© Knowles 2014 SafetyDatasheet Issue 9 Release Date 10/01/17 Page 7 of 9
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Ageing of Ceramic Capacitors
Capacitor ageing is a term used to describe the negative,
logarithmic capacitance change which takes place in ceramic
capacitors with time. The crystalline structure for barium
titanate based ceramics changes on passing through its Curie
temperature (known as the Curie Point) at about 125ºC. The
domain structure relaxes with time and in doing so, the
dielectric constant reduces logarithmically; this is known as the
ageing mechanism of the dielectric constant. The more stable
dielectrics have the lowest ageing rates.
The ageing process is reversible and repeatable. Whenever the
capacitor is heated to a temperature above the Curie Point the
ageing process starts again from zero.
The ageing constant, or ageing rate, is defined as the
percentage loss of capacitance due to the ageing process of
the dielectric which occurs during a decade of time (a tenfold
increase in age) and is expressed as percent per logarithmic
decade of hours. As the law of decrease of capacitance is
logarithmic, this means that for a capacitor with an ageing rate
of 1% per decade of time, the capacitance will decrease at a
rate of:
a) 1% between 1 and 10 hours
b) An additional 1% between the following 10 and 100
hours
c) An additional 1% between the following 100 and 1000
hours
d) An additional 1% between the following 1000 and
10000 hours
e) The ageing rate continues in this manner throughout
the capacitor’s life.
Typical values of the ageing constant for our MLCCs are
Negligible capacitance loss
through ageing
Capacitance Measurements
Because of ageing it is necessary to specify an age for
reference measurements at which the capacitance shall be
within the prescribed tolerance. This is fixed at 1000 hours,
since for all practical purposes there is not much further loss of
capacitance after this time.
All capacitors shipped are within their specified tolerance at the
standard reference age of 1000 hours after having cooled
through their Curie temperature.
The ageing curve for any ceramic dielectric is a straight line
when plotted on semi-log paper.
Capacitance vs. Time
(Ageing X7R @ 1% per decade)
Tight Tolerance
One of the advantages of Knowles’s unique ‘wet process’ of
manufacture is the ability to offer capacitors with exceptionally
tight capacitance tolerances.
The accuracy of the printing screens used in the fully
automated, computer controlled manufacturing process allows
for tolerance as close as ± 1% on C0G/NP0 parts greater than
or equal to 10pF. For capacitance value less than 4.7pF
tolerances can be as tight as ± 0.05pF.
Periodic Tests Conducted and Reliability Data
For standard surface mount capacitors components are
randomly selected on a sample basis and the following routine
tests conducted:
Load Test. 1,000 hours @ 125˚C (150˚C for X8R).
Applied voltage depends on components tested
Humidity Test. 168 hours @ 85˚C/85%RH
Board Deflection (bend test)
Test results are available on request.
Conversion Factors
FIT = Failures In Time. 1 FIT = 1 failure in 10
9
hours
MTBF = Mean Time Between Failure
Example of FIT Data Available
Component type: 0805 (C0G/NP0 and X7R)
Testing Location: Knowles reliability test department
Results based on: 16,622,000 component test hours
