# IGBT Modules - Technologies, Driver and Application (Second Edition) - page 525

513
T
C4
needs to be considered, since the time of a complete load cycle is in the order of
the time constant, which is relevant to the stress of the system solder layer. The smaller
the effective cycle time, the less relevant it is in this calculation (this can also be seen in
the large cycle values which result, when smaller time intervals are considered, as
described further below).
For each of these temperature differences an associated cycle valuemay be read from
theTC diagram. The following results:
T
C1
= 24K
n
1
=
6
10 13.1
cycles
T
C2
= 18K
n
2
=
6
10 11.4
cycles
T
C3
= 11K
n
3
=
6
10 72.37
cycles
T
C4
= 31K
n
4
=
6
10 36.0
cycles
Since each cycle shortens the lifetime of themodule, the respective values according to
ave to be combined.
1
k
1i
k
cyc
n
1
n
=
=
Eq. 14.12
For the lifetime in cycles referenced to the system solder, therefore, the following value
results from theexample above:
3
1
4
3 2 1
cyc
10 254
n
1
n
1
n
1
n
1
n
= 

+ + + =
cycles
As the whole load cycle takes 300s, the entire operational lifetime is accordingly
21167h.
It has to be noted that in the calculation the TC chart sets a lower temperature limit of
25°C, while this was 50°C in the example above. Accordingly, an effective, but
unspecified reductionof the calculatednumber of cycles has to beassumed.
In the second step, the operational life of the bondwire connections is investigated. For
this, thePC charts fro
and
reused:
Three load cycles can be distinguishedwith the load cycle profile. These are defined by
the respective load factors. In contrast to the considerations of the system solder layer,
in the investigation of the bond wire connections the maximum temperature lift for a
complete load cycle is not taken into account, as this is significantly above the time
constant, which is relevant for this connection:
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