Explanation of Aging of MLC
CAPACITANCE MEASUREMENTS
AGING
Because of aging it is necessary to specify an age for reference
measurements at which the capacitance shall be within the prescribed
tolerance. This is fixed at 1,000 hours, since for practical purposes there is
not much further loss of capacitance after this time.
Capacitor aging is a term used to describe the negative, logarithmic capacitance
change that 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 125°C. This domain structure
relaxes with time and in doing so, the dielectric constant reduces logarithmically;
this is known as the aging mechanism of the dielectric constant. The more stable
dielectrics have the lowest aging rates.
All capacitors shipped are within their specified tolerance at the
standard reference age of 1,000 hours after having cooled through
their Curie temperature.
The aging process is reversible and repeatable. Whenever the capacitor is heated
to a temperature above the Curie Point, the aging process starts again from zero.
The aging curve for any ceramic dielectric is a straight line when
plotted on semi-log paper.
The aging constant, or aging rate, is defined as the percentage loss of
capacitance due to the aging process of the dielectric that 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 in a capacitor with an aging rate of 1ꢀ per decade of time, the
capacitance will decrease at a rate of:
CAPACITANCE vs. TIME
(Aging X7R (2R1) @ <2ꢀ per decade)
Standard reference time
24
20
a) 1ꢀ between 1 and 10 hours
16
12
8
b) An additional 1ꢀ between the following 10 and 100 hours
c) An additional 1ꢀ between the following 100 and 1,000 hours
d) An additional 1ꢀ between the following 1,000 and 10,000 hours, etc.
e) The aging rate continues in this manner throughout the capacitor’s life.
C0G/NP0 (1B)
4
0
X7R (2R1)
1,000
Typical values of the aging constant for our Multilayer Ceramic Capacitors
are:
-4
10,000
0.1
1
10
100
Age (Hours)
Dielectric class
Typical values
TIGHT TOLERANCE
Ultra Stable C0G/NP0 (1B)
Stable X7R (2R1)
Negligible capacitance loss through aging
<2ꢀ per decade of time
One of the advantages of Knowles’ 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 +/-1ꢀ on C0G/NP0 (1B) parts greater than or equal to 10pF. For
capacitance values below <4.7pF, tolerances can be as tight as +/-0.05pF.
12
KNOWLES [ KNOWLES ELECTRONICS ]
