Ceramic capacitor failure

I am seeing some failures of ceramic capacitors in a product I designed. Th ese are 1206 22uf 10V ceramic caps, we do not specify a manufacturer. The c aps are on the output of a 3.3V 1amp 2MHz dc to dc converter (5 amp inducto r 2.2uH), there are two in parallel. They become resistive, around 200 ohms . The ripple current is around 0.4 amps total, so I cant see that it is exc essive ripple current, they are not near mounting holes so I cant see mecha nical issues, and they initially pass test but fail in the field. Heating t he cap does not have much effect on the resistance. Any pointers would be appreciated. Thanks

Reply to
steve
Loading thread data ...

designed. These are 1206 22uf 10V ceramic caps, we do not specify a manufacturer. The caps are on the output of a 3.3V 1amp 2MHz dc to dc converter (5 amp inductor 2.2uH), there are two in parallel. They become resistive, around 200 ohms. The ripple current is around 0.4 amps total, so I cant see that it is excessive ripple current, they are not near mounting holes so I cant see mechanical issues, and they initially pass test but fail in the field. Heating the cap does not have much effect on the resistance.

Do the capacitors get warm?

--
Uwe Bonnes                bon@elektron.ikp.physik.tu-darmstadt.de 

Institut fuer Kernphysik  Schlossgartenstrasse 9  64289 Darmstadt 
 Click to see the full signature
Reply to
Uwe Bonnes

I don;t know, it doesnt in the lab.

Reply to
steve

SMPSs can hammer components; the devil is in the details. Ceramic caps can have very variable specifications and construction, e.g. notoriously w.r.t. the capacitance as a function of DC voltage.

The obvious question is whether you are using the capacitors within the datasheet and/or application note limits.

The former will be difficult to ascertain if the manufacturer is not specified.

Reply to
Tom Gardner

Have looked at a lot of data sheets very few give ripple current specifications

Reply to
steve

Also have life tested with 1amp load, no noticeable temperature rise, most of the rise appears to be from the inductor warming up (3 degress rise)

Reply to
steve

In which case I presume you are using those capacitors, and are operating well within the limits.

If not, especially if you are using something like Farnell's "multicomp", then you will have to qualify the caps yourself.

Reply to
Tom Gardner

The life testing was done on the current manufacturing batch that have the issue and I assume the same capacitors fitted, they look the same.

Reply to
steve

If the OP's ripple waveform has a lot of nasty steps in it, he could be exciting a piezoelectric resonance, leading to fatigue failure.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs 
Principal Consultant 
 Click to see the full signature
Reply to
Phil Hobbs

d.

ee

ers

The unit has a sleep current of 40uA, a running current of 70mA with a 1 am p load whenever the GSM modem kicks in, this has a 4700uF cap on it and whe n turned on and is driven by a 1amp current limited switch which takes arou nd 15ms to charge up, then current is around 350mA. Unit runs approx every

15mins and uses the modem once a day for a couple of minutes.
Reply to
steve

You say you've got two caps in parallel. What's the correlation between one failing and the other failing on a given unit? Do they tend to both go, or does it tend towards one dead, one live?

--
Rob Gaddi, Highland Technology -- www.highlandtechnology.com 
Email address domain is currently out of order.  See above to fix.
Reply to
Rob Gaddi

Are you able to identify the manufacturer?

Reply to
Dave Nadler

See if your exceeding the cap voltage during turn on. That can cause a failure rather quickly, even if it does not travel to the rest of the circuit because of some inductance.

Cheers

Reply to
Martin Riddle

It's a common failure mode at high temperatures, which probably doesn't apply here. Board flex can also cause this and Flexiterm caps help along with not flexing the board or mounting in a different place/orientation or just using through hole parts.

I have also seen this apparent partial short where there was nothing wrong with the capacitor, just some burnt on flux underneath. Worth taking a capacitor off to measure it just in case...

Cheers

--
Clive
Reply to
Clive Arthur

Where are you fabbing your product?

I have heard manufacturers in certain countries have a tendency to 'optimise' your product by replacing your parts with cheaper ones which can be of dubious quality.

I'd start by chatting with the factory and seeing if you can find out what parts they fit.

Theo

Reply to
Theo

.

an

Thanks, I will try and get further information from give that a try. I think of the re was over voltage on the cap, the semiconductors on the board would fail too, the output goes straight to a power plane, and also the external supp ly is 7.2V which is below the voltage rating of the capacitor, and also onc e powered the units stay powered.

Reply to
steve

. These are 1206 22uf 10V ceramic caps, we do not specify a manufacturer. T he caps are on the output of a 3.3V 1amp 2MHz dc to dc converter (5 amp ind uctor 2.2uH), there are two in parallel. They become resistive, around 200 ohms. The ripple current is around 0.4 amps total, so I cant see that it is excessive ripple current, they are not near mounting holes so I cant see m echanical issues, and they initially pass test but fail in the field. Heati ng the cap does not have much effect on the resistance.

Only one of the two fails, then the output feeds the 3v3 power plane, which also has a lot of capacitance/decoupling caps etc.

Reply to
steve

The factory its put together in is in the UK.

Reply to
steve

. These are 1206 22uf 10V ceramic caps, we do not specify a manufacturer. T he caps are on the output of a 3.3V 1amp 2MHz dc to dc converter (5 amp ind uctor 2.2uH), there are two in parallel. They become resistive, around 200 ohms. The ripple current is around 0.4 amps total, so I cant see that it is excessive ripple current, they are not near mounting holes so I cant see m echanical issues, and they initially pass test but fail in the field. Heati ng the cap does not have much effect on the resistance.

I have taken the caps off and the fault is with the cap, I have cleaned the cap of flux residue and tried heating and fault stays. It measures 186 ohm s, another cap reads 400 ohms. Only one of the two parallel caps fail (Both same part) and no other caps fail (Different parts and smaller). I have lo oked for ceramic cracks and cant see any but I am not sure if I would see t hem even if they were there! I have checked power up and the power supply does not over shoot either, Ri ses in 10ms to 3.3V as expected

Reply to
steve

ed. These are 1206 22uf 10V ceramic caps, we do not specify a manufacturer. The caps are on the output of a 3.3V 1amp 2MHz dc to dc converter (5 amp i nductor 2.2uH), there are two in parallel. They become resistive, around 20

0 ohms. The ripple current is around 0.4 amps total, so I cant see that it is excessive ripple current, they are not near mounting holes so I cant see mechanical issues, and they initially pass test but fail in the field. Hea ting the cap does not have much effect on the resistance.

he cap of flux residue and tried heating and fault stays. It measures 186 o hms, another cap reads 400 ohms. Only one of the two parallel caps fail (Bo th same part) and no other caps fail (Different parts and smaller). I have looked for ceramic cracks and cant see any but I am not sure if I would see them even if they were there!

Rises in 10ms to 3.3V as expected

Just curious: how do you detect the failure. There are two ceramic caps in parallel, both with very little resistance. The equivalent resistance is ha lf the resistance of a single cap. With the failure, no matter how high is the resistance of that cap, the equivalent resistance is still less than th e resistance of the single good cap. Is it not enough? I suppose you use tw o caps to double the capacitance not lower the resistance. btw, 47uF/10V at 1210 (B size) tantalum is quite common. Is it not enough? Too expensive? I n this case I bet you can find poscap capacitors these days with better res istance (in fact VERY low) and probably less expensive than tantalum. Hard to say though if less expensive than two ceramics of this size.

Reply to
raimond.dragomir

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.