strange burning on mains PCB - Page 1
Forum section: electronics / projects

author: julianhigginson
author_rank: Frequent Contributor
timestamp: February 28, 2020, 07:36:03 am
content: so.. I've been shown some building management product PCBs that look very interesting. While I've got some of my own ideas about what happened they're not complete, and it's a pretty unusual thing, so maybe people might find it fun to chat about in here?first up, they are certified products, and from an install in a ceiling of a commercial building (in a bathroom - ie toilets and basins) . There are 2 units from one bathroom that have both failed this way, but other units in the building haven't. they are powering LED lighting ballasts (if that's the correct term?) looks like about 300mA is drawn from each device when on and the devices are nominally rated for 800mA continuous load.Secondly the mains circuit here is a SSR mains switch... not much to it really, but there's an SSR switching active that has a MOV to neutral, and a snubber (50R, then 33nF mains rated film cap) also to neutral.. switched mains also has an NTC in series to the output of the device, to limit switch on current. the NTC is right up against the snubber cap.the PCB is a 4 layer job with what seems to be prepreg/core/prepreg stackup. measures as a nominal 1.6mm thickness spec.obviously inner layers all clear of metal around the mains circuit and mains only on top/bottomblack soldermask.The PCB has two mains areas.. the other mains area is for mains input, where there is a standard mains to DC encapsulated module for the control circuitry. this area seems to have no burning or anything else. the input filtering cap for the mains to dc module is the same manufacturer and model and voltage (but different size) as the snubber cap.Only the output area has this failure in 2 units I've seen.As for the damage -The PCBs have been pretty much burned through, like I've seen underneath components like MOVs that had obviously died in the past... but the burn through is not under any parts.. it's NEXT TO a part (a mains rated cap that's in there as a load snubber alongside a 50R resistor)The mains rated cap is in bad shape - in both cases split open at the bottom and swollen and the cover is melted and generally looking pretty badly wrecked (though on the board with the less split open cap and less PCB burned, this cap still maintains its 33nF value.. on the worse of the 2 boards it's down towards 0nF)The SMT resistor right next to it which makes up the rest of the snubber is fine.. the board damage has delaminated its pads and tracks.. but the chip resistors themselves from both units measure 50R..the NTC is completely fine. looks like nothing went wrong at all. not even any burn residue on it.With the burn area, you have exposed fibreglass weave on each side that looks almost silver, and if you push into the weave with a ballpoint pen, the inner layer is all crumbly and black - it breaks apart and falls out of the weave in small chunks and dust.both boards are burned their worst just near the node where the 50R, 33nF join. they both have an actual small penatrating hole.Anyway - the best understanding I can manage right now, comes from the cap being the most heavily damaged part on the board, and less damaged than things that were actually on and next to the burning PCB.... so I'm expecting that it's the snubber cap failing for some reason and ejecting burning debris onto the board which is burning the board initially and causing a bunch of conductive residue to coat the board.. this is then conducting enough between active and neutral to heat up and slowly/progressively carbonise the PCB all around that area?of course this relies on the cap doing something I've never seen a film cap do before, and something I'd really not expect from a mains filtering cap.. also it leaves me with no cause for this 400V rated part to fail like that in the first place... I don't really expect this circuit will be creating anything particularly interesting across the snubber cap when turning on and off a LED ballast (very capacitive itself) through an NTC.if people are needing better info, I can try to get some photos and could definitely draw the basic circuit/layout.. let me know.

author: thinkfat
author_rank: Unknown
timestamp: February 28, 2020, 08:31:35 am
content: Could there be a resonance causing overvoltage and destroying the cap?

author: julianhigginson
author_rank: Frequent Contributor
timestamp: February 28, 2020, 12:41:23 pm
content: I was wondering that, though it doesn't really make sense.. load is mostly capacitive. Short cable. Just a few metres. Maybe it could have been coiled, but even then, the 50r resistor would damp that a fair bit... Might have to try some modelling.

author: capt bullshot
author_rank: Super Contributor
timestamp: February 28, 2020, 02:39:14 pm
content: Do you have a good (non-burned) specimen?My guess would be: Somehow an arc has formed between two traces through some series impedance (otherwise the upstream fuse would have been blown out) and was sustained long enough to burn through the board. You'd need quite a lot of heat and time to burn through a four layer board. I don't thing a capacitor spitting fire was the root cause (maybe except for the well known RIFA bombs).

author: duak
author_rank: Super Contributor
timestamp: February 28, 2020, 06:03:18 pm
content: Julian, you mentioned snubber. To me, that implies an RC network to reduce the peak amplitude of some sort of fast pulse. If there's a 50 R resistor in series with the cap, it's not going to do much to reduce the typical power line spike. Rather, it's maybe to help with EMI from the device, eg. cut down interference with AM radios.A schematic of the affected section would help me visualize this correctly. The type and ratings of the cap would also be helpful.To make sure we're on the same length of wave here, this circuit is powered from line to neutral giving 230 VAC and the cap is rated for 400 V? Is that AC or DC? If 400 VDC, then it's perilously close to its limits.

author: james_s
author_rank: Super Contributor
timestamp: February 28, 2020, 07:37:21 pm
content: This is one of those cases where a picture is worth a thousand words.For burned PCBs the usual culprits are lightning strikes and flashovers caused by contamination or debris on the board. I've seen adhesives become conductive with age, leaked electrolyte from capacitors, flux residue and external contaminates cause arcing or smoldering.

author: Gyro
author_rank: Super Contributor
timestamp: February 28, 2020, 08:16:24 pm
content: RIFA metallized paper cap by any chance?Agreed, a photo would help solve it.

author: ocset
author_rank: Super Contributor
timestamp: March 01, 2020, 10:05:19 am
content: QuoteThis sounds like it to me.....eg, the resonance between emi filter inductor and capacitor....as said, schem is needed.If there is no big input electrolytic, then you need some kind of LC ringing management circuitry, to dampen the switch on ringing...........unless of course all your HV connected circuitry is rated to mains peak x 2, eg at the very least 540V (for 265vac)...but you already say the cap is 400v......and this is downstream of the bridge i presume.

author: julianhigginson
author_rank: Frequent Contributor
timestamp: March 02, 2020, 12:25:14 pm
content: sorry, I thought I was going to have time to respond here over the weekend, but my weekend was pretty crazy...I'll make up a drawing of the schematic and the layout as I remember it, and see if I can manage a photo.there's not much in the way of (deliberate) inductors around this mains circuit, unless it's in the mains power module that powers the control and comms circuitry..cables were short and load (LED lighting ballasts) would look mostly like a capacitor, I expect... but I don't have the insides of one of those.The only time I've seen a burn like this, away from any parts, was a pool pump controller that someone I know had... the case had cracked, and water had gotten in.. a whole section of the PCB burned (black charcoal, left a great big hole.. kind of the same as the charcoal from the core layer in this 4 layer PCB) in that case I thought the PCB had actually absorbed moisture before becoming conductive... One of my first thoughts on these boards was moisture, but I wouldn't expect *that* much moisture in the ceiling of a commercial office bathroom - no showers, just toilets and urinals and hand basins) Also this board does have that totally destroyed cap just to the side of the burn in the PCB... it really seems related.

author: Psi
author_rank: Super Contributor
timestamp: March 02, 2020, 12:47:34 pm
content: How are they cleaning the bathroom.Are they hosing down the floor, walls and ceiling with strong chemicals and then again with water?Sometimes they design bathrooms to be cleaned like this because it is very time efficient.Maybe some caustic fumes are getting to the PCB and damaging parts, Making them fail shorted or semi-shorted.

author: julianhigginson
author_rank: Frequent Contributor
timestamp: March 02, 2020, 11:16:23 pm
content: Quote from: Psi on March 02, 2020, 12:47:34 pmgood question! I'll ask that and see if my guy knows. the devices are pretty well sealed though.. 2 part plastic case that clups together and is glued at a few points for good measure... cable grommets around mains cable going in and out of the box... any ingress points will be very small and thin.

author: james_s
author_rank: Super Contributor
timestamp: March 03, 2020, 11:02:00 pm
content: I've seen boards burn that got wet, or got leaked electrolyte (which is just another type of wet), and I've seen bad solder joints on high current connectors arc and burn. I've seen bad connectors oxidize and burn. Once something gets hot enough to carbonize something, carbon is conductive and will self sustain additional burning.

author: Phoenix
author_rank: Frequent Contributor
timestamp: March 04, 2020, 11:00:09 pm
content: Quote from: Gyro on February 28, 2020, 08:16:24 pmSounds like the "good old" RIFA caps... Does it look anything like thesehttps://www.badcaps.net/forum/showthread.php?p=382644