Creepage/Clearance between wire wound inductor /pcb tracks - Page 1
Forum section: electronics / projects

author: max_torque
author_rank: Super Contributor
timestamp: October 24, 2019, 12:44:10 pm
content: 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)

author: Neilm
author_rank: Super Contributor
timestamp: October 24, 2019, 06:34:40 pm
content: 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.

author: MagicSmoker
author_rank: Super Contributor
timestamp: October 24, 2019, 06:55:51 pm
content: Quote from: max_torque on October 24, 2019, 12:44:10 pmYou 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.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 24, 2019, 07:09:47 pm
content: 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

author: TimNJ
author_rank: Super Contributor
timestamp: October 24, 2019, 08:12:03 pm
content: Quote from: T3sl4co1l on October 24, 2019, 07:09:47 pmAre 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 insulation2.) 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.

author: floobydust
author_rank: Super Contributor
timestamp: October 24, 2019, 08:19:30 pm
content: Soldermask is not safety-approved as an insulator- the thickness, curing and composition are not controlled.It's bad practice to run PCB traces under inductors due to mutual-coupling, which will induce noise on that track. A few places I have worked for fail a pcb layout if that is found.If this is HV and safety approvals needed, I'd get a different inductor package, or use insulating tape (Kapton, UL yellow) or an insulator - all with agency approvals.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 24, 2019, 09:47:30 pm
content: Quote from: TimNJ on October 24, 2019, 08:12:03 pmFunctional, that's the word I should've been thinking of, thanks.Off the top of my head, I can't recall what the difference is between that and "basic". I know doubled "basic" is "reinforced"... I suppose it depends on the quality of the material, if it counts or not, which makes more sense.Tim

author: TimNJ
author_rank: Super Contributor
timestamp: October 24, 2019, 10:00:38 pm
content: Quote from: T3sl4co1l on October 24, 2019, 09:47:30 pmIt's really confusing, and I'm pretty sure some companies even get it wrong in their documentation.Basic + Supplementary = ReinforcedBasic + Basic = ReinforcedTriple Insulated Wire = ReinforcedBasic, Supplementary, and Reinforced insulation are all levels of "real" insulation, recognized by international safety agencies.Functional or operational insulation is not recognized as a means of protection. It is only to allow the product to function as intended. So, enameled wire allows us to make inductors that don't instantly short themselves out. Its "function" is to prevent turn-to-turn shorts, but from a safety perspective, it doesn't count.

author: max_torque
author_rank: Super Contributor
timestamp: October 25, 2019, 11:30:08 am
content: It's all "low" frequency (50Hz) stuff, and the inductors are there for noise suppression. Voltage is 250VAC nominal max, and is surge limited to around 750V peak by MOVs/GDTs etcI wonder if i could get some thick adhesive backed, laser cut "stickers" that could simply be stuck to the pcb before the inductors were soldered down? Would have to look at the material properties and thicknessesSounds like it's definitely worth putting something physical in the gap to avoid any issues, even if not strictly called for by the regs....Build volumes are low, perhaps 100's, so having to hand assy a laser cut insulator onto the inductors before they are soldered is not an issue

author: MagicSmoker
author_rank: Super Contributor
timestamp: October 25, 2019, 12:08:13 pm
content: Quote from: max_torque on October 25, 2019, 11:30:08 amSure, as long as the sticker is made of a material listed in the Electrical Insulation System for the magnet wire on the inductor and it has a temperature rating at least as high as the wire's. A sheet or adhesive tape made of mylar (polyester/PET), kapton (polyimide) or nomex (aramid) are all good candidates. Slapping a vinyl sticker down on the board absolutely won't fly if the product has to meet safety agency approval.

author: TimNJ
author_rank: Super Contributor
timestamp: October 25, 2019, 01:33:37 pm
content: Yeah just to reiterate...on this dark side of the electronics world, you can't just say, "We tested it and it works!" with regards to mains safety insulation/isolation. If there's a component/material intended to serve as a means of protection, then it has to be UL approved material compatible with the insulation system used by the inductor.

author: Neilm
author_rank: Super Contributor
timestamp: October 25, 2019, 05:03:42 pm
content: Quote from: TimNJ on October 25, 2019, 01:33:37 pmThe test I use when I am making decisions like on safety like this - "would I feel comfortable explaining that in court to a new widow?"

author: ZaphodBeeblebrox
author_rank: Contributor
timestamp: October 25, 2019, 06:20:15 pm
content: Soldermask can indeed not be regarded as insulation. I advise to place an extra layer of insulation in between the PCB and the component. Mind that outside of the US, other regulations than UL apply. For example glow wire requirements (especially if you're aiming for the 60335) and comparative tracking index of the material used influence which materials are appropriate or not.

author: jonpaul
author_rank: Super Contributor
timestamp: October 25, 2019, 07:21:03 pm
content: Hello allIf the 250 VAC is power mains connected, the creep and strike insulation hypot must be compliant with the applicable safety regulations, UL, VDE, EU, etc.Neither solder mask, nor magnet wire insulation are safety rated.Notice that the peak transients on a power line can be several KV, line to earth.So this design seems to be marginal.Suggest that you add an additional insulation eg 5 mil mylar or kynar.Just the ramblings of a retired EEJon

author: max_torque
author_rank: Super Contributor
timestamp: October 25, 2019, 07:38:15 pm
content: Quote from: Neilm on October 25, 2019, 05:03:42 pmIn this case, the insulation in question is not safety insulation, but functional. In the event of a short, the only effect will be a loss of function and not HV getting somewhere it shouldn't!However, for the minimal cost i'll add a extra layer of something, i just need to decide what the best 'something' actually is

author: jonpaul
author_rank: Super Contributor
timestamp: October 25, 2019, 08:43:01 pm
content: Hello againI have used 2-5 mil mylar sheet, or 1-2 layers 3M #56 electrical tape.In production, the material vendor (eg, EIS) can die-cut the mylar to and designed pattern.Finally the inductor vendor may have a variation with insulated bottom or can supply an insulation pad.Have you contacted the inductor manufacturer?Jon