AN5948 Datasheet, PDF (1/11 Pages) Dynex Semiconductor – Reliability of High Power

INTRODUCTION
We are often asked “What is the MTBF or FIT
rating of this diode or that thyristor?” We
cannot answer this without knowing how the
customer intends to use these devices in a
system and what the conditions of the
operation of the devices are. In other words
we would need to know the “Mission Profile”.
MTBF is the “Mean Time Between Failure”
and is the measure of an average time for a
second component to fail after the failure of a
first component in a system. MTBF usually
applies to a repairable system consisting of
many components. Knowing the MTBF allows
the system designer to recommend repair or
maintenance schedule for the system and
thus deduce the running cost of the system.
For semiconductor devices MTTF (Mean Time
To Fail) is generally appropriate, however
MTBF and MTTF have the same value if the
time to repair a system is negligible. Thus
MTBF is loosely used to mean MTTF for
semiconductor devices.
FIT (Failure unIT or Failure In Time) is a unit
for the measure of failure rate (λ) of the
components, and is equal to one failure per
billion hours (10⁹ hours). Both MTBF and λ are
statistical quantities and if the failure pattern
assumes normal distribution then one is the
reciprocal the other (MTBF = 1/λ). The failure
rate is useful for predicting the life of a
device.
The purpose of this Application Note is to
discuss the reliability of the high power
bipolar devices (diodes, thyristors and GTOs)
which is related to the different failure
AN 5948
Reliability of High Power
Bipolar Devices
Application Note
AN5948-2 September 2009 LN26862
Authors: Dinesh Chamund, Colin Rout
mechanisms, materials used in packaging the
devices and the manufacturing processes
used. Also different methods used to predict
the reliability and pros and cons about each
method are discussed.
DEVICE CONSTRUCTION
Fig. 1 Pressure contact thyristor construction
Fig. 1 shows a typical construction of a fully
floating pressure contact thyristor and the
materials used. The silicon wafer is
sandwiched between a molybdenum washer
and disc, thus providing electrical contact to
the active parts of the device. These are
further sandwiched between two copper pole
pieces one in the ceramic housing and the
other in the lid. The housing is backfilled with
inert gas and the copper lid is cold welded to
the ceramic housing. In non-fully floating
construction the molybdenum disc is alloyed
to the silicon wafer. Electrical and thermal
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