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Recently I've been preparing to manufacture a product which exclusively uses MLCC capacitors across the board. It integrates an onboard buck converter which uses them, and MLCC are also used for local decoupling.

My prototypes have consisted of "dodgy" reflow techniques using a hot plate. Generally, 10% of the time after doing this, I find a shorted MLCC on the board, usually found because when power is applied, the cap will smoke.

However, just right now I was replacing one of these caps with a soldering iron and after I had replaced it, it was still shorted. I verified there was no other short circuit on the board (because when removed 3.3V showed a few kohms of resistance.) It seems the simple operation of soldering the cap has caused it to fail.

I've also recently repaired an LCD monitor which had a shorted MLCC on the T-con board and a few other users on a popular forum have reported this issue as being surprisingly common. Now, in this case, a monitor gets warm or hot, but no where near as hot as a soldering iron - so why could these be failing?

I am planning to offer a five year or perhaps longer warranty on these boards, but I can only do so if I am confident the board is capable of surviving normal conditions.

Caps are 0603 (100n, 10u 6.3V), 0805 (22u 6.3V) and 1206 (10u 35V). All are X5R or X7R. There are some 18pF caps for the crystal, but I've never seen those fail - I suspect they are a different technology to MLCC though.

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  • \$\begingroup\$ Brand matters with capacitors. Do you know the brand of caps that you are using? Who is your supplier. \$\endgroup\$ – Russell McMahon May 18 '12 at 17:46
  • \$\begingroup\$ @RussellMcMahon These aren't electrolytic caps, but if it matters, they are mostly AVX with a few Multicomp (i.e. cheap made-in-china). The multicomp are 100n exclusively. \$\endgroup\$ – Thomas O May 18 '12 at 17:52
  • \$\begingroup\$ Do you flex the board at all? MLCC's are very sensitive to flex unless you spec the $$$ "flexible termination" ones. Even the flex caps aren't great but at least they fail open. \$\endgroup\$ – markrages May 18 '12 at 17:56
  • \$\begingroup\$ @markrages In general I don't. However, the thermal expansion might be causing them to flex? \$\endgroup\$ – Thomas O May 18 '12 at 18:00
  • \$\begingroup\$ What size caps? 1206 and bigger are most likely to have a flex problem. 0603 and smaller are much less likely to have a problem (but still could if you're abusing the board). If it's 0402, though, I'd be more likely to think you just got a solder bridge. \$\endgroup\$ – The Photon May 18 '12 at 18:08
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Some cap vendors make their own parts. Some buy caps from a smaller manufacture with the reels re-branded at the fab. Watch out. I got into a mlcc failure investigation in 2002 and started inspecting caps on a reel under a microscope. 3/10 came off the reel cracked. A crack will sooner or later lead to a short. Cracks are not obvious even under a microscope. They may present as a subtle colour shift if the crack is below the surface layer. Some cracks may be sufficient to present a short immediately. Not all. The vendor's manufacturer in this case eventually identified a hopper where the caps were getting cracked.

MLCC are very sensitive to mechanical stress. Especially bigger than 1210 sizes. I found a big power bypass cap adjacent to a heavy mechanical connector once. The nearest mounting hole was 2" away! They were cracking at a rate of 5/10 during install of the unit. A fraction of those were catching fire. The fire would continue to burn until it melted the copper, breaking the power connection.

Another effect of a crack is a reduction of the max working voltage of the cap. It might be spec'd for 200 V. But once cracked it may break down at 40 V. Cracked caps burst into flames in my lab when tested - even below their rated voltage.

Another way to warm up caps is exceeding their max ac current. It's easy to think of caps as zero power dissipation devices. Especially the higher Q mlcc. But they're not. Calculate the power dissipated in caps and do not exceed power/ac current limits. Shows up in power circuits and converters commonly.

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MLCCs fail for a variety of reasons, including mechanical stress (board flex) and thermal shock. They are very sensitive to the manufacturing process. Type "ceramic capacitor failure modes" into Google and you'll find all the data you'll ever need.

As for the CRT issue that you described, most likely the capacitors in question are being latently damaged during assembly, which prematurely shortens their life leading to early field failures. The warm ambient may contribute a bit to the rate of failure, but I doubt that a properly-sized, properly-soldered part would fail solely because of this.

MLCC rework should not be done with an iron. A hot-air rework tool should be used to uniformly heat the entire part so that it 'reflows' on its pads, then remove it with tweezers or some other tool. Replacement is similar - evenly heat the part on the pads so that it reflows, then remove the air and let the solder solidify. Too much heat with an iron can damage the part both on removal and on replacement. I believe that IPC mandates hot-air rework for MLCCs, as both my current and former employer strictly enforced this policy on shippable goods.

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We have adopted the practice of stacking 2 MLCC in series and mounting them at right angles for any decoupling caps that are powered continuously. We also mount them at right angles so that singulation stresses that crack one cap will not crack the other. Since they virtually always fail short you are left with 1 of the caps still in the circuit.

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MLCC capacitor manufacturers have lots of information concerning soldering and mounting do's and don'ts. Hand soldering is a large contributor of failure mechanism,(25 years of contract manufacturing). The larger the cap, 1206, 2512, 2225 etc. the more susceptible to thermal shock and failure. If hand soldering, heat components with hot plate to reduce the Tshock. Many large corporations forbid the hand soldering of MLCC capacitors on their products. Mechanical stress, especially depanelizing with a rotary cutter creates lots of bad components, LED's and MLCC's. Design orientation of MLCC length should be perpendicular to PCB potential flex length. Keep away from any potential stress points, i.e. mounting holes, I/O connectors etc. BUY Quality components. If pennies and nickels make or break the product, then it probably is not important if it survives anyway!

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