Fit Rate / Equivalent Device Hours

λ = Proportion allowed system failures

X

1

X

10

9

=

FITS

Time period

No. of devices

In the case of the example,

λ =

0.01 Failures

X

1

=

10

9

=

14 FITS

720 Hours

1000 Devices

or 14 FITs or 14 failures per 10

Traditionally, reliability results have been presented in terms of Mean-Time-To-Failure

or Median-Time-To-Failure.

While these results have their value, they do not

necessarily tell the designer what he most needs to know. For example, the Median-

Time-To-Failure tells the engineer how long it will take for half a particular lot of

devices to fail. Clearly no designer wishes to have a 50% failure rate within a

reasonable equipment lifetime. Of greater interest, therefore, is the time to failure of a

much smaller percentage of devices say 1% or 0.1%. For example, in a given

application one failure per hundred units over five years is an acceptable failure rate

for the equipment, the designer knows that time to accumulate 1% failure of that

components per unit, then no more than 0.1% of the components may fail in five

years. Therefore, the IGBT / CoPack reliability or operating-life data is presented in

terms of the time it will take to produce a prescribed number of failures under given

operating conditions.

To obtain a perspective of failure rate from an example, let us assume that an

electronic system contains 1,000 semiconductor devices, and that it can tolerate 1%

system failures per month. The equation for the device failure is:

IGBT / CoPack

Quarterly Reliability Report

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