Ceramic Capacitor Failure
in [Design]
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From: eeboy on 30 Mar 2010 16:50 I've got a situation where boards are being returned all of the sudden. This particular board has been in service well before my time so it seems as if it is perhaps related to a particular batch. The problem is with an 0805 ceramic capacitor. The same capacitor seems to have failed on each. The result is a 56 ohm DC resistance. The capacitor is a simple decoupling cap for a logic IC. Once removed, the board behaves nicely again. Is this a common failure mode for ceramic caps? What's the probability that we simply got a bad reel of the component? What potential causes are there for this failure? I can rule out over voltage. The cap is rated at 25V but the maximum voltage it would see during operation is 5V. I think I can rule out ESD as there are many more sensitive components that would have fried before this cap. The circuit is a low power logic circuit... no large switching currents. Any thoughts? --------------------------------------- Posted through http://www.Electronics-Related.com
From: Vladimir Vassilevsky on 30 Mar 2010 17:06 eeboy wrote: > I've got a situation where boards are being returned all of the sudden. > This particular board has been in service well before my time so it seems > as if it is perhaps related to a particular batch. The problem is with an > 0805 ceramic capacitor. The same capacitor seems to have failed on each. > The result is a 56 ohm DC resistance. The capacitor is a simple decoupling > cap for a logic IC. Once removed, the board behaves nicely again. Is this a > common failure mode for ceramic caps? What's the probability that we simply > got a bad reel of the component? What potential causes are there for this > failure? > > I can rule out over voltage. The cap is rated at 25V but the maximum > voltage it would see during operation is 5V. I think I can rule out ESD as > there are many more sensitive components that would have fried before this > cap. The circuit is a low power logic circuit... no large switching > currents. > > Any thoughts? I've seen that problem before. Certain low quality makes or batches of ceramic caps can fail into short or resistance even if they look good initially. The failure happens when a cap is been soldered to the PCB; especially if the cap is soldered by hand. Thermal stress destroys it. Vladimir Vassilevsky DSP and Mixed Signal Design Consultant http://www.abvolt.com
From: Joerg on 30 Mar 2010 17:10 eeboy wrote: > I've got a situation where boards are being returned all of the sudden. > This particular board has been in service well before my time so it seems > as if it is perhaps related to a particular batch. The problem is with an > 0805 ceramic capacitor. The same capacitor seems to have failed on each. > The result is a 56 ohm DC resistance. The capacitor is a simple decoupling > cap for a logic IC. Once removed, the board behaves nicely again. Is this a > common failure mode for ceramic caps? What's the probability that we simply > got a bad reel of the component? What potential causes are there for this > failure? > > I can rule out over voltage. The cap is rated at 25V but the maximum > voltage it would see during operation is 5V. I think I can rule out ESD as > there are many more sensitive components that would have fried before this > cap. The circuit is a low power logic circuit... no large switching > currents. > > Any thoughts? > Take a good look at it under a microscope, from all sides. Stress fractures would be very unusual for a 0805 size but who knows? -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
From: eeboy on 30 Mar 2010 18:19 >Take a good look at it under a microscope, from all sides. Stress >fractures would be very unusual for a 0805 size but who knows? > >-- >Regards, Joerg > Interesting... under the microscope I see something towards one end on only one side. I can't say that it is a fracture but it certainly appears as such. A darker jagged line of sorts. What causes fractures. Overheating as indicated in another post? The board flexing while the component is fixed? --------------------------------------- Posted through http://www.Electronics-Related.com
From: Joerg on 30 Mar 2010 18:46
eeboy wrote: >> Take a good look at it under a microscope, from all sides. Stress >> fractures would be very unusual for a 0805 size but who knows? >> >> -- >> Regards, Joerg >> > > Interesting... under the microscope I see something towards one end on only > one side. I can't say that it is a fracture but it certainly appears as > such. A darker jagged line of sorts. > Oh-oh, jagged lines are not a good thing there :-( > What causes fractures. Overheating as indicated in another post? The board > flexing while the component is fixed? > Mostly thermal load changes but flexing can add to it. RoHS solder processes can really make things worse. This is one reason why really fat footprints like 2220 are often avoided and why there are cap arrays on "stilts", with L-brackets at both ends. -- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM. |