Diagnosing A PCB Fire - Page 1
Forum section: electronics / projects

author: waymond91
author_rank: Regular Contributor
timestamp: April 08, 2024, 01:19:53 pm
content: Hello all,I have an embarassing problem.I am designing what was supposed to be a simple circuit board that acts as a power distrubtion board.However, during its first test it caught on fire in a bad wayThe projects runs directly off a 12V lead acid car battery, and we estimate a 16A current draw. However, at the time of the "incident" it was probably about 5A.We do havesomeprotection in place (reverse polarity mosfet, fuse - and now Im adding a MAX6399 as an overvoltage cut-off in case someone plugs in the wrong kind of battery).HOWEVER, before the main power distribution board was a simple pcb to connect the power input connector at the input panel to the main power board.Input Connector: SP-2-MDH -https://www.digikey.com/en/products/detail/amphenol-sine-systems-corp/SP-2-MDH/10443792Output Connector: Molex 0430450800 -https://www.digikey.com/en/products/detail/molex/0430450800/252529?s=N4IgTCBcDaICwGYAMcCsSAcSkgLoF8gSo the board literally just connects the SP-2-MDH with the Molex 0430450800.The whole top of the board is a copper plane for the 12V input. Likewise, the entire bottom is a ground plane.One unfortunate oversite on my end is that the ground plane clearance is only0.5mm,Unfortunately, I designed the board while working remotely so I was not the person to assemble and test. However, I like to imagine my co-workers do a good job.There a few things that stand out to me as wrong and things that seem like a good idea to improve - but I really want to try and understand the root cause instead of just making hopeful improvements.Any insights as to what happened? Or what design rules were not followed that led to this?The design rules Im looking to would be:Min via sizeMin trace width sizeTrace / pad spacingBut Id still feel better if I could look at the connector pad and say "oh well at 5 amps this has a thermal rise of 100 C - which is why it failed"OR"At this current / voltage the min pad spacing must be greater than X or we get this failure"What do y'all think? Take it easy it on meIt looks to me like the Molex connector seemed to catch first. I also know the device ran fine for a at least few hours before the failure.

author: inse
author_rank: Super Contributor
timestamp: April 08, 2024, 01:44:30 pm
content: What does the connected device look like?

author: waymond91
author_rank: Regular Contributor
timestamp: April 08, 2024, 02:11:50 pm
content: Quote from: inse on April 08, 2024, 01:44:30 pmRather than making the entire load pass through the power switch, we instead use a switch (panel mounted) to energize a 12v relay.This board has quite a few other connectors that all suffer from the low clearance power & ground planes...

author: inse
author_rank: Super Contributor
timestamp: April 08, 2024, 03:48:26 pm
content: I meant in terms of destruction…But why not take another set of PCB and cables and hook it to a lab supply and run 5-10-15A through it?Regarding the clearance - 0.5mm should be sufficient for 12V as long as you keep it in a clean environment…

author: waymond91
author_rank: Regular Contributor
timestamp: April 08, 2024, 03:52:58 pm
content: Quote from: inse on April 08, 2024, 03:48:26 pmAh, the connected device was actually ok. Although I find it to be suspect as it was designed as part of the same board file.So all of the design "rules" carried over.

author: max_torque
author_rank: Super Contributor
timestamp: April 08, 2024, 05:03:50 pm
content: Pretty clearly there has been a gross and continuous short circuit. The fact that both connectors have experienced significant thermal heating suggests there was a hard short circuit somewhere. Given the pcb substrate damage, i'd guess a short at the soldering of the Molex through hole pins;Some lessons should be:1) car batteries always win - use a current limited bench PSU or suitable upstream fusing for tests, especially early ones where your system confidence is low2) car batteries always win - make sure when you use a battery you have suitable fast acting current interuption device that can break the likely fusing currents experienced. For a car battery this will be hundreds of amps (a dead short circuited car battery can do over 1,000 amps). Don't rely on manual disconnect, nobody wants to grab red hot smoking cables and connectors to try to disconnect things. Trying to find a 10mm spanner to remove a car battery terminal clamp whilst your workshop burns to the ground around you is not a fun experienceand3) who was monitoring this test? pretty clear this thing smoked for a while before it was disconnected........

author: max_torque
author_rank: Super Contributor
timestamp: April 08, 2024, 05:05:11 pm
content: oh yeah, and in terms of "lessons learnt" a good one to to ensure your overcurrent protection is as close as possible to the power input connector.......

author: jkostb
author_rank: Regular Contributor
timestamp: April 08, 2024, 05:48:58 pm
content: I observed that you use thermal relieves. Please note that these thermal relieves increase resistance and therefore create heat with high currents. I would omit those thermal relieves (for repair then you need to preheat the board). I assume that the fire started at connection between connector and PCB?

author: waymond91
author_rank: Regular Contributor
timestamp: April 08, 2024, 06:21:01 pm
content: Quote from: max_torque on April 08, 2024, 05:03:50 pmOk so I wasnt there and I really dont want to point fingers, but it really does look like that to me.Im just confused as to why it would have functioned for a few hours before blowing up like this?Yeah there has been some discussion with the person organizing the test - for the most part my hands are tied except for revising the design.Im definitely keen to move the fuse - I have a 20A fuse downstream, but as you say, as close as possible to the power source.

author: waymond91
author_rank: Regular Contributor
timestamp: April 08, 2024, 06:21:54 pm
content: Quote from: jkostb on April 08, 2024, 05:48:58 pmOk. Long term our plan is to have these assembled at a pcb manufacturer. Im wondering if the the lack of thermal relief will tick them off?

author: jkostb
author_rank: Regular Contributor
timestamp: April 08, 2024, 06:31:29 pm
content: No. For PCBA manufacturer it is not an issue because they preheat the board (I have done this with production PCB's). Only problem is if you repair the board it will very hard to desolder the connector. Is there no way to use a connector with more pins for power distribution? What is the copper thickness? May good to use 70um

author: PCB.Wiz
author_rank: Super Contributor
timestamp: April 08, 2024, 08:08:02 pm
content: Quote from: waymond91 on April 08, 2024, 01:19:53 pmLooks like the classic e-schooter connects failures.With any sort of decent current, a high resistance point can thermally cycle to become a higher resistance point and thermal runaway occurs.One tip is tonever rely on hole plating for high current connections, especially on mechanical items like connectors.We use a star fanout of vias on all high current tracks now.Addit : it looks like the tracks that got hottest, were those where you relied on plating.Might be a coincidence, or could be a message ?

author: Someone
author_rank: Super Contributor
timestamp: April 08, 2024, 11:09:52 pm
content: Plated castellation and board lock holes? Which do not have the full (and already unnecessarily tight) clearance.

author: Siwastaja
author_rank: Super Contributor
timestamp: April 09, 2024, 06:18:32 pm
content: Fuse should be located closer to the battery, schematically speaking. Of course, you always havesomeamount of un-fused circuit, but the closer the battery the fuse is, the more it protects. The non-fused sections needs super rigorous design practices. Like, massive clearances, special qualification of any parts used. You need to prove nothing can short out. After the fuse it's much easier for you.

author: tooki
author_rank: Super Contributor
timestamp: April 09, 2024, 08:36:56 pm
content: Quote from: PCB.Wiz on April 08, 2024, 08:08:02 pmHuh? What do you mean, relying on plating? The main conductor is the component lead that’s in the hole. Even if this board had unplated holes, it would still work because the leads are soldered to the pads, which form the electrical connection.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: April 09, 2024, 10:40:37 pm
content: Quote from: tooki on April 09, 2024, 08:36:56 pm... and how does the current make it to the top side tracks ?

author: Jeff eelcr
author_rank: Frequent Contributor
timestamp: April 09, 2024, 11:46:21 pm
content: I have seen a 14V run arc between a 5V run at .5mm distance with the connector corner over them.That took several years to form. What is the current on each pin?Jeff

author: tooki
author_rank: Super Contributor
timestamp: April 10, 2024, 12:06:50 am
content: Quote from: PCB.Wiz on April 09, 2024, 10:40:37 pmI asked you to clarify. Asking me to explain the thing I am not seeing is not helpful.I already stated the reason for my confusion about your statement: the component leg is in the hole (whether plated or not). Current flows through that. Current gets to whichever side you want by virtue of the leg being soldered to the pad on that side of the board. Or to both, if there are pads and traces on both sides. So the hole plating, if present, is at most auxiliary to the component leg itself.So what is the situation where you’re relying on a through-hole’s plating?How are vias (which are tiny plated through-holes, but without a component leg through them) superior to a plated hole with a component leg in it?I assume you are not talking about unpopulated plated through holes, where the component leg is absent.On this board, current flows through the 12V plane one one side of the board, and through GND on the plane on the opposite side of the board. The connectors are soldered to both sides of the board. So I don’t see where any current at all is flowing through the plating, it’s flowing directly from the connector leg to the planes through the pads present on both sides of the board. Every leg has a direct connection to both sides of the board.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: April 10, 2024, 12:14:22 am
content: Quote from: tooki on April 10, 2024, 12:06:50 amYou seem to have assumed full solder wicking to the top side component leg, which of course does not always occur. (and may even be actively hindered by mouldings etc)In that case you arenow relying on the plating, and connectors we have found have additional mechanical/thermal stresses, that make plating even more vulnerable.Hence our rule to always have many additional 'redundant' vias, which removes relying on one holes plating, and a single point of failure.Vias are essentially free, so this is easy to include in most cases.

author: tooki
author_rank: Super Contributor
timestamp: April 10, 2024, 12:20:01 am
content: Quote from: PCB.Wiz on April 10, 2024, 12:14:22 amOK, that makes sense now that you explain the specific circumstance that causes it to happen.Though yes, I think that complete solder flow is a reasonable expectation for high-current joints. It may not be required for typical low-current joints, but for power connections it seems to me a reasonable thing to expect (and specify accordingly).