conformal coatings are not a moisture prevention application; they are barriers but at high enough humidity levels some moisture will end up underneath reducing the effective dielectric strength (unless you use parylene which is both extremely costly and virtually impossible to remove). \$\endgroup\$
    – Peter Smith
    Commented Dec 3, 2019 at 16:01
  • \$\begingroup\$ @PeterSmith thanks, so this is the reason why I'm letting more spacing than it was suggested by the safety standar tables. I'm trying to get larger safety margins. Furthermore I will use something similar to these headers with 3KV dielectric strength material for the 9 HV wire crimpage connectors: te.com/global-en/product-293043-1.html \$\endgroup\$
    – Suvi_Eu
    Commented Dec 3, 2019 at 16:10

2 Answers 2

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You said

"... So I use clearance and creepage at the PCB design that matches with conformal coating usage. ..."

DON'T.
6 kV is astoundingly unforgiving.
If you do not utterly HAVE to minimise distances, don't do so.
If you HAVE to, still don't do so! :-) :-(.
Regardless, have your finished actual layout checked by somebody skilled in not carbonising PCBs and circuitry at say 10 kV.

There is seldom any reason apart from cost to not use conformal coating in other locations on the PCB. Components subject to wicking or contact masking may need to be avoided or masked.

A generally superb conformal coating is Dowsil (formerly Dow Corning) 1-2577 datasheet here

Can be dipped, brushed or sprayed. Rated at 13 kV / mm - but not what you'd rely on as your primary EHT insulator :-). Annoyingly costly in small quantities, but gets much cheaper per volume in litre+ sizes.

You will need to confirm that it suits your need. The manufacturers are generally highly competent and willing to provide technical advice.
I have no association with Dow Corning apart from being satisfied user of a number of their products.

Related Dow pages here

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  • \$\begingroup\$ Thanks @Russell but, what does it mean "other locations on the PCB" here: "There is seldom any reason apart from cost to not use conformal coating in other locations on the PCB." Where are those "other locations"? Other than "which ones"? I would like to put conformal coat over my PCB because I'm trying to fit the tables values. For 6KV and coated conductors, CTI0, I will need 30mm of creepage and spacing. So, obviously I will need to cover some parts with coating, but I don' t know how and what parts. That was my firstly doubts. \$\endgroup\$
    – Suvi_Eu
    Commented Dec 3, 2019 at 10:23
  • \$\begingroup\$ And once again thanks for the solution product suggestion. I will check it as soon as possible. \$\endgroup\$
    – Suvi_Eu
    Commented Dec 3, 2019 at 10:26
  • \$\begingroup\$ @EugeniaSuarez " ... other locations ..." -> I meant that unless the cost of conformal coating was a major issue (and it probably ISN'T) that coating the whole PCB usually makes sense. You need to mask eg contacts and some connectors, and some components such as sockets MAY wick up the conformal coating so that it forms an insulating layer between the socket and plug or component. So conformal coating needs to used with intelligence, but it should not be too hard to use. \$\endgroup\$
    – Russell McMahon ♦
    Commented Dec 3, 2019 at 19:37
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40 kV/mm at 50 µm is only 2 kV. Those numbers are entirely abstract.

Aside from the quality of the coatnig you're looking at a lot of other factors as well.
- Cleanliness. The boards will require thorough washing and some cleanroom drying time before applying the coating.
- Trapped air. At 10kV air can become conductive and the coating will breakdown inside-out. Vacuum application and baking is probably required.
- 10 kV dc might be fine, but what about surges? Does the geometry of your conductors cause any problems here? Sharp edges or points might weaken the insulation.
- Will your insulation or board withstand 10 kV for years?

I'd definitely recommend hiring a consultant for this, or completely outsourcing this part of the design.

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  • \$\begingroup\$ I tryied to use rounded pads every where there was a conductive point, and I talk to manufacturer in order to choose that material with the lower degradation per years ratio that they can provide us. I would like to put this search in consultant hands, but it seems to be an I+D work for us. \$\endgroup\$
    – Suvi_Eu
    Commented Dec 3, 2019 at 11:30
  • \$\begingroup\$ Regarding 2kv per 50um, does it mean that if I let 150um I will have 3x2KV of dielectric strength? On the board, is more than 3cm spacing between a hv part and the next low voltage conductive point, now. Furthermore I add some air gaps slots and opened coductive parts will have conformal coating. \$\endgroup\$
    – Suvi_Eu
    Commented Dec 3, 2019 at 11:36

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