Ken

I have seen several causes for leakage in chip capacitors:

Material:

- Choice of the dielectric. Some ceramic materials degrade over time releasing free ions entrapped in the matrix or change their structure ( NPO- type ). Some ceramics such as titanate can have semiconductor characteristics.
- Spacing . Capacitors with high capacitance have more electrodes and thinner dielectrics which makes leakage more likely than in capacitors with low capacitance.
To see this you can make microsections, analysis of the material, measurement of the dielectric resistance, characterisation of the resistance ( or capacitance )- temperature behaviour.


Damage:

Cracks ( don't have to be visible outside ) in the dielectric either by mechanical stress or thermal stress ( soldering ) are a path where filaments can grow. Voids in the ceramic matrix have the same effect.
We had the best success by applying a controlled current to the capacitor while we watched for local heating with an IR- microscope. Once the heated point was localised we where able to show the failure with a microsection.

Another important analysis tool is some luck. However I try never to rely on that.

Have a great day 

Guenter

Guenter Grossmann
Swiss Federal Institute for Materials Testing and Research EMPA
Centre for Reliability
8600 Duebendorf
Switzerland

Phone: xx41 1 823 4279
Fax :      xx41 1823 4054
mail:     [log in to unmask]

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