In a message dated 8/2/99 9:42:04 AM US Eastern Standard Time,
[log in to unmask] writes:
<< Could anyone enlighten me on the difference between Thermal shock and
Temperature cycling test. I know the former is more severe but what is each
test looking at?
>>
David,
In a former life, I dealt with validating proto/pilot automotive products.
What I often found was that our customers and even our own design folks did
not have a clue as to 1). what fail mode is a particular reliability test
supposed to create, 2). what test parameters best simulates the real-life
conditions in the field for my product, and 3). whether or not a given test
is applicable to my product. There is even disagreement amongst reliability
gurus as to what certain tests are designed to yield in terms of product
reliability. More times than not we found our customers creating a
reliabilty test specification that is simply a cut and paste from past
documents. "It sounds good. The more testing and the longer the test the
better." Often times I found myself in a position trying to pursuade a
customer that a certain test or tests doesn't make sense for its product.
My whole point is that there is no "golden" test that apply to all products,
particularly when talking about temperature cycling. It is indeed a gray
area. Here is my experience with the thermal shock and temperature cycling
to which you eluded.
TEMPERATURE CYCLING
1). All test samples are typically powered and functionally exercised
(inputs and outputs) throughout the course of the test.
2). In recent years, industry has pushed TC to include continous functional
monitoring of test samples in order to better determine an exact time and
condition of failure.
3). Test samples are tested under specified loads (e.g., motors, coils,
lamps, etc.).
4). TC testing is an accelerated life test that uses a mathematically
derived acceleration factor. For automotive products, TC would simulate 10
years or 100,000 miles. Theoretically, the acceleration factor is design
dependent.
5). The test is designed to electrically stress the product in order to
flush out latent component defects and to promote solder fractures.
6). Full functional tests are typically performed at the product's minimum,
maximum, and ambient operating temperatures (e.g., -40C, +25C, and +85C)
before TC (to ensure good product enters the test) and afterwards. Prior to
continuous monitoring, we performed full functional tests were at convenient
points in the test (e.g., 168 hours, 504 hours, etc.).
7). Ramp rates are usually slow and vary quite a bit. The fastest I have
enountered is 10degC/min. Many TC chambers will find it difficult to achieve
anything faster.
THERMAL SHOCK
1). The test samples are not typically tested under power.
2). The test samples are exposed to two sometimes three dwell temperatures.
These temperature extremes are typcially identical to those of TC (e.g.,
-40C, +25C, and +85C).
3). Ramp rates are fast, usually 1 minute or less.
4). TS is designed to the stress mechanical integrity of the design.
5). I have never seen a thermal shock failure!
6). Test duration could range from 96 hour up to 1000 hours (a bit of an
overkill).
Hopefully this helps you and also stimulates an interesting discussion on the
topic.
Good luck!
David Morrow
Project Engineer
CLS, Inc.
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