Thermoelectric devices are highly reliable due to their
solid state construction. Although reliability is somewhat
application dependent, MTBFs calculated as a result of
tests performed by various customers are on the order
of 200,000 to 300,000 hours at room temperature. Elevated
temperature (80°C) MTBFs are conservatively reported
to be on the order of 100,000 hours. Field experience
by hundreds of customers representing more than 7,500,000
of our CP type modules and more than 800,000 OptoTEC™
type modules during the last ten years have resulted in
a failure return of less than 0.1%. More than 90% of all
modules returned were found to be failures resulting from
mechanical abuse or overheating on the part of the customer.
Thus, less than one failure per 10,000 modules used in
systems could be suspect of product defect. Therefore,
the combination of proper handling, and proper assembly
techniques will yield an extremely reliable system.
Historical failure analysis has generally shown the cause
of failure as one of two types:
Mechanical damage as a result of improper handling or
system assembly techniques.
Moisture:
Moisture must not penetrate
into the thermoelectric module area. The presence of
moisture will cause an electro-corrosion that will degrade
the thermoelectric material, conductors and solders.
Moisture can also provide an electrical path to ground
causing an electrical short or hot side to cold side
thermal short. A proper sealing method or dry atmosphere
can eliminate these problems.
Shock and Vibration:
Thermoelectric modules in
various types of assemblies have for years been used
in different Military/Aerospace applications. Thermoelectric
devices have been successfully subjected to shock and
vibration requirements for aircraft, ordinance, space
vehicles, shipboard use and most other such systems.
While a thermoelectric device is quite strong in both
tension and compression, it tends to be relatively weak
in shear. When in a severe shock or vibration environment,
care should be taken in the design of the assembly to
ensure "compressive loading" of thermoelectric
devices.
Mechanical Mounting:
A common failure mode
for thermoelectric modules is un-even compression forces
induced by improper torqing, bolting patterns, and mechanical
conditions of heat exchangers. The polycrystalline thermoelectric
material exhibits less strength perpendicular to the
length (growth axis) than the horizontal axis. Thus,
the thermoelectric elements are quite strong in compressive
strength and tend to be weak in the shear direction.
During assembly, un-even torquing or un-flat heat exchangers
can cause severe shear forces. Recommended compression
value is 150 PSI. (See assembly instructions for proper
mounting techniques.)
Inadvertent overheating
of the module.
The direct soldering process
does result in temperature restriction for operation
or storage of the modules.
At temperatures above 80°C two phenomena seriously
reduce useful life:
Above 80°C copper diffusion
into the thermoelements occurs due to increasing solid
solubility in the thermoelectric material and increasing
diffusion rate. At 100 - 110°C the combined solubility
and diffusion rate could result in approximately 25%
loss of device performance within 100 hours.
Above 85°C in the soldering
process (using Bismuth-Tin Alloy) small amounts of selenium,
tellurium, antimony and nickel are inherently dissolved
into the bismuth-tin solder. Although the melting point
of the base solder is 136°C, the combined mixture
of all elements results in either a minute eutectic
phase or a highly effective solid state reaction occurring
at above 85°C that starts to delaminate the ends
of the thermoelements by physical penetration between
cleavage planes in the thermoelectric material. This
results in a mechanical failure of the interface.
