How are creepage and clearance distances affected by PCB soldermask, according to regulations? (having 62368-1 in mind)

And are the CTI of soldermask different from CTI of the FR4 board?

Two wires under soldermask requiring isolation, does the safety distance depend on the worst CTI of soldermask vs FR4 board?

What if board is covered with epoxy or some conformal coating? Does the worst CTI of any of these insulation apply.

If possible, please refer to any clause in the standard. I did not find it.

 

Hello Butthead.



In 62368-1, I suggest you look at 5.4.3 Creepage Distances, and G.13.3, Coated printed boards.



Most of the answers are there.



For coated boards,



G.13.3 Coated printed boards

The requirements for separation distances before the boards are coated are specified below.

An alternative method to qualify coated printed boards is given in IEC 60664-3.

For printed boards whose outer surfaces are to be coated with a suitable coating material, the minimum separation distances of Table G.13 apply to conductive parts before they are coated.

DOUBLE INSULATION and REINFORCED INSULATION shall pass ROUTINE TESTS for electric strength of 5.4.9.2.

Either one or both conductive parts and the entire distances over the surface between the conductive parts shall be coated.

The minimum CLEARANCES of 5.4.2 and the minimum CREEPAGE DISTANCES of 5.4.3 shall apply:

– if the above conditions are not met;

– between any two uncoated conductive parts; and

– over the outside of the coating.

Compliance is checked by inspection and measurement, taking Figure O.11 and Figure O.12 into account, and by the tests of G.13.6.



Also, most conformal coatings don’t have a CTI.



Per 5.4.3.3,



If the material group is not known, Material Group IIIb shall be assumed.



Material Group IIIb   100 ≤ CTI < 175



Best regards,



Tom

 

The CTI rating of a PCB (or other material) would be tested as per IEC 60112, which would include any surface coating such as solder mask on a PCB. So although the solder mask and base PCB material may have differing CTI values, the CTI test is applied to the external surface (see clause 5.4.3.3).

Where tracks on a PCB do not meet the minimum separation distance requirements for the class of insulation under consideration, adding a coating (such as epoxy or a conformal coating) can be used to achieve a micro environment of pollution degree 1 – with the resultant reduction in creepage distance required. However the applied coating must pass the test specified in clause 5.4.1.5.2, including the thermal cycling tests of clause 5.4.1.5.3.

If the creepage distance is at issue, then one possible solution is to cut a slot in the PCB, effectively removing the creeapge path, and applying the clearance requirement only.

If the insulation under consideration is functional, then 62368-1 permits the substandard creeapge/clearance distances on the proviso that no hazards results from short circuiting the insulation.

 

Soldermask is NOT considered an insulator in a safety function.
They have nothing for UL/CSA approval as a dielectric. Mainly because the application is not well controlled, the coating thickness can be all over the place. Soldermasks do have UL 94V-0 flammability rating though. So don't consider soldermask as anything other than cosmetic when doing design. It's a bad habit I've seen junior engineers screw up and think they are covered by it as an insulator. Not so when safety is involved.

 

Thanks for your answers.

If soldermask should be seen as cosmetic only, then tracking can occur under the soldermask, I assume? Is that just because of the imperfect parameters of the soldermask? Will tracking not occur if comformal coating like epoxy is used?

Speking of that. Expoy coating gives pollution degree 1. Hermetically sealed also? Is that the reason why many smpsu are ultrasound welded?

 

A layer of epoxy placed on a surface does not necessarily result in a pollution degree 1 environment – it must pass the tests as per clause 5.4.1.5.2 (including thermal cycling) – see my post above.

Ultrasonic welding of enclosures is probably a cheaper mass production option (than screws); the integrity of the weld can be observed by splitting the weld apart and inspecting the weld surface texture.