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

−

=

=

∑

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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