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Re: Ceramic capacitor failure

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On Tuesday, February 26, 2019 at 3:27:37 AM UTC-6, steve wrote:
> On Monday, February 25, 2019 at 2:21:30 PM UTC, raimond....@gmail.com wrote:
> > luni, 25 februarie 2019, 14:58:44 UTC+2, steve a scris:
> > > On Friday, February 22, 2019 at 11:56:12 AM UTC, Clive Arthur wrote:
> > > > On 21/02/2019 10:07, steve wrote:
> > > > > I am seeing some failures of ceramic capacitors in a product I 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.
> > > > > Any pointers would be appreciated.
> > > > > Thanks
> > > > > 
> > > > 
> > > > 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
> > > 
> > > 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 ohms, 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 looked for ceramic cracks and cant see any but I am not sure if I would see them even if they were there! 
> > > I have checked power up and the power supply does not over shoot either, 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 half 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 the resistance of the single good cap. Is it not enough? I suppose you use two 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? In this case I bet you can find poscap capacitors these days with better resistance (in fact VERY low) and probably less expensive than tantalum. Hard to say though if less expensive than two ceramics of this size.
> 
> When the caps fail the load current of the board in sleep mode goes from 20uA to 5mA, hence batteries go flat. When I remove the two caps board current returns to 20uA. If I measure the resistance of the caps one is high impedance, and one around 186  ohms.

I would suggest you contact the capacitor manufacturer.  They are intimately familiar with the various failure modes of the caps and can likely provide you with some insight.  

Rick C.

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Ceramic capacitor failure steve <steve.jones@scannex.co.uk> - 2019-02-21 02:07 -0800
  Re: Ceramic capacitor failure Uwe Bonnes <bon@hertz.ikp.physik.tu-darmstadt.de> - 2019-02-21 11:09 +0000
    Re: Ceramic capacitor failure steve <steve.jones@scannex.co.uk> - 2019-02-21 04:00 -0800
  Re: Ceramic capacitor failure Tom Gardner <spamjunk@blueyonder.co.uk> - 2019-02-21 12:27 +0000
    Re: Ceramic capacitor failure steve <steve.jones@scannex.co.uk> - 2019-02-21 04:46 -0800
      Re: Ceramic capacitor failure steve <steve.jones@scannex.co.uk> - 2019-02-21 05:22 -0800
      Re: Ceramic capacitor failure Tom Gardner <spamjunk@blueyonder.co.uk> - 2019-02-21 13:32 +0000
        Re: Ceramic capacitor failure steve <steve.jones@scannex.co.uk> - 2019-02-21 06:16 -0800
    Re: Ceramic capacitor failure Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2019-02-21 09:33 -0500
      Re: Ceramic capacitor failure steve <steve.jones@scannex.co.uk> - 2019-02-21 07:32 -0800
  Re: Ceramic capacitor failure Rob Gaddi <rgaddi@highlandtechnology.invalid> - 2019-02-21 09:47 -0800
    Re: Ceramic capacitor failure steve <steve.jones@scannex.co.uk> - 2019-02-25 02:14 -0800
  Re: Ceramic capacitor failure Dave Nadler <drn@nadler.com> - 2019-02-21 11:20 -0800
  Re: Ceramic capacitor failure Martin Riddle <martin_ridd@verizon.net> - 2019-02-21 20:52 -0500
  Re: Ceramic capacitor failure Clive Arthur <cliveta@nowaytoday.co.uk> - 2019-02-22 11:56 +0000
    Re: Ceramic capacitor failure steve <steve.jones@scannex.co.uk> - 2019-02-25 04:58 -0800
      Re: Ceramic capacitor failure raimond.dragomir@gmail.com - 2019-02-25 06:21 -0800
        Re: Ceramic capacitor failure steve <steve.jones@scannex.co.uk> - 2019-02-26 01:27 -0800
          Re: Ceramic capacitor failure gnuarm.deletethisbit@gmail.com - 2019-02-26 06:43 -0800
            Re: Ceramic capacitor failure steve <steve.jones@scannex.co.uk> - 2019-02-27 01:17 -0800
              Re: Ceramic capacitor failure steve <steve.jones@scannex.co.uk> - 2019-03-11 03:36 -0700
                Re: Ceramic capacitor failure gnuarm.deletethisbit@gmail.com - 2019-03-12 18:13 -0700
  Re: Ceramic capacitor failure Theo <theom+news@chiark.greenend.org.uk> - 2019-02-22 23:33 +0000
    Re: Ceramic capacitor failure steve <steve.jones@scannex.co.uk> - 2019-02-25 02:13 -0800
    Re: Ceramic capacitor failure steve <steve.jones@scannex.co.uk> - 2019-02-25 02:16 -0800

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