Highly Accelerated Life Testing – Testing With a Different Purpose
Neill Doertenbach
QualMark Corporation

Biography
Mr. Doertenbach holds an Electrical Engineering degreefrom CSU in Fort Collins, Colorado. He has a wide
range of industry experience, including digital hardware
design, software and Quality Assurance, and is the OVS
Technical Sales Manager of QualMark Corporation.

Abstract
This paper describes the technique of HALT – Highly
Accelerated Life Testing – and the advantages gainedby using the technique. HASS – Highly Accelerated
Stress Screening –is also introduced and described. The
paper begins with a discussion of the HALT philosophy
and how it differs from traditional Design Verification
Testing (DVT). The advantages of the technique arehighlighted. The process of HALT is described indetail, with emphasis on contrasting HALT with DVT
and the logic behind the differences. The discussion of
the technique will include preparing for the test,
fixturing, the sequence of the applied stresses and thepost-test activities. HASS is introduced, including thedevelopment of a screen, proof of screen and fixture
mapping.
Keywords: HALT, HASS, Accelerated Life Testing,Accelerated Stress Screening, DVT, ESS, Proof of
Screen.
Overview and Definitions
In recent years, the test techniques known as HALT(Highly Accelerated Life Testing) and HASS (Highly
Accelerated Stress Screening) have been gaining
advocates and practitioners. These test methods, quite
different from standard life testing, design verification
testing and end-of-production testing, are becoming
recognized as powerful tools for improving productreliability, reducing warranty costs and increasing
customer satisfaction. This paper provides a basic
description of these techniques, highlights thedifferences between these techniques and moreconventional testing and provides a guideline for theirimplementation.

© IEST, 2000 proceedings, February, 2000.

HALT is a test that is performed on a product as part ofthe design process. Typically it is performed on a
product when pre-pilot or pilot run units are available,
before the design verification testing begins. During
HALT, a product is stressed far beyond its
specifications as well as far beyond what the productwill encounter in a typical use environment. The actual
functional and destruct limits of the product are foundand pushed out as far as possible. These limits are usedas the basis for the implementation of HASS during the
production of the product. HASS is a production screen
test, performed on products built as part of the
production process. Since HALT is required for the
implementation of HASS, HALT will be discussed first.
HALT vs. DVT - the difference is the purpose
When first exposed to the concept of HALT, many
design engineers are skeptical of the method. Much ofthis skepticism stems from the fact that these engineers
are used to doing standard life testing and design
verification testing, and the HALT methods differ sodramatically from these conventional methods that they
seem to be almost at odds with them. The key tounderstanding the value of HALT lies in understanding
the basic difference in the purpose of the testing being
done. The basic purpose of Design Verification Testing(DVT) is well understood - it is to demonstrate that the
product meets its specifications, and to demonstrate thatthe product will function in its intended environment.
DVT is considered successful when all the tests are
passed, with no failures detected.
The purpose of HALT is dramatically different. In
HALT, the goal is to over-stress the product and to very
quickly induce failures in the product. By applying
these stresses in a controlled, stepped fashion, while
continuously monitoring the product for failures, thetesting results in the exposure of the weakest points inthe design. At the completion of HALT, the functionaland destruct limits of the product are known, and a“laundry list” of design and process limitations aredefined, with corrective actions often defined as well.
In short, the goal of HALT is to quickly break the
product and learn from the failure modes the productexhibits. The key value of the testing lies in the failuremodes that are uncovered and the speed with which they
are uncovered. HALT is considered a success when

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failures are induced, the failure modes are understood,
corrective action has been taken, and the limits of the
product are clearly defined and pushed out as far as
possible. Unlike DVT, HALT is not a pass/fail test. It
is a process of discovery and design optimization.
Although these failure modes are induced by stresses inexcess of specification, they are typically valid failuremodes that would show up in the product in the field. A
full failure analysis of all modes found will help
confirm this. The important thing to remember is that
HALT is finding the weakest parts of the design. These
weak links will be the source of warranty problems in
the field. The controlled over-stresses applied during
the HALT process simply accelerated the precipitationof these failures to allow early detection and correction.
The advantage of HALT is that it quickly finds failuremodes that would not be brought out in DVT. A typical
HALT will take only 3 to 5 days.
Because the purpose of the tests is so clearly different,
HALT is not intended to replace DVT. It is true that
HALT will find most, if not all, of the failure modes
that would show up in DVT (along with many more).
However, HALT will not provide you with the
documented evidence that you often need to prove thatyour product meets specification. By doing HALT
before DVT is started, you help insure that your DVT
will be completed in one pass, with no defects found.
This will greatly speed your time to market, avoiding
the slow process of repeating DVT until no more
failures are precipitated and detected.
Choosing HALT stresses and equipment
The basic concept of HALT can be implemented using
many different stresses. However, the stresses mostoften used are thermal extremes, extreme thermal rates
of change, vibration and the combination of thermal andvibration. Other stresses, such as voltage margining,frequency margining, power supply loading and power
cycling can also be applied, resulting in additional valid
failure modes being exposed.
It is worth remembering that HALT is not intended todemonstrate that a product will function in its intended
environment. Consequently, the stresses do not attemptin any way to duplicate those expected in “real life”.
Rather, the stresses are specifically designed to quickly
bring out failure modes. This logic affects the choice ofchamber used to apply the stresses as well as the type ofvibration fixturing used and the routing of the air flow
through the product. Given that extreme stresses are tobe applied, the chamber must be capable of reachingboth hot and cold thermal extremes, executing very fast

© IEST, 2000 proceedings, February, 2000.

thermal ramps and providing high vibrational energythat will quickly bring out failure modes. This, ofcourse, precludes the use of mechanical refrigeration
systems.
The vibration system that has been proven to be themost effective for HALT is a Repetitive Shock (RS)system with a wide frequency and acceleration range
and 6 degree-of-freedom vibration. In order to rapidly
and effectively bring out failure modes it is important toexcite the product at the resonant frequency of all
assemblies, sub-assemblies, components and leads andlegs of components in the product, regardless of whatthat resonant frequency, or the orientation of theassembly or component may be. An RS shaker,
designed to provide energy from 2 Hz to 10,000 Hz will
do this most effectively.
Preparing for HALT and planning the test
Once the purpose of HALT is understood and accepted,
the process and stresses used during the testing begin to
make more sense. Because the stresses applied areincreased until failure occurs, it is not necessary to test alarge population of product to insure that a failure mode
will be found. A relatively small sample - typically 4 to
6 units - is adequate.
This number will allow
verification of a failure mode in more than one unit aswell as providing for a spare or two in the event of acatastrophic failure of a unit under test. In order topreserve these samples and get as much information as
possible from them, the stresses are applied startingwith the least destructive and going to the mostdestructive. For the thermal and vibration stresses, thismeans starting with cold step stressing, then hot, thenrapid thermal extremes, then vibration, followed by afinal combined thermal/vibration environment.
If the product being tested is more complex than simply
a single board or small system, then one of the firstquestions to consider is what level of the product to test.
In general, the goal of HALT is most effectively met bytesting at the lowest possible subassembly. Card cages
or other assemblies can dampen vibration and block air
flow, reducing the stresses applied to subassemblies
inside them. Of course, the trade-offs of functionality
and testability must be considered. Also, there will beinterconnect circuitry and connections that may not be
tested at the subassembly level. An ideal HALT on acomplex product would include HALT on all
subassemblies, with a final HALT on the upper level
assembly as well.
The functional test equipment used during HALT isextremely important. Since the value of HALT is the

Page 2detection of failure modes induced, it is critical to beable to detect the failures when they happen. Thismeans that the units under test must undergo complete
diagnostics while they are being stressed. Much
valuable information will be lost if the product isstressed without being monitored, then removed fromthe stress and tested at ambient. By testing understress, you will be able to detect “soft” failures that only
show up under a particular stress or combination ofstresses. These soft failures define the operating limit ofyour product, and can be the source of troublesome “nodefect found” failures when the product reaches thefield.
The vibration fixturing used in HALT is very different
from that used when testing with typical ElectroDynamic (ED) or hydraulic shakers. In HALT, thefixture is not designed to mimic the real-life mountingof the product. Instead, it is designed to maximize thetransmission of energy into the product to speed theprecipitation of failures.
This results in simple,
inexpensive fixturing with the goal of simply clamping
the product to the vibration table as tightly as possible.
Figure 1 shows a typical product fixtured in a HALT
chamber. To maximize air flow through the product aswell as to improve the transmission of the low
frequency energy, the product is set up on aluminum uchannel rather than being placed directly on the table
top. The u-channel across the top of the product and the
all-thread rod and nuts clamp the product to the table.

Air flow through the product is also planned with the
HALT goal in mind. Using flexible air ducts, the air
flow is routed to maximize the temperature rate ofchange on the thermally sensitive parts of the productand to insure that all parts of the product experiencemaximum temperature extremes. The normal air flow

© IEST, 2000 proceedings, February, 2000.

through the product during use is not considered when
the ducting is designed. If necessary, holes should be
cut in the product’s case to allow sufficient air flow
across its components.
To aid in failure analysis and to insure that the stresses
are being coupled into the product effectively, it isimportant to instrument the product under test.
Thermocouples should be placed at key points on the
product, and accelerometers can be placed on boards
and subassemblies to evaluate the transmission ofenergy into the product.
However, the actual
accelerometer placement should be delayed until after
the thermal portion of the stressing is complete, since
the accelerometers would be exposed to stress levels
that may shorten their life.
A final, important part of the HALT setup is to clearly
define what parameters in the product will bemonitored, and what constitutes a failure. This fairly
obvious step in the test process can be easily missed,
making the interpretation of HALT findings moredifficult.
Margin Discovery – the core of HALTWith the test set up, the process of Margin Discovery
can begin. As mentioned above, HALT will uncover
the operational and destruct limits of your product.
During testing, the stress is steadily increased in a
stepwise fashion, with a complete functional test done ateach step. The operational limit is defined as the stressnecessary to cause a product to malfunction, but the
product returns to normal operation when the stress isremoved. Essentially, it is the point of “soft” failure.
The destruct limit, as you may guess, is the level ofstress necessary to cause a permanent, or “hard” failure
to occur. The difference between these limits and youroperating specifications is your margin for thatparticular stress. As the failure modes are found andeliminated that are responsible for these limits, you push
the limits further and further out, maximizing yourmargins and increasing your product’s life andreliability.
Figure 2 graphically represents these limits. The stressapplied is shown in the X axis, with number of failures
shown in the Y axis. The curve drawn around each of
the limits represents the distribution of the failure that is
responsible for that particular limit. The operating
specifications and margins are also shown.
This figure can be helpful in gaining an intuitive
understanding of the value of HALT. Consider a failuremode – say, a high ripple on the output of a power

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supply – that causes a unit under test to fail. If you wereable to test hundreds of units, you could see andunderstand the distribution on that failure mode, assketched on the graph. However, you do not typicallyhave that luxury. By increasing the stress until the

failure is seen, then it doesn’t matter where in that
distribution the unit under test falls – the failure mode
will be detected. If the tail of that distribution happens
to fall in the operating specifications, then the failuremode would have been an out-of-box failure mode onsome fraction of your products. By doing HALT andstressing to failure, you will find the failure mode
without having to hope that your sample size is big
enough to exhibit the failure within operating
specifications.
But, what if the tails of the distribution are well outsideof the product specification, as shown on the graph? Is
the high ripple a failure mode that can be ignored?
Consider for a moment what happens to this distribution
and limits as your product ages in the field.
Components fatigue and begin to drift out ofspecification, power cycles and lightning strikes stressthe product, and these limits begin to creep in. If you
have chosen to ignore the failure, then you will find that
it is one of the first failures to begin showing up in
warranty issues. By pushing the stress until the failure
occurs, you have effectively accelerated time,
precipitating a failure mode in just a few days that could
have taken months to come up in the field.
As illustrated in the above example, a failure mode
found beyond the operating limits of the product can,indeed, be a “valid” failure mode that could cause
warranty problems in the future. However, it is also
clear that you may find a failure mode that is completely
due to the extreme stress applied, and would neveroccur in the field. Consider a failure mode precipitatedby the softening of a plastic boss at high temperature. A
brief failure analysis will reveal that the distribution on© IEST, 2000 proceedings, February, 2000.

this failure mode is clearly understood, will never havea tail that is in the product specification, and will not
shift with time and fatigue. Consequently, this failuremode can be safely ignored. Of course, the distribution
on most failure modes is not that easily understood.
This is one reason why a complete failure analysis isalways necessary on HALT failures. In general, it isunusual when a HALT failure can be safely ignored. It
is important to resist the urge to ignore a failure mode
simply because it happened outside of the specification
for the product.
As you test to higher and higher extremes of stress,
pushing limits further and further, an obvious question
comes up – When do I stop testing? The stopping point
will be either the limit of the test equipment, or thefundamental limit of the technology.1 This fundamental
limit is the point where multiple failures begin to occur
with small increases in stress. Failure analysis reveals
fundamental and catastrophic failures across several
devices, with corrective action being prohibitive or
impossible. In vibration testing, multiple components
are coming off the board.
With this understanding of the margin discovery
process, the process of margin discovery can begin. As
described earlier, stresses are applied starting with theleast destructive and progressing to the most destructive.
This helps conserve samples. Cold step is done first.
Cold step testing begins at ambient temperature The
temperature is dropped in 5 Co steps. At each step thetemperature is allowed to stabilize for 10 minutes. This
dwell helps insure that the entire product is stabilized at
this temperature, and makes the testing more repeatable.
At the end of 10 minutes, a full functional test of the
product is done. If the product passes, the temperatureis dropped again, and the process repeated. When afailure occurs, the testing is stopped and an
investigation into the failure is done. Often, once thefailure mode is defined, it is possible to “work around”
the failure with a quick patch and continue testing,saving the intensive failure evaluation for later. As
described above, this step process is continued until you
reach the limits of your test equipment or until you
reach the fundamental limit of the technology.
After the cold step is completed, hot step testing is donein a similar manner. Again, testing is started at ambient,then increased in 5 Co steps . The dwell and functioal
testing are identical to those done in cold step testing.
The third stress applied in HALT is rapid thermal
extremes. Now, the product is functionally tested
continuously while the product temperature is changed
as rapidly as allowed by the chamber. The upper andPage 4lower limits of these ramps are determined by the
results of the step stressing, and stay within theoperating limits found there (there is no point inrepeating failures that were found earlier). If the
product cannot tolerate these maximum thermal ramps,
then the ramp rate is decreased, and then increased in a
stepwise fashion, similar to the thermal step stressing.
When failures are encountered, they are addressed in asimilar fashion as before.
With the thermal only portion of the testing completed,the product is now exposed to vibration.
Withaccelerometers applied to the product to verify adequate
energy transmission to the product, vibration testing is
begun at a stress level of 3 to 5 GRMS. Just like in thethermal phase, there is a 10 minute dwell, then acomplete functional test of the product is executed.
Again, the stress is stepped up, in 3 to 5 GRMS
increments, until the chamber limit is reached or you
begin to see the catastrophic failures indicative of thefundamental limit of the technology.
The final environment is combined thermal andvibration. Now, the temperature is ramped as it was
during the “rapid thermal extremes” portion of thetesting, while the vibration is stepped up as it was
during the vibration only portion.
It is important to remember that the HALT will be mademore effective if additional stresses can be incorporated.
By combining more and more stresses, you will bringout failure modes that may occur in the field only undera unique stress situation. This can eliminate a failuremode that could cause a lot of headaches if you wereforced to look for it using traditional methods, after the
product was released.
At the completion of the step stress testing, you will
have found many valuable failure modes for yourproduct. You will have a clear understanding of themargins in your product. You will know not only what
your limits are, but WHY they are where they are,giving you a unique understanding of the weaknesses inyour product. After doing a root cause failure analysis
on all failures found and implementing corrective
action, you can do a verification HALT to test yourfixes and make sure you have not introduced any new“weak links” in the design with your changes. In theend, you will have optimized the design of your productso that it will last as long as possible in the field.

anyone who has seen a product into production knows,
the production process can introduce many failuremodes that are not related to a faulty design, and thesustaining process can certainly introduce new design
problems. HASS is intended to catch these new failuremodes more quickly and more effectively than burn-in
or other ESS testing done in production.
Once again, an understanding of the purpose of the testis helpful. Burn-in is designed to weed out infantmortality in a product, aging it to induce early life
failures before the product ships. HASS has a broader
purpose. The goal in HASS is to verify that no new“weak link” has crept into the product since HALT thathas shifted either the operational or destruct limits foundin HALT.
An important first step to setting up HASS is thecompletion of HALT on the product. The HASS limits
will be set based on the operational and destruct limits
found in HALT. Prior to setting up HASS, it isimportant that corrective action has been implemented
on all HALT failures and a verification HALT has beendone.
The HASS process and equipment
The equipment used to do HASS is similar to that used
in HALT, although often a larger chamber is used toaccommodate production quantities. The fixturing canbe quite different in HASS, simply to accommodate theproduction flow. The speed with which product can be
fixtured in the chamber becomes important, as well as
maximizing the number of products in the chamber.
Quick release clamps are often used in lieu of nuts andbolts for securing the product.

HASS – maintaining optimization

An important part of designing a fixture for HASS is the
mapping of the fixture. The goal is to insure that thevibration and thermal stresses at each point in the
fixture are roughly equal (although precise uniformity is
not important). Mapping the fixture involves taking
accelerometer and thermocouple readings on a productin each of the fixture locations. It is important the
fixture is completely loaded with product for the test,
since the load will affect the vibration characteristics.
Thermal inconsistencies can be corrected by changing
air flow through baffling or other air distributionchanges. Vibrational inconsistencies can be corrected
through fixturing changes, with the introduction ofdampening materials or changes in clamping
mechanisms.

After your design is ruggedized through HALT and you
have completed DVT, you will begin production. As

During HASS, the stresses are applied simultaneously.
Typically, the product is subjected to continuous© IEST, 2000 proceedings, February, 2000.

Page 5

vibration while the temperature is ramped between its
limits, with short dwells at the extremes.

distribution looks like on these limits or where the tails
may be. Consequently, a more empirical method is

Defining the screen
The levels of the stresses to be applied during the screen
are based on the limits found during HALT. There are
two parts to the screen.3
The first part is thePrecipitation screen. This screen stresses the productbeyond the operational limits and near the destructlimits found in HALT. It is intended to precipitate
failures in the product due to latent defects. Because theproduct is being stressed beyond its operational limit,
you do not expect it to function properly, so no testing isdone on the product at this point. The product should bepowered, however, since applied power can be asignificant stress for the product in itself whencombined with the other stresses of HASS. The second
part of the screen is the Detection screen. During the
Detection screen the product is stressed to near theoperational limit found in HALT. Now, the product isbeing functionally tested. Any hard failures induced
during the Precipitation screen will be detected, as wellas any soft failures that may be induced by the stresses.
Figure 3 can provide an overview of the purpose andused. A baseline for the stresses is derived byguardbanding the limits found in HALT. Typically,
vibration is reduced by 50% and thermal excursions are
reduced by 20%.1,2 These limits can be used as astarting point for the Proof of Screen process.
Proof of Screen (PoS) is a critical part of HASS
implementation. The goal of PoS is to demonstrate thatthe screen will reliably find defects without inducing
failures or significantly reducing the life of the product.
The process of PoS is fairly straightforward. A sample
of product – typically a full chamber load – is run
through the proposed HASS multiple times. The
sample includes some seeded failures – perhaps some“no defect found” failures from field trials. The final
configuration of the screen will depend on two factors –the number of cycles through the screen necessary to
precipitate the seeded failures, and the number of cycles
good product is able to tolerate before exhibiting endof-life failures.
Figure 5 demonstrates the logic behind PoS.

limits of these screens. It shows the margin discovery
curves, overlaid with the Precipitation and Detection
screens. The limits on the screens are set so that they
are outside of the tails of the distribution of the failuremode(s) that define the operational and destruct limits
for the product. Consequently, product which has no
new latent failure modes should pass the screen
undamaged. Any new failure mode, however, will be
exposed. Figure 4 illustrates a typical thermal profile
for a HASS screen.
There is one key problem with setting up the limits on
the screens from this data – the small sample size used
in HALT means that you really have no idea what the© IEST, 2000 proceedings, February, 2000.

Page 6

Ideally,one or two cycles through the screen will precipitate all
the seeded failures. This will yield a short, efficient
screen, typically lasting less than 2 hours. As Figure 5
shows, if seeded failures are not precipitated until
several passes through the screen, then the severity ofthe screen should be increased. This part of the PoS
verifies that the screen will reliably find defects.
Multiple repetitions of the screen will demonstrate thatthe screen is not taking an unacceptable amount of life
out of the product. Ideally, good product will tolerate
20 to 50 passes through the screen without exhibiting
failures. If end-of-life failures are seen before 20 ormore cycles are complete, the screen may need to be
reduced in severity. A rough estimation can be made of
the amount of life being removed from the product by
the screen by simply comparing the number of cycles in
the proposed production screen to the number of cycles
necessary to cause end of life failures to occur. For
example, if your production screen consists of 2 passes
through the precipitation and detection screens, andyour proof of screen showed that 20 cycles through thescreen induced no end-of-life failures, then your screen
is removing less that 2/20, or 10%, of the useful life ofyour product.

Summary
A clear understanding of the unique goals of HALT and
HASS provides the basis necessary for introducing thetechniques into an R&D and production process. This
understanding will also enable you to intelligently makechanges in the process. If carefully executed, the end
result will be increased product life and reliability,
reduced warranty expenses, faster time to market anddelighted customers.
1

Hopf, A.M., “Highly Accelerated Life Testing for
Design and Process Improvement”, Sound andVibration, November, 1993, pp. 20-24
2McLean, H., “Exceeding the Limits of Traditional
Reliability Tests”, Medical Device & Diagnostic
Industry, April, 1994
3
“HALT and HASS, The New Quality and Reliability
Paradigm”, G.K.Hobbs, 1996. Published by Hobbs
Engineering Corporation, Available Upon Request

The stress levels can be adjusted, or the vibration duty
cycle can be changed, to achieve the proper balance
between the number of cycles necessary to bring out
defects versus the amount of life being taken out of the
product. If stresses are increased as a result of the PoS,
the PoS must be repeated on new, unstressed samples.
In reality, in can often be difficult to seed failures
sufficiently to accurately verify that the screen will finddefective units. Consequently, it is typically necessary
to make a conservative estimate of the number of passes
through the screen that are necessary, then tune thescreen after a reasonable population of product has been
through it. If you find that all of your failures are being
precipitated in the first one or two passes through thescreen, then no more than two passes should benecessary. Conversely, if you are running 3 passes
through the screen and are seeing equal failures in eachpass, you should either make the screen more aggressive
or increase the number of passes through the screen.
Once your HASS process is defined and proven, it is notnecessarily “set in stone”. Product changes can bring
acceptable changes in the limits, if they are understood.
However, it is always important to base your decisions
on a complete failure analysis and a thoroughunderstanding of the impact of the change. Rememberthat a verification HALT is a useful tool when
considering these changes.

© IEST, 2000 proceedings, February, 2000.

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