Multilayer ceramic capacitors are widely used in electronics design. SMD MLCCs are used for example as filter and bypass capacitors. As the name implies the MLCC has multi layers connected inside to form the desired capacitance. Unfortunately, MLCC could crack internally under mechanical stress.
Multi-layer capacitors placed near or behind a terminal exposed to mechanical stress or in any area of the PCB are expected to bend. A portion of this stress will be transferred to the multi-layer capacitor and cause an internal crack which could cause a short circuit or an open circuit.
Stress sources vary, it could be from a terminal with screws, a plug/connector or even a bend made by human action like cutting PCBs from the panel after assembly. That’s why it’s not recommended to break the assembled PCBs from panels with bare hands.
David Jones, over EEVBlog, made an amazing video explaining about strain crack mechanism and prevention. Dave demonstrated a collection of recommendations while placing an MLCC in the PCB.
In this demo, C1 will express a stress while using the plug. PCB designers can take action to avoid the stress by adding an extra mounting hole next to C1 to take the mechanical stress away (1), or by moving the plug near to mounting holes (2). Another smart suggestion is to rotate the capacitor which will reduce the stress on it (3). This is just like holding a stick’s two ends between your two hands to crack it, which will be easy to crack, but if you rotate it 90 degree it will be very hard to crack it. Adding stress relief slots could also take away stress (4).
As mentioned by Dave in the same video, designers can also use safety techniques in the design to avoid short circuit failure when a capacitor fails:
- Connecting two capacitors in series, thus if one made a short circuit the other will handle it. In this way you have double capacitance because of series connection.
- The previous technique will save the design from a short circuit not open circuit. So you can add another two capacitors in parallel for this purpose.
- Using smaller MLCC in packages. Smaller in packages less affected by stress.
- Using new technologies proposed by capacitors manufactures like AVX MLCC FLEXITERM technology, where the component it self is designed to absorb the stress.
These consideration can be very useful even for other types of capacitors. Please refer to James Lewis from Kemet amazing presentation “They’re ‘Not’ JUST Capacitors” which presents a lot of capacitors types and some defects of each type. And of course, try to watch the main reference of this micro-blog, Dave’s video “Solving Ceramic Capacitor Cracking“.
