|
Reliability Testing
Reliability testing is the cornerstone of a
reliability engineering program. It provides the most detailed form of
reliability data because the conditions under which the data are collected can be carefully
controlled and monitored. Furthermore, reliability tests can be designed to
uncover particular suspected failure modes and other problems. The type of
reliability testing a product undergoes will change along different points of
its life cycle, but the overriding goal is to insure that data from all or most
of the tests were generated under similar enough conditions so that an
"apples to apples" comparison can be made of the product's reliability
characteristics at different points in the product's life. It is for this
reason that consistent and thorough reliability specifications and a standard
definition of failure are up-front requirements to implementing reliability
testing.
A properly designed series of tests,
particularly during the product's earlier design stages, can generate data
that would be useful in the implementation of a reliability growth tracking
program. This will provide information that will be helpful in making management
decisions regarding scheduling, development cost projections and so forth.
This information will also be useful in planning the development cycle of
future products.
There are several different kinds of tests,
including: Customer Usage Profiling,
Development Testing and
Manufacturing Testing.
Customer Usage Profiling
An important
requirement for designing useful reliability tests is to have a good idea of
how the product is actually going to be used in the field. The tests should
be based on a realistic expectation of the customer usage, rather than
estimates or "gut feelings" about the way the customer will use the product.
Tests based on mere speculation may result in a product that has not been
rigorously tested and consequently may run into operational difficulties due
to use stress levels being higher than anticipated. On the other hand, tests
that are designed with a strong basis of information on how the product will
be used will be more realistic and result in an optimized design that will
exhibit fewer failures in the field.
Customer usage profiles can be designed to
actively gather information on how the customers are actually using an
organization's product. This design can range from a simple questionnaire to
sophisticated instrumentation within the product that feeds back detailed
information about its operation. An incentive is often useful to get
customers to sign on for a usage measurement program, particularly if it is
an intrusive process that involves the installation of data collection
equipment. Additionally, customers are often eager to participate in these
programs in the knowledge that the information that they provide will
ultimately result in a more reliable and user-friendly product.
Developmental Testing
Developmental testing occurs during the early phases of the product's life
cycle, usually from project inception to product design release. It is vital
to be able to characterize the reliability of the product as it progresses
through its initial design stages so that the reliability specifications
will be met by the time the product is ready for release. With a multitude
of design stages and changes that could affect the product's reliability, it
is necessary to closely monitor how the product's reliability grows and
changes as the product design matures. There are a number of different test
types that can be run during this phase of a product's life cycle to provide
useful reliability information:
- Component-level
Testing: Although component-level testing can continue
throughout the development phase of a product, it is most likely to occur
very early in the process. This may be due to the unavailability of parts
in the early stages of the development program. There may also be special
interest in the performance of a specific component if it has been
radically redesigned or if there is a separate or individual reliability
specification for that component. In many cases, component-level testing
is undertaken to begin characterizing a product's reliability even though
full system-level test units are unavailable or prohibitively expensive.
However, system-level reliability characterization can be achieved through
component-level testing. This is possible if sufficient understanding
exists to characterize the interaction of the components. If this is the
case, the system-level reliability can be modeled based on the
configuration of components and the result of component reliability
testing.
- System-level
Testing: Although the results of component-level tests can be
used to characterize the reliability of the entire system, the ideal
approach is to test the entire system, particularly if that is how the
reliability is specified. That is, if the technical specifications call
out a reliability goal for a specific system or configuration of
components, that entire system or configuration should be tested to
compare the actual performance with the stated goal. Although early
system-level test units may be difficult to obtain, it is advisable to
perform reliability tests at the system level as early in the development
process as possible. At the very least, comprehensive system-level testing
should be performed immediately prior to the product's release for
manufacturing in order to verify design reliability. During such
system-level reliability testing, the units under test should be from a
homogeneous population and should be devoted solely to the specific
reliability test. The results of the reliability test could be skewed or
confounded by "piggybacking" other tests along with it and this practice
should be avoided. A properly conducted system-level reliability test will
be able to provide valuable engineering information above and beyond the
raw reliability data.
- Environmental
and Accelerated Testing: It may be necessary in some cases to
institute a series of tests in which the system is tested at extreme
environmental conditions or with other stress factors accelerated above
the normal levels of use. It may be that the product would not normally
fail within the time constraints of the test and, in order to get
meaningful data within a reasonable time, the stress factors must be
accelerated. In other cases, it may be necessary to simulate different
operating environments based on where the product will be sold or
operated. Regardless of the cause, tests like these should be designed,
implemented and analyzed with care. Depending on the nature of the
accelerating stress factors, it is easy to draw incorrect conclusions from
the results of these tests. A good understanding of the proper
accelerating stresses and the design limits of the product are necessary
to be able to implement a meaningful accelerated reliability test. For
example, one would not want to design an accelerated test that would
overstress the product and introduce failure modes that would not normally
be encountered in the field. Given that there have been a lot of
incredible claims about the capability of accelerated testing and the
improbably high acceleration factors that can supposedly be produced, care
needs to be taken when setting up this type of reliability testing
program.
- Shipping Tests:
Although shipping tests do not necessarily qualify as reliability tests
per se, shipping tests or simulations designed to test the impact on the
product of shipping and handling should be a part of the reliability
testing program. This is because the effects of shipping will often have an impact
on the reliability of the product as experienced by the customer. As such,
it may be useful to incorporate shipping tests alongside the normal
reliability testing. For example, it may be a good idea to put the units
of a final design release reliability test through a non-destructive
shipping test prior to the actual reliability testing in order to better
simulate actual use conditions.
Manufacturing Testing
The testing that
takes place after a product design has been released for production
generally tends to measure the manufacturing process rather than the
product, under the assumption that the released product design is final and
good. However, this is not necessarily the case, as post-release design
changes or feature additions are not uncommon. It is still possible to
obtain useful reliability information from manufacturing testing without
diluting any of the process-oriented information that these tests are
designed to produce.
- Functionality
Testing and Burn-In: This type of testing usually falls under
the category of operation verification. In these tests, a large
proportion, if not all, of the products coming off of the assembly line
are put on a very short test in order to verify that they are functioning.
In some situations, they may be run for a predetermined "burn-in" time in
order to weed out those units that would have early infantile failures in
the field. Although it may not be possible to collect detailed reliability
information from this type of testing, what is lost in quality is made up
for in quantity. With the proper structuring, these tests can provide a
fairly good picture of early-life reliability behavior of the product.
- Extended
Post-Production Testing: This type of testing usually gets
implemented near the end or shortly after the product design is released
to production. It is useful to structure these types of tests to be
identical to the final reliability verification tests conducted at the end
of the design phase. The purpose of these tests is to assess the effects
of the production process on the reliability of the product. In many
cases, the test units that undergo reliability testing prior to the onset
of actual production are hand-built or carefully adjusted prior to the
beginning of the reliability tests. By replicating these tests with actual
production units, potential problems in the manufacturing process can be
identified before many units are shipped.
- Design/Process
Change Verification: This type of testing is similar to the
extended post-production testing in that it should closely emulate the
reliability verification testing that takes place at the end of the design
phase. This type of testing should occur at regular intervals during
production or immediately following a post-release design change or a
change in the manufacturing process. These changes can have a potentially
large effect on the reliability of the product and these tests should be
adequate, in terms of duration and sample size, to detect such changes.
|
