What is HALT?
THE BASIC PRINCIPLES OF HALT
is performed prior to commencing volume production. It is used to
find weaknesses and product stress
limits through the techniques of thermal
cycling and omni-axial random vibration
stress. Weaknesses in the design and construction of a product
are stimulated by the application of progressively higher levels
of cyclic mechanical stress. In a relatively short (i.e. accelerated)
time, failures can be precipitated that would otherwise have occurred
in the field, probably during the warranty period.
HALT is not a compliance test nor is it an attempt
to simulate a worst-case operating environment. It is a process
that stimulates the UUT to the point of failure in order to precipitate
latent defects. Stresses are progressively applied in order to eventually
exceed the UUTs design limits.
HALT should be viewed as an additional tool in
the reliability engineers tool kit. It complements
existing reliability test methods and should not be viewed as universally
During HALT products must be stimulated and monitored
to determine when failures occur. When a failure is detected, root
cause analysis is performed to understand the failure mechanism.
The defective unit is repaired and strengthened, then returned for
further applications of progressively higher stress levels.
Failures precipitated by HALT will be associated
with one or more of the following categories:
- the design of the product
- the components from which it is made
- the quality of the assembly process
THE HALT PROCESS
There is no definitive, universally agreed method
of implementing HALT. However, most users will carry out most of
the following steps on one or more UUTs:
Steady state temperature step stress:
The UUT is tested at progressively lower temperatures,
until it ceases to function, thus determining a lower operational
The UUT is tested at progressively higher temperatures,
until it ceases to function, thus determining an upper operational
The UUT is subjected to progressively higher
levels of cyclic thermal stress by the application of increasing
numbers of thermal cycles at high and linear rates of change of
temperature. The temperature limits are determined from the levels
established during the previous steady state temperature stress
Multi-axis (6 DoF)
is mounted on a vibration
table using appropriate test fixturing. Starting at a relatively
low energy level the UUT
is subjected to multi-axis random vibration in steps of several
minutes duration. The vibration level is incremented at each
step until failures are stimulated.
Combined environment (thermal with vibration)
Depending on the UUT,
suitable combinations of vibration
and thermal stresses are used to further investigate product robustness.
After completing HALT and analysing the results,
suitable modifications may be introduced to the product design to
enhance its robustness, prior to commencing volume manufacturing.
These changes are comparable to the retrospective engineering changes
that are often required once a product has been released to market.
HALT results also provide the manufacturer with
a reference robustness benchmark for his specific product.
provides data from which a suitable production HASS
profile may be determined.
In order to be successful, HALT
must be properly managed and integrated into a new product development
program. Attempting to add HALT
at the last minute in the design validation process is unlikely
to be satisfactory.
How can I start to use HALT?
Thats where using Reliability Plus will
Start by thoroughly understanding the principles
of accelerated stress screening as applied to HALT. A short training
session can be arranged on your own premises.
Analyse the product(s) that you wish to evaluate
and develop a tailored strategy.
Design and manufacture the necessary (usually
straightforward) mechanical fixturing.
Determine how best to stimulate and monitor your
products during HALT. Make sure that you can have access to the
necessary equipment when you need it.
Ensure that a suitable number of product samples
will be available for HALT.
If you dont have your own HALT system,
arrange to use a properly equipped test house. Typically you will
initially need to budget for a week of test house time.
Take expert advice at every stage of the process.
Thats where Reliability Plus can make the difference. We offer
training, a HALT planning service and hands-on assistance for your
first exercise. Working with your own engineers we can make sure
that you get the maximum benefit from HALT.
HALT also provides the basic information you
will need to develop an effective production screening (or HASS)
process. Again, Reliability Plus will help you develop the right