Failure of CIN MLCC Capacitor in STKNX IC peripheral circuit

[SV.11]

Hi @Peter BENSCH , 

We are using the STKNX IC to generate the 12V supply (for latching relay operation) in our products since 2023. Its KNX UART is not utilised. 

The value of CIN capacitor used is 0805 10uF/50V MLCC Capacitor. In Datasheet, recommended CIN value is 10uF/>=35V MLCC Capacitor. It has been observed that, after working in the field for 6-8 months, the CIN capacitor fails (short) hence, the 12V output is absent. Due to this, the relay operation stops. This has been observed in many of our products. 

Could you tell what could be the reason of this CIN Capacitor failure? What corrective action should be taken to avoid this failure. 

[Peter BENSCH]

Most likely cause is stress on the MLCC capacitor, rather than a normal STKNX functional issue. Transient stress is a strong candidate - have you perhaps carried out ESD tests on the faulty systems?

In addition, an 0805 10µF MLCC can be vulnerable to microcracks from PCB flex, assembly stress, or repeated electrical stress, which may later develop into a short circuit. It might be a good idea to use a soft-termination MLCC if possible.

Although you haven't shown a layout excerpt, it may also be possible that you haven't moved the CIN close enough to the STKNX, which can lead to overloading of CIN and STKNX. Have you carried out a physical damage analysis of the failed capacitors?

Regards
/Peter