Another statistical value is "ppm" (parts per million). This describes the ratio of the
of all components delivered to the number of failures n
for the same
period. However, only failures are counted which occur during operation under
component specification. To calculate the ppm rate, all components are entered
beginningwith series production. Prototypes, engineering samples or other samples are
not taken into account.
14.2 Failuremechanisms in the application
As describedabove, two general types of failuremechanisms can bedistinguished:
Sudden failures, i.e. spontaneous, unpredictable failures and
Drift failures, i.e. predictable failures, whichdevelop slowly over time.
One of the main tasks of the application engineer is to prevent sudden failures. The
application has to be designed such that no failure occurs considering all relevant
parameters. This could be done by redundant systems, enhanced protectionmeasures,
etc. The manufacturer of the component used in the target application has, in practice,
virtually no way to design the component accordingly. Instead, the correct selection of
the component and the engineer's system design determine the risk of a sudden failure.
The manufacturers of power devices offer support for the choice of the appropriate
component. And often they offer direct or indirect support by internal or external
application engineers, and calculation and simulation software for the design process of
the target application.
With regard to drift failures, the long-term stability of the component that is determined
by the manufacturer and subsequently cannot be changed, plays a decisive role. For
the design engineer, it is therefore necessary to obtain sufficient data on the
characteristics of the component with respect to its long-term stability in the target
application. Manufacturers typically provide data such as power cycling and thermal
cycling diagrams. Based on these data, together with the individual load profile of the
application, a statement can be made with regard to the life of the component and its
depicts an overview of different failuremechanisms, all of which can ultimately
lead to the failure of an IGBT power semiconductor. The gray boxes show the causes of
sudden failures. The shaded boxes stand for causes that can lead to drift failures. A
special point here is "improper handling", which can lead to ESD induced failures.
Depending on the degree of damage, the failure might show up immediately or only
after a longer period (pre-damage).
A special failuremechanism that can occur due to cosmic particle radiation is not listed
in the summary. This is dealt with separately at theendof this chapter.