Electronic – High short circuit failure rate of aluminum electrolytic capacitors during the production processes

electrolytic-capacitorshort-circuit

I have produced 1000 units of our new developed energy meter. Almost 20 aluminum electrolytic capacitors (2%) have failed short when we test them during the production processes. The Capacitor is a 470uF 35V (UPW1V471MPD) made by Nichicon.

The failure itself varies between total short circuit to small variable resistance. One of the caps spit out its electrolyte.

I have 3 other different aluminum electrolytic caps from the same manufacturer that didn’t fail. A year ago, I have produced 300 units (same design and part no.) and they running perfectly and didn’t find any failed capacitors.

My question has 3 parts:

  1. How this occurred?

    Is it normal to have an infant mortality of 2% with short circuit failure for aluminum electrolytic caps?

    Or, may be the cap is counterfeited and the con manufacturer didn’t do enough quality testing. (I bought them through a PCB and components supplier in Shenzhen). If they are counterfeit, how to verify that?

  2. What should I do with the rest of the batch?

    If I release the batch, would I guarantee that there will be no more short circuit failure? If it didn’t die during the manufacturing, it will not die in the near future! (U shape/bathtub graph).

    Or it is better to run my meters for a period of time (24 hours) if didn’t fail it will not fail in the future.

    Or I should go the hard way and change them all?

  3. How to avoid this in the future, because I will be going for large quantities > 100K.

    I didn’t find very useful notes about electronics parts quality and reliability other than MIL-HDBK-978B. But it is out dated and didn’t have the electrolytic caps.

Added

Operation condition:

  1. Voltage = 20V.

  2. Inrush and ripple current = the capacitor is used to store energy to supply a latch/rely only when operate. it operated about 5 times during production. latch/relay impedance 60 ohm. it draw its voltage from a capacitive power supply capable of supplying a 17mA at maximum.

  3. Temperature: about 30C.

  4. there is a guarantee that no cap have been reversed biased.

Best Answer

That high a failure rate is unheard of for a top-quality supplier like Nichicon when properly assembled and operated conservatively. Even for no-name parts it’s not at all usual- one in 10,000 might be plausible, but that’s on the high side. Short circuit failures are very rare for aluminum electrolytics. I did once see a few in a bag of 1,000 from a Taiwan supplier that were completely missing the rubber seal so the electrolyte also had gone AWOL- that was actually funny.

You can contact Nichicon directly to confirm the parts are genuine (or not). They may be able to tell just from photos or you might have to courier samples.

You can review their application information to make sure you are not abusing the parts in some way- not only voltage but also ripple current, possible reverse voltage or reverse installation (that is one thing that will cause shorts). Poorly made counterfeits might be marked incorrectly so they are reversed even though they appear to be installed correctly. Your transformerless supply might be stressing the part upon application of power.

Also confirm that the chemicals and processes used in the PCBA and any subsequent operations such as cleaning are approved.

I would definitely pull 100% of the parts from that batch of boards and replace them with known good ones. Use good tools and skilled technicians so that reliability isn’t unduly compromised by the rework. Field failures are extremely expensive in dollars and in reputation. Give them a good visual inspection under a microscope, or at least with a magnifier, and see if you can identify differences between batches or within a batch.

As far as prevention in the future, that is a bit off-topic, but there are a few approaches to control “quality fade” and substitutions of inferior parts - one of which is third party inspections. The assembly house or supplier you used is suspect if they allowed counterfeit parts to be procured. You may have a better choice of suppliers at higher quantity levels and you can ask how they intend to guarantee genuine parts are used. The Shenzhen markets are a bit of the Wild West, so you need to take care. Nichicon will have authorized distribution channels there, but it’s also possible to procure parts of unknown history at the many retail shops in Huaqiangbei or online at Taobao etc.

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