Electronic – Reliability and failure mode of MLCC (chip capacitors)


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.

Best Answer

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.