Ceramic Capacitor Failure

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

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?

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?

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I had that problem on 0805 caps.

One particular cap failed regularly from one reel of capacitors, open circuit not low R as in this case. It was a boost cap on a simple switcher. It was a little thicker than normal and the placement force was cracking it. After several months the psu failed as the switcher lost the boost cap.

If it had been a case of being cracked when placed we would have found it on test. I concluded that the cap had been stressed and then failed due to normal expansion and contraction due to temperature variation.

Oh-oh, jagged lines are not a good thing there :-(

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.

Probably as everyone said its a fracture that wicked in some contaminant that caused the short. The cracks usually ocur on soldering but also cna occir due to board flex and are aggrivated by vibration and thermal cycles.

As carefully as you can remove a few and send them in the the manufacturer to let them have a look.

Cap manufacturers specify a maximum rate of change of temperature vs time during the soldering process. This is, most likely, where your problem is.

We had a lot of problems with high density ceramics. We modified the reflow profile and the problems went away.

Bob

simply

I've seen similar failures in wire leaded high voltage ceramic caps. Z5R material, 470pf, caps rated at 10,000 volts failed at about 4,500 volts. It seemed to be batch related with a number of failures.

Guess where the caps were made?.......Of course kids, China, where else?

I'll bet that batch they were wondering of anyone would notice they labeled someones 1Kv caps at 10Kv. :)

Z5U at 10kV? Wow, it might actually make a good bypass down under 100V ;-)

Tim

-- Deep Friar: a very philosophical monk. Website:

is.

reflow

One thing that I just realized is the very same capacitor is used in two other places on the board. This particular board is a 4 layer board with the inner planes devoted to power and ground. Examining the three locations I noticed the two other locations where this cap is used are tied to the inner planes with short traces and a via. The particular cap that is failing has one pad tied to ground through a short trace but the other pad travels about 2cm (7mil trace) before tying to anything significant. Perhaps if this trace was short like the other two locations then the problem would disappear?

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How are these boards soldered. Hand, reflow, etc?

;-)

That's Z5R

This is an uncommon failure unless you are using crappy or counterfit parts. Shorts can be caused by stress if it cracks the capacitor. Is this part stressed during assembly or mounting? Depaneling a board can stess parts near the edge of the board, especially if the parts aren't aligned properly. Stress from the mounting points can crack nearby parts if there is vibration, shipping, or the system is dropped, especially if the board has insufficient support.

Google Z5R and find NOTHING. Google Z5U and find that it is an IEDC Class 2 dielectric medium.

Maybe you should "try again".

Z5U is a common HV cap dielectric. Never heard of Z5R.

How was it attached? By hand? Then yes, MLCCs can be put in jeopardy of infant failure modes by the application of excessive heat. If you glue it on and wave solder that side of the board, you can also do it.

Then, there is brand name. Independent of materials used in its construction, buying from a not so well known "maker" can yield very poor lot-to-lot quality consistency.

Stick with Vishay, who bought up a lot of the other makers. Philips is another good one. there are, of course others that are just as good.

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With the rather minimal data presented it sure sounds like bad parts. =20 Admittedly it was ~20 to 30 years ago I did space qualification testing=20 of MLC capacitors.

Well then, since you can't find it on the internets, it must not exist! Idiot!

Z5R is a 10deg C., to 85 deg C, +/- 15% Class 2 dielectric.

Table 3 EIA Codes For Non-Temperature-Stable Ceramic Capacitors (Class 2 and Higher)

Low Temp. Limit: High Temp. Limit: Capacitance Change over Range:

X -55C 4 +65C A ± 1.0%

Y -30C 5 +85C B ± 1.5%

Z +10C 6 +105C C ± 2.2%

P ± 10%

R ± 15%

S ± 22%

T +22% -33%

U +22% -56%

V +22% -82%