A ceramic capacitor of too low voltage may have been fitted on all our boards?

[treez]

We have an LR8 linear regulator downstream of our mains diode bridge (240vac). The capacitor at the bridge output (at the input to the LR8) is just a 100nf, 1812, x7r, 630vdc capacitor. It sometimes (one in 50 power ups) blows up at switch on.
It surely cannot be blowing up because of over voltage above 630v, because the sensitive LR8 would surely have blown up first.
What we suspect is that our board stuffer has accidentally placed a 250V capacitor there, and most often these are surviving in circuit, but every now and again the product is switched on at mains peak, and this erroneously placed 250v part blows up. What do you think?
Do you think this is likely?...ie the board stuffer has accidentally placed a 250v capacitor here, instead of the required 630vdc capacitor?
Surely it’s the only explanation?
Strangely, the capacitor , when blown up, has a hole in it and there’s tarnish on the PCB near it, but when desoldered the capacitor actually still measures 100nf.
LR8
https://www.microchip.com/wwwproducts/en/LR8

 

[barry]

Um, why don't you ask your board stuffer what they used instead of asking a bunch of strangers on the internet to make wild guesses??

 

[treez]

Thanks, they mightn't 'fess up though.

- - - Updated - - -

they previously put down the wrong color leds, and denied it even though you could see they were different on the board.

 

[barry]

Thanks, they mightn't 'fess up though.

- - - Updated - - -

they previously put down the wrong color leds, and denied it even though you could see they were different on the board.

You have posted previous issues about problems with your board stuffer. You need to do YOUR due diligence and get a better vendor. That's absurd that you don't think the vendor will help resolve a problem like this.

 

[KlausST]

Hi,

Did you consider
* over-current
* or voltage peaks caused by series inductance (wiring)

Try to do realistic measurements:
* use different wiring, with different length, cables, single wires
* use mains directly, because laboratory AC equippment may have different output impedance ... and thus not cause voltage peaks of the same magnitude.

Do you know that X7R ceramics is not suitable for high voltage AC applications.
Read this **broken link removed**
to find out if this applies to you.


Klaus

 

[FvM]

Question is, what's the LR8 load and how much is the ripple voltage. If the ripple voltage is too high, you'll get at the edge X7R AC rating.

I would also suggest a small series resistor to limit inrush current when connecting to the mains.

 

[treez]

Thanks, LR8 load is 2V5 and 5mA. The voltage on the LR8 is , as you know, rippling at 100Hz, going from around 200V to 339V

 

[Relayer]

Hello treez,

Strangely, the capacitor , when blown up, has a hole in it and there’s tarnish on the PCB near it, but when desoldered the capacitor actually still measures 100nf.

To me, this smells of a manufacturing failure on some of these caps, especially if found to be the correct
working voltage.
You should insist on obtaining the faulty caps and doing a full inspection on them. This would include
opening a few entirely by splitting them in half to see if they had been formed with impurities in them.
Regards,
Relayer

 

[asdf44]

1812 Ceramic capacitors are somewhat large (though not huge) and also susceptible to stress cracks during manufacture.

Of particular hazzard are caps near the board edges which can be stressed when the boards are de-panelized.

Over a few years with one design we went through multiple episodes exactly as you describe: sudden high failure rate of a particular ceramic cap and cracking was a major reason (improper original specification was another reason). In the end we re-spun to replace as many large ceramics with film as possible.

If nothing else comes up consider multiple 1210's and/or look for flexible termination types.