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Air-to-air
thermal shock testing employs a very high rate of
temperature change, typically 30 degrees C per
minute or higher. To do this with a single
chamber would require either a very high
performance refrigeration system or a direct
coolant like liquid nitrogen (LN2), both of which
are very expensive to purchase and maintain. To
avoid these problems, typically a two or three
chamber system is used. In the case of the
two-chamber design, one chamber is hot, the other
is cold, and they are stacked on top of each other
(can also be side by side). Three chamber systems
can have a hot, room temperature, and cold
arrangement, or a hot, cold, hot design permitting
two different tests to be run simultaneously
provided they share the same cold temperature.
For all cases, a carriage is used to move the
product under test between the boxes. This
transition is typically done in as little time as
5 seconds. Of note is that thermal shock is
designed to "relieve" stress inherent in
incorrectly designed or manufactured components as
opposed to the fatiguing mechanisms at work in
thermal cycling stress.
Liquid-to-liquid systems employ a two-vat system
and a mechanized basket arrangement to move
product between the hot and cold sides of the
equipment. The baskets are typically small,
perhaps large enough to hold a number of small
printed circuit boards. Other than the difference
in size and thermal medium, the process works
basically the same.
What are
the variables for these tests?
-
Hot box
temperature
-
Cold box
temperature
-
Soak time
in the hot box
-
Soak time
in the cold box
-
Transition time (for some equipment, this cannot
be modified). Usually specified as a maximum
time permitted to move the product from one box
to the other.
-
Number of
cycles (a cycle is defined as the combination of
one hot and one cold soak)
Soak times
range from as little as a few minutes to hours.
From the standpoint of optimizing the test, the
only purpose of the soak is to ensure that the
device under test has stabilized at the box temp.
Almost all of the stress occurs during the rapid
transition as opposed to being attributable to the
storage effects at the high or low temperature.
For this reason it is best to characterize the
device by instrumenting the component on the
device that has the highest thermal mass (that
which will take the longest time to thermally
stabilize). Record the times required after both
a hot and cold transition for stabilization noting
that the two may be somewhat different. Add a
small margin of a few minutes to account for
variation and use those times for your hot and
cold soaks. A significant amount of time can be
cut from your testing resulting in significant
savings to your test budget.
Number of
cycles depends on the device. It is best to try
and draw some analogies to the product's expected
field usage. If this is not clear, look at some
of the standard specs. We typically see cycles
varying from as little as 1 to as many as 200.
Test
Specification Template (Thermal Shock Testing)
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