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Failure mechanisms in ceramic capacitors

SAQ

There are two main ways in which ceramic chip capacitors can fail in a surface mount assembly:

from internal stresses resulting from rapid temperature change (during soldering, for example), the result of differences in CTE between the capacitor materials and between capacitor and board
from external over-stress, applied during placement or post-soldering handling.

Both of these produce cracks in the capacitor, and may actually break the part.

The significance for reliability is that, whilst immediate detectable changes in both value and insulation resistance may be experienced in a percentage of parts, even these will often not be picked up by final testing. As a result, damaged components may be shipped in otherwise good assemblies. In the long term, cracks may lead either to parts that are open-circuit or, more likely, leaky or short-circuit. Depending on their position in the circuit, failures may be catastrophic, or perhaps just affect functionality under adverse conditions.

Apart from buying properly-made capacitors which are fit for the intended soldering method, the key precautions to minimise the risk of component failure are:

in all soldering operations, to avoid excessive temperature or high rates of change of temperature
in all component and assembly handling, whether by hand or machine, to ensure that capacitors are not subjected to strains greater than 1000 micro-strain.

This second precaution means designing the assembly to minimise strain on the parts, supporting the assembly during processing to avoid board flexure, especially during placement, and choosing a suitable depanelling process.

Cracks are often not visible to the naked eye or at low-power magnification. Whilst high-power microscopes are usually successful in detecting surface cracks, hidden cracks can only be found with some difficulty by:

observing their long-term effect on insulation resistance under accelerated conditions of heat and humidity – cracked parts will deteriorate faster than undamaged capacitors
using acoustic microscopy to detect cracks and voids, which reflect acoustic energy.