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Electronics => Projects, Designs, and Technical Stuff => Topic started by: Jester on October 04, 2018, 11:51:51 pm

Title: PCB tweak: Pollution Deg 2 ->3
Post by: Jester on October 04, 2018, 11:51:51 pm
I have been asked to tweak the design of a control board used in an industrial application so that it will meet pollution degree 3 specifications. The only problem area I see is the connectors for measurement of 240Vac nominal.

The creepage to other circuits appears adequate, however functional creepage for 240V is 151mil or 3.8mm and the connector has 5.08mm pin spacing, so even with oval pads the creepage is only 97mil or 2.5mm (Okay for PD2 but not PD3).

I would prefer to keep using the same connectors  Weidmuller 1594540000    https://www.digikey.com/product-detail/en/weidmuller/1594540000/281-3145-ND/492619 (https://www.digikey.com/product-detail/en/weidmuller/1594540000/281-3145-ND/492619)

IF I add a slot between the connector pin pads, the creepage would be 2.5 +1.6 = 4.1mm

Does anyone see any issues with this approach?

From a manufacturing perspective, what is an ideal slot width?

Title: Re: PCB tweak: Pollution Deg 2 ->3
Post by: Niklas on October 05, 2018, 09:27:22 pm
Adding routed slots to increase the creepage distance is common and perhaps easier to handle than selective conformal coating. If I remember correct, then the slot must be at least 1 mm wide to count as a slot according to the EN-standard and that also matches with minimum routing bit diameter for many PCB manufucturers.

Also remember to check the creepage distance on the traces that leads up to the connector. The solder mask is just a solder mask so it does not decrease the pollution degree in the same way as a layer of conformal coating does.
Title: Re: PCB tweak: Pollution Deg 2 ->3
Post by: jbb on October 06, 2018, 01:32:21 am
If you have the room, slots are great.

Typically, PCB fabricators like a 2.0mm routing bit.  They can usually do 1.0mm by special request.  I would first look at 2mm rout and evaluate whether it will fit, and how much mechanical strength you sacrifice.  Now is the time to call your PCB fab and find out a) what they can do without getting expensive and b) how you should prepare the design files (e.g. mechanical layers).

You should also have a look at creepage and clearance inside the connector itself.  You may have to chop a few up to understand how all the metal and plastic parts go together.  Creepage is especially important because the connectors likely have a lower CTI than the PCB.

Conformal coat, on the other hand, can be very effective.  It can let you really squeeze a design down (especially when PD3 or altitude is considered), but you need to keep an eye out for:
Title: Re: PCB tweak: Pollution Deg 2 ->3
Post by: Jester on October 06, 2018, 04:08:02 pm
Quote from: jbb on October 06, 2018, 01:32:21 am
If you have the room, slots are great.

Typically, PCB fabricators like a 2.0mm routing bit.  They can usually do 1.0mm by special request.  I would first look at 2mm rout and evaluate whether it will fit, and how much mechanical strength you sacrifice.  Now is the time to call your PCB fab and find out a) what they can do without getting expensive and b) how you should prepare the design files (e.g. mechanical layers).

You should also have a look at creepage and clearance inside the connector itself.  You may have to chop a few up to understand how all the metal and plastic parts go together.  Creepage is especially important because the connectors likely have a lower CTI than the PCB.

Conformal coat, on the other hand, can be very effective.  It can let you really squeeze a design down (especially when PD3 or altitude is considered), but you need to keep an eye out for:
  • Coating equipment - does you manufacturer have it? How much would it cost?
  • How is the coating cured? UV cures require UV ovens, which can be very aggressive on parts (including straight-up temperature rise).  Thermal cures may require excessive temperatures or times.  Solvent coatings may present safety hazards.
  • Coating wicking into connectors can ruin your day
  • Coating tends to creep a bit before/during curing, so the placement accuracy is limited. Imagine you have a must-coat part right next to a must-not-coat connector...
  • Coating adds thermal resistance and could make hot spots worse
  • Coating may add leakage current to sensitive nets

Thanks for the detailed response.

This particular connector is rated for 300V PD3, so I think I'm covered there.