What causes the degradation?

-----Original Message-----
From: Guenter Grossmann [mailto:[log in to unmask]]
Sent: Wednesday, December 13, 2000 4:44 AM
To: [log in to unmask]
Subject: [TN] Antw: [TN] Failure analysis of cap: Type analysis and any
lab that cando the job in San Jose Bay Area


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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