What caused this pcb to blow out?

[ElectricSpecs]

Hi. A few days ago my cooker extractor fan made a loud bang and sparked when I tried to turn it on.

The trip had tripped on the fuse box.

I turned off the electric and it dissembled it enough to remove the control box which is basically just a pcb with 5 switches mounted to it.

On inspecting the pcb it was clear that there had been a major spark/arcing which had left some scorch marks on the pcb. The track looked bad in a couple of places.



I tested for continuity and that confirmed it had burnt the track so I soldered some wires to bridge the damaged tracks then added arrive hot glue to insulate and secure everything. Put it back together and it all works fine.

So that's all good but I'm wondering what caused this. I read that tracks can degrade with age, humidity which would be appropriate to the situation.

Bit perplexed that the tracks have out in 2 places at once and also AFAIK it's on the circuit which is the light switch hence low current and the bulb hadn't even blown.

I should mention that at the time the spark/bank happened I was just turining on the appliance, had slightly wet fingers altough very much doubt any water got past the buttons, switches and through into the pcb. Was also possibly trying to turn the light on and the fan at the same time ie pressing 2 buttons at once...


Thoughts on the cause and safety of continuing to use?

 

[Geoff1946]

PCB faults adjacent to mechanical parts like switches often start as bad soldering during manufacture. Eventually arcing starts and then anything can develop. However, in your case the tracks look to have simply blown, - effectively acting as a fuse. I suspect a short in one of the lamps or its holder. Check the wiring and replace both lamps to be on the safe side.
I would have repaired it in a similar way, except there is no need for the heavy cable. A single strand from (say) a bit of 1mm would be better, and wrapped around the switch terminal post. The metalisation should be removed from the PCB with emery. Also one of the switch terminals still isn't properly soldered.

 

[UNG]

Also one of the switch terminals still isn't properly soldered.

A lot of the soldering looks like it needs some attention it looks to blobby as if it has not had enough flux to flow properly

 

[Geoff1946]

A lot of the soldering looks like it needs some attention it looks to blobby as if it has not had enough flux to flow properly

I agree. The whole thing should have been rejected by any QA inspector who was awake, (if there was one).
This post prompted me to google the price of a replacement PCB.
https://www.espares.ie/product/es1395442/cooker-hood-pcb-module
You would think they could afford to solder it properly at that price!
Seriously though, there is much hot air these days about making things repairable rather than just disposable. This simple assembly, which probably contains about £3 of components suggests this is a pipe dream at the moment.

 

[ElectricSpecs]

@Geoff1946 Hmm interesting idea that the bulb /wiring could have shorted. That would indeed leave the bulb fine but have caused a massive current surge which would take out the track. Only thing is it all seems fine now which would imply an intermittent fault in which case how do you effectively check it and know its good?

You said "The metalisation should be removed from the PCB with emery. Also one of the switch terminals still isn't properly soldered."
WHat is the reason for removing the metalisation from the PCB? I'm guessing it could become loose /flake off and cause a short?

WHich swtich terminal? one of my repair joints or the original factory solders?

And thanks for looking at replacement PCBs and yes it was a sense of complete mickey taking on the price that led me to repair rather than replace.

The appliance btw is made by AEG which AFAIK is a good german brand - looked at replacing the whole extractor and we are talking best part of £400! - but it's lasted well over 10 years although poor solder joints are inexcusable IMO

 

[UNG]

The problem is that a lot of PCB's are flow soldered these days and if they are not properly cleaned before hand the soldering invariablely fails or can cause dry joints

WHich swtich terminal? one of my repair joints or the original factory solders?

I would resolder all the joints if you have the soldering iron ready to go it won't take long to go through them all

The appliance btw is made by AEG which AFAIK is a good german brand - looked at replacing the whole extractor and we are talking best part of £400! - but it's lasted well over 10 years although poor solder joints are inexcusable IMO

The AEG brand name has been owned by Electrolux of Sweden since 2005 so your appliance might not be as German as you think

 

[binky]

A lot of the soldering looks like it needs some attention it looks to blobby as if it has not had enough flux to flow properly

Definitely rubbish soldering, orange peel surface suggests it wasn't hot enough and therefore is probably a dry joint. I find modern solder doesn't have enough flux in it for my liking and flows badly. I use silver solder paste at home, it works far better than solder wire.

 

[johnb2713]

Definitely rubbish soldering, orange peel surface suggests it wasn't hot enough and therefore is probably a dry joint. I find modern solder doesn't have enough flux in it for my liking and flows badly. I use silver solder paste at home, it works far better than solder wire.

I still have loads of multicore solder from back in the 70's acquired from several projects, proper solder with lead in it LOL. It does seem to work better than the modern stuff.

 

[Geoff1946]

You said "The metalisation should be removed from the PCB with emery. Also one of the switch terminals still isn't properly soldered."
WHat is the reason for removing the metalisation from the PCB? I'm guessing it could become loose /flake off and cause a short?

WHich swtich terminal? one of my repair joints or the original factory solders?

The metalisation won't flake off but could lead to unwanted conduction across its surface.
I f you look how the switch is wired you will see its a double pole switch with the two poles connected in parallel. i.e four connections. One of them has little or no solder on it.

 

[Geoff1946]

I still have loads of multicore solder from back in the 70's acquired from several projects, proper solder with lead in it LOL. It does seem to work better than the modern stuff.

Yes, lead free solder is harder to use and as it needs a higher temperature is more prone to dry joints. Some say that you shouldn't mix the two when doing repairs but I have done so with no obvious problems.

 

[binky]

I still have loads of multicore solder from back in the 70's acquired from several projects, proper solder with lead in it LOL. It does seem to work better than the modern stuff.

Just don't sniff the fumes

 

[johnb2713]

Just don't sniff the fumes

Why man...... it's kind of cool. Why drink and drive when you can sniff and fly

 

[binky]

I see you've sniffed too much already

 

[stuboy]

Yes, lead free solder is harder to use and as it needs a higher temperature is more prone to dry joints. Some say that you shouldn't mix the two when doing repairs but I have done so with no obvious problems.

Don't listen to who ever told you that its pure rubbish. For repairing items leaded solder has a much better wetting ability so makes a firmer joint on second hand stuff. New shinny surfaces with modern flow solder are now better than the old leaded solder. I did a test for the company i used to work for many moons ago when the first unleaded solder emerged. It was truly useless, it has however come a long way since then ;-)
Been sniffin it for over 40 years, and it hasn't affecccttiiiiiiiddd me
Cheers
Stu

 

[Bruspark]

I still have loads of multicore solder from back in the 70's acquired from several projects, proper solder with lead in it LOL. It does seem to work better than the modern stuff.

The key to lead free soldering rework is flux. Flux in the solder wire. Gell on the board, on the component legs. Everywhere.
Then it works pretty well.

Leaded solder wire is still available with a range of agressive through to No clean flux cores and is ‘legal’ should you consider your item to be originally lead soldered, medical, military or high risk (which confirms what everyone in industry knows about lead free)

 

[johnb2713]

medical, military or high risk (which confirms what everyone in industry knows about lead free)

Care to share??

 

[Bruspark]

Care to share??

Which bit ?

Lead free solder is bad news under vibration and thermal cycling

Or look under the pile of lead free at RS ?

 

[johnb2713]

Which bit ?

Lead free solder is bad news under vibration and thermal cycling

Or look under the pile of lead free at RS ?

Interesting to hear.

For the last part of my working life I was an ROV senior pilot/tech. The ROVs operated in very hostile environments etc and not once did we have a failure due to solder related issues.

I’ll have to read up, every day a school day.

Thanks

 

[binky]

The key to lead free soldering rework is flux. Flux in the solder wire. Gell on the board, on the component legs. Everywhere.
Then it works pretty well.

Leaded solder wire is still available with a range of agressive through to No clean flux cores and is ‘legal’ should you consider your item to be originally lead soldered, medical, military or high risk (which confirms what everyone in industry knows about lead free)

When I was taught to solder to MIL Std 2000A, a fair while ago as a British Aerospace apprentice, we used flux on everything, non aggressive type. I find soldering with out additional flux both annoying and frequently disappointing in the results, which is why I have taken to using solder paste at home. It flows far better than modern solder.

I suspect a lot of 'high integrity ' gear suffers issues from lack of 'wetting' using modern solders, leaving many dry joints.

 

[Bruspark]

When I was taught to solder to MIL Std 2000A, a fair while ago as a British Aerospace apprentice, we used flux on everything, non aggressive type. I find soldering with out additional flux both annoying and frequently disappointing in the results, which is why I have taken to using solder paste at home. It flows far better than modern solder.

I suspect a lot of 'high integrity ' gear suffers issues from lack of 'wetting' using modern solders, leaving many dry joints.

Similarly trained

PCBs on lead-free production lines are so minutely controlled that they usually work for ages unless there’s vibration, contamination or heat cycling

On the other hand, large hand assembled components on boards and other assembly joints have always been a problem.

Sadly the hand-assembly workers now often lack skill, time and suitable materials.