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I am trying to make a fault analysis of a bunch of ceramic capacitors.

Short description of the application:

10 220 µF ceramic capacitors 1210 package are placed in parallel with a 3.6 V battery. A MCU wakes up periodically (maximum once per minute) and draws current (maximum peak 10-15 mA for a few milliseconds). Total time before going back to extremely low power sleep is 130 ms. The capacitors are supposed to hold enough energy to cover this without dropping below 1.6 V (minimum supply voltage for the MCU).

This is needed since the operating temperature is low, and the battery cannot deliver. The battery has enough time to recharge the capacitors while the MCU sleeps.

I am suspecting shorts in the capacitors. Because:

  • The battery has drained very quickly on some of my PCBs
  • From what I have read ceramic capacitors, especially in large packages, are sensitive to mechanical stress and can crack, causing shorts

To see this for myself I have attempted making cross sections, but I have a hard time understanding what I am seeing.

How I made the cross section:

  • Used a dremel to cut off the corner of the PCB where the capacitors are placed
  • Molded the cut off PCBs in epoxy glue to make handling easier
  • Used a diamond circular saw blade to make a cross section approximately in the middle of the capacitors (lengthwise)
  • Wet grinding and polishing down to 1 micron and then 1 µ lapping film

I repeated this on two PCBs.

There are 3 capacitors next to each other: Overview

Here you can see a color difference between the capacitors, top right and bottom middle are darker in color. But as you can see, not in the same position.

I don't have enough rep to add all images. I will comment links to all the images. Would appreciate if someone could edit and add the images to the post.

The darker colored ones (top right, bottom middle) look like this close up.dark1 third

Almost what I was expecting a ceramic capacitor to look like. At least you can see some kind of layering. But the layers are not solid as I expected. Can this be damage caused by the grinding and polishing?

The distance between the layers is 2 µm.

The lighter colored ones look like this: fourth fifth

What is this?! Can e.g. high currents cause the layers to melt together like this? Or can this also be caused by my grinding and polishing?

Here we can see an air bubble in the solder. But the gap close to the bottom, can that be damage caused by mechanical stress?

sixth

I later tried grinding and polishing a bit further into the capacitors. It looks exactly the same. If the strange wavyness and/or the broken off layers had been caused by the grinding and polishing I expect that the characteristics would have changed. E.g., a wavy one now has broken off layers instead, and the other way around.

The exact capacitors used are Taiyo Yuden JMK325ABJ227MM-T

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    \$\begingroup\$ "I am suspecting shorts in the capacitors." should be easy to test with a multimeter. Also you gone to quite some effort looking into the caps, you could have used a known healthy one as a comparison. \$\endgroup\$ – PlasmaHH Jun 26 '17 at 8:28
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    \$\begingroup\$ especially if you suspect a high Ω short, the fault (if visible at all) may not be in the layer you sanded down. But still there you could measure with a multimeter, you just need to wait a while until it settles down. Or you apply a voltage and measure the leakage current directly if that is more what you are intrested it. \$\endgroup\$ – PlasmaHH Jun 26 '17 at 8:44
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    \$\begingroup\$ Now I am more curious for an explanation of the weird circling and landscape-like structure in image 4 and 5 \$\endgroup\$ – Filippa Jun 26 '17 at 9:00
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    \$\begingroup\$ +1 for the outstanding quality of both the structure, content and photography. A superb question. \$\endgroup\$ – Wossname Jun 26 '17 at 10:43
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    \$\begingroup\$ The wavy patterns look like you somehow managed to slice the cap parallel with the layers. You see waviness because the planes are perfectly parallel, and your process isn't perfectly aligned with the planes either. If these caps are square in crossection, then it's a tossup which way they get soldered onto the board. \$\endgroup\$ – Olin Lathrop Jun 26 '17 at 11:30
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It looks to me like the grinding/polishing has been done fairly well (with more care you could have less scratches), and you're looking at an accurate and undamaged image of the capacitor cross section.

The "dark" images are more or less what I'd expect to see from a capacitor cut across the planes of the electrodes. Metal electrodes in a darker ceramic matrix. For lower value capacitors I'd expect to see thicker parallel lines, but for the lines to be slightly wavy and broken isn't a huge surprise. I expect that this results from the special steps they've taken to get the very high capacitance in a tiny package. Possibly a combination of grid electrodes rather than planes, and squashing/forming the ceramic after building the layers but before final firing in order to get the layers thinner.

The "pale" images are more or less what I'd expect for a capacitor sectioned parallel to the electrode planes. Assuming you've used a metallographic grinder (looks like it) then your section plane is flat, but the electrodes aren't. So you get contour-like features where the electrode crosses the section plane.

I doubt you'll find your leakage in these images. Other places to look:

  • Check the datasheets for the expected resistance. Is it as high as you thought? Check the conditions under which it is given in the datasheet, see if your environment is likely to make it worse.
  • Check a batch of new capacitors to see what the resistances are
  • Check a bunch of capacitors from your warranty returns to see if the capacitance or resistance has changed.
  • Measure the resistances on your PCB before assembly (should be nice and high)
  • Measure the resistance on a completed PCB (maybe sans MCU). Look for evidence of flux which has not been cleaned well enough and could reduce the resistance.
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  • \$\begingroup\$ Oh! This explains everything! Please feel free to expand your answer with more details on MLCC development and structure. What do you have to say about these failure analyses, for example? Were they simply extremely lucky to find these faults using this method? gideonlabs.com/posts/failure-analysis-mlccs, gideonlabs.com/posts/…, gideonlabs.com/posts/leakage-current-mlcc-pcb \$\endgroup\$ – Filippa Jun 26 '17 at 18:52
  • \$\begingroup\$ Those analyses show delaminaton and cracking which looks familiar to me, not from MLCCs, but from piezos, which are actually very similar materials and construction. I don't see anything similar in your images, which is what I meant by saying I didn't think you'd find it. Damage could be quite small and localised, so you might not necessarily see it on a single section. To get those images they have probably ground the whole capacitor away 50um at a time, and picked the best image for the writeup. \$\endgroup\$ – Jack B Jun 26 '17 at 21:24
  • \$\begingroup\$ My advice to you would be to take some caps off the failed boards and actually measure them, and compare to new ones. Only if you are certain the cap has degraded would I spend much time on sectioning and examining. And if you are going down that route you'll probably need to take a lot of sections to find the damage. The second one in your link is a very severe failure. I would expect that to have a resistance of only a few ohms. That is probably visible in every section, the other one not so much. \$\endgroup\$ – Jack B Jun 26 '17 at 21:28
  • \$\begingroup\$ A visual inspection might miss some kinds of flaws: if these capacitors use tin metallization, tin oxide is both conductive and transparent. \$\endgroup\$ – Whit3rd Jun 27 '17 at 8:02
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I assume the purpose of this exercise is to source capacitors that are OK.

Unless you are buying gazillions of the things, and so would have the purchasing clout to make the manufacturer listen, doing physical analysis of the capacitors is not going to advance you along the road of being able to get good parts, even if you can figure out what's wrong and how to change the manufacturer's process.

First, identify all the different capacitor manufacturers. Then buy a few samples of a suitable capacitor from each. Measure their leakage before soldering. Solder to boards and remeasure their leakage. Identify specific part number that are OK to buy, or should not be bought, as a result of these tests. Then stick to good part numbers.

Warning, leakage measurements are difficult to do well, wait long enough, check parasitic currents like DMM and amplifier input currents, make sure surface contaminants aren't making the board leak.

220uF is a lot for an SMD capacitor. You might get better results by using a less extreme capacitance/volume ratio, even if it means using more parts. Manufacturers use different ceramics for different C/V ratios, and you might find that leakage has been sacrificed for capacity in the particular size ratio you bought. Note that designations like X7R, Y5U etc do not identify the ceramic, only the tempco and tolerance spec. They don't identify the voltco (a very bad feature of high C/V ratio ceramics) and will also not identify any leakage specs.

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    \$\begingroup\$ At this point this is more of experimenting out of curiosity and wanting to understand. It is also fun to use some cool equipment you don't get to use very often \$\endgroup\$ – Filippa Jun 26 '17 at 9:03
  • \$\begingroup\$ @Filippa go for it then, but I doubt you'll see anything that accounts for the difference in leakage. I'm also quite interested to see what they look like inside. I guess the wavyness is due to perhaps using 'puff-pastry' techniques to get very thin layers? But take the type of ceramic very seriously, it varies as they try to cram more uFs into the same footprint, and compromises will have been made. \$\endgroup\$ – Neil_UK Jun 26 '17 at 9:21
  • \$\begingroup\$ But since some of the capacitors have the strange wavyness, while others do not... Feels like they have been damaged somehow \$\endgroup\$ – Filippa Jun 26 '17 at 9:25
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    \$\begingroup\$ Using the TDK capacitor tool, the effective capacitance at 3.6V can be expected to be no higher than 100uF for these devices. product.tdk.com/info/en/products/capacitor/ceramic/mlcc/… \$\endgroup\$ – Peter Smith Jun 26 '17 at 9:56

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