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Creepage/Clearance between wire wound inductor /pcb tracks
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max_torque:
I have a pcb onto which are mounted some wire wound inductors. For packaging reasons, they are "flat" rather than "vertical" orientation, ie look like this:







Because of the size of the parts, i need to run traces under those parts, and those traces will be covered in solder mask, and the inductors winding is lacquered.

Where does this leave me in terms of creepage and clearance requirements?

I've seen similar inductors that have thin plastic / fibreglass insulation sheets mounted under them to form an insulated boundary, but are these actually necessary?  The inductors are through hole soldered, and also held with 2 zip ties to the pcb as an additional mechanical retainment, but effectively the winding could be "touching" the trace running under it?  And that trace could be at a 250VAC potential difference to the inductor winding itself.

Any opinions?

(application is 'static' industrial, so no excessive vibrations etc)
Neilm:
If there is any sort of vibration there could be wear on the soldermask that would expose the track. Also, laquered wire is usually specified with a number of pin holes in the insulation per metre. If one of these lines up with the track you will get an arc. I would put the layer of plastic in or find another component.

I have been involved in a safety recall where a transistor was lying flat and secured to the board. A PCB modification (that didn't go past the safety guys) moved a track under this transistor. We ended up having to recall about 9 months of production to rectify it.
MagicSmoker:

--- Quote from: max_torque on October 24, 2019, 12:44:10 pm ---I have a pcb onto which are mounted some wire wound inductors. For packaging reasons, they are "flat" rather than "vertical" orientation, ie look like this:
...
Because of the size of the parts, i need to run traces under those parts, and those traces will be covered in solder mask, and the inductors winding is lacquered.

Where does this leave me in terms of creepage and clearance requirements?

--- End quote ---

You will almost certainly have to put additional insulation between the choke and the board, usually a sheet of polyester or the like, but check with your compliance people to see if 2-3 layers of transformer margin/layer tape (e,g, - 3M #56) can be used instead, as it will be cheaper/easier to apply.

T3sl4co1l:
Counts as double insulated, i.e. reinforced.

To what voltage, is the question...

Suspect it may be adequate by the letter of the law, but I agree with others that a layer of tape or film would be more comfortable.

If applying a sticker or tape is a PITA, you might consider getting the board with extra soldermask or silkscreen layer(s), or with conformal coating applied through a stencil.  Or the parts could be similarly pre-treated (probably the bigger pain though?), or customized from the manufacturer if quantities permit.  :-+

Anecdote: once had a series inductor jumpered out on an inverter board.  This was a large chip style inductor, with ground poured around it.  I did the mods on our existing prototypes by bending some wire into jumpers and spanning between the pads; the protos we got from the assembler, they just took some bits of 0.025" square pin header and slapped those down.  So, my version had height over the ground pour between pads, theirs was flush over the pads and pour.  Several of the assembler's boards did indeed work at up to 650VDC; one did however fail shorted, in that location.

On a related note, another proto we played with (earlier on that job), a 4-layer ExpressPCB board, where the inner layers didn't have any special clearance assigned, just the usual 7 mils.  Similar failure rate, one shorted out, a few others worked fine.

Tim
TimNJ:

--- Quote from: T3sl4co1l on October 24, 2019, 07:09:47 pm ---Counts as double insulated, i.e. reinforced.

--- End quote ---

Are you sure about that? As far as I know, standard enameled wire only has "operational" or "functional" insulation that doesn't count for any means of protection when used in a critical location. And solder-mask does not officially count as insulation, although it is an epoxy with insulative properties.

Reinforced implies either:

1.) Basic insulation + supplementary insulation
2.) Fully reinforced insulation (triple insulated wire)

As the OP mentioned, a small piece of FR-4 (with holes for the leads) is pretty straightforward. Epoxy the inductor to the FR-4.
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