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Qualmark HALT Testing Guidelines
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Qualmark
HASS/HASA
Guideline

The information contained in this paper is Qualmark proprietary information and is copyrighted. No portion
may be copied, modified, distributed or published without the express written permission of Qualmark
Corporation.

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Revision History
Rev

Originator

Description of Change

3/22/04

C. Drake (QualMark Corporation)

Initial release10/12/07

M. Feero (Qualmark Corporation)

Updated logo

N. Doertenbach (Qualmark Corporation)

Document number assigned.
Placed in Document Control.
Released: ECO #3119

00

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Table of Contents
Section Title

Page

1.

Purpose ................................................................................................ 4

2.

Scope.................................................................................................... 4

3.

Definitions ............................................................................................. 44.

HALT for HASS Development ............................................................... 6

5.

Equipment Requirements ..................................................................... 7

6.

Functional Testing and Troubleshooting Requirements ........................ 9

7.

Fixture Design and Qualification ......................................................... 10

8.

Profile Development ........................................................................... 13

9.

Profile Type ......................................................................................... 14

10.

Proof-of-Screen .................................................................................. 17

11.

HASS Profile Evolution ....................................................................... 19

12.

Failure Analysis in HASS .................................................................... 20

13.

Variations on HASS ............................................................................ 20

14.

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Qualmark HASS/HASA Guideline
1.

Purpose

1.1.

This instruction describes the method for performing HASS (Highly
Accelerated Stress Screen) Development. It is intended for the use ofcompanies as a guideline to perform the HASS Development process within
their facility with qualified personnel and fulfill the requirements as definedin this procedure. The result of adherence to this document is thatcompanies will achieve optimal HASS results and delivery of morerobust/rugged products to the marketplace.

1.2.

This instruction also describes the method for performing HASA(Highly Accelerated Stress Audit) Development. With the exception ofdefining the auditing process and levels, and the resultant changes inthroughput requirements, HASA development is nearly identical to HASS
development. Unless indicated otherwise, all statements in this instructionthat refer to HASS also apply to HASA.

2.

Scope

2.1.

The process elements to successfully implement and perform HASS
are defined in this document. Adherence to this document will provide
management and test personnel the fundamental guidelines to implement
and operate a successful HASS test process. This guideline applies to adiversified mix of product segments, including electronic assemblies,
electro-mechanical assemblies, and some mechanical assemblies as well.

3.

Definitions

3.1.

Destruct Limit (DL) – The stress level where one or more of the
product’s operating characteristics are no longer within specification. The
product does not recover when the stress is reduced (i.e. hard failure).


Upper Destruct Limit (UDL)
Lower Destruct Limit (LDL)th

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3.2.

HASA (Highly Accelerated Stress Audit) – An environmental stress test
that is used in production on a sample of the production units. It is often
used in high volume production environments.

3.3.

HASS (Highly Accelerated Stress Screen) - A production screen using
the same accelerated techniques as HALT, but derated. Its purpose is tomonitor the manufacturing process for deviations, by screening production
units.

3.4.

HASS Development - A process used to determine and define theappropriate HASS profile.

3.5.

HASS Profile - The temperature, vibration levels, SSR control, andnumber of cycles performed to provide an effective screen using the HASS
chamber.

3.6.

NTF, (No Trouble Found) - A product failure mechanism that has
occurred in use but cannot be reproduced on the bench when returned to
manufacturer.

3.7.

Operational Limit (OL) – The stress level prior to where one or more of
the product’s operating characteristics are no longer within specification.
The product recovers when the stress is reduced (i.e. soft failure).


Upper Operating Limit (UOL)
Lower Operating Limit (LOL)

3.8.

Parametrically Marginal unit – A unit that has been determined to be
marginal based on functional testing results.

3.9.

Product Specific Stresses – Stresses that are applied to the productduring the HASS screen in addition to the thermal and vibration stresses
provided by the HASS system. For example, applying variations to input
voltages or to load levels. These are used to introduce new stresses andstress combinations that can improve the effectiveness of the screen by
forcing more and different failure mechanisms to be detected.

3.10.

Production HASS Test - The process of screening manufactured
products using accelerated test techniques.

3.11.

Seeded Defects - Defects that are intentionally inserted into a test unit
to determine the effectiveness of the HASS screen.

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HASS Development Procedure
HASS Development includes the following processes:

4.



HALT to determine operating and destruct limitsEquipment and Functional Testing Requirements

Fixture Design Qualification



Fixture Integration Qualification

Profile DesignProof of ScreenScreen EffectivenessProduct Life Valuation
HALT for HASS Development

4.1.

It is very important that HALT be done on a product before HASS is
implemented. HALT provides the basic information on the product’s true
functional and destruct limits that is used to determine the starting point for
the HASS screen levels. Without this accurate starting point, screen
development cannot be safely or effectively executed. In addition, if a
product has latent design defects that are not detected and corrected in
HALT before starting HASS, fallout rates in HASS will be unacceptably highand field returns and warranty expenses will suffer.

4.2.

For the screen levels determined for HASS to be valid, they must be
measured in the same way as the HALT stresses were measured. Thismeans that it is particularly important to carefully document and duplicate
sensor placements and measurement methods used in HALT when
beginning HASS development. In all cases, the stress values used must be
the stress as measured on the product, and not the stress input levels.
Typically there are changes, sometimes significant ones, in the productfixturing, orientation or configuration that will change the effect of an input
stress. Measuring the actual stress experienced by the product at specific,thQualmark Corporation | 10390 East 48 Avenue | Denver, CO 80238 | phone 303-254-8800 | fax 303-254-8343

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repeatable locations helps ensure that the HASS screen will provide thedesired stress levels.
5.

Equipment Requirements

5.1.

HASS Chamber Requirements

5.1.1.

The equipment required to perform HASS testing must be capable ofproducing thermal and vibration energy stresses as defined below. In
addition, it must have the capability to create these stresses in a
combined environment; i.e. the chamber must have thermal and vibration
capability within one chamber.

5.1.2.

5.1.3.

5.2.

Vibration: Chamber technology for HASS is:

Repetitive shock vibration that is 6 degree of freedom (3 linear and 3
rotational) multi-axis and quasi-random

Broadband energy exists from 20 Hz up to 5,000 Hz



35 Grms minimum vibration table output with no load
Thermal: the goal is to force rapid thermal rates of change on the
product. It is additionally important that the chamber has sufficient air
velocity to produce the desired rapid thermal rates of change asmeasured on the product and to maintain thermal stability.
High rate of change (minimum air temperature average of
50ºC/minute)

Recommended thermal range from -60ºC to 120ºC
Test Equipment

5.2.1.

Product response data is acquired during the HASS Development
process. This data includes thermal, vibration and product functional
performance.

5.2.2.

The collection and storage of thermal data is required to provide
credible evidence that thermal stresses of the correct levels were applied
to the product. This may be achieved by utilizing available thermocouplethQualmark Corporation | 10390 East 48 Avenue | Denver, CO 80238 | phone 303-254-8800 | fax 303-254-8343

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monitoring channels of the HASS test system or the use of a dataacquisition instrument capable of multiple channel measurements.
5.2.3.

The collection and storage of vibration data is required to provide
credible evidence that vibration stresses of the correct levels wereapplied to the product. This may be achieved by utilizing available
accelerometer monitoring channels of the HASS test system or the useof a spectrum analyzer instrument capable of measuring sensors (e.g.
accelerometers) and displaying the data.

5.2.4.

Accelerometers for the measurement of product response fromvibration. These accelerometers should be low mass type (e.g. 4grams), with frequency response capability of 2 Hz to 10KHz, and ameasurement range of ± 500 g’s. The accelerometers should be smallenough to be mounted in the desired location and light enough that theirmass does not significantly impact or alter the vibration dynamic
characteristics of the sample tested.

5.2.5.

Thermocouples for the measurement of product thermal response.
Thermocouples used for HASS should have sufficient stability
characteristics through the temperature range of the chamber(approximately -60ºC to +120ºC). Lighter weight thermocouple wire (22gauge or less) should be used to minimize the effect of the thermocouple
thermal mass on the ramp rate readings.

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6.

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Functional Testing and Troubleshooting Requirements

6.1.

The goal of HASS is to force latent product defects to fail before theproduct is shipped. If those failures are not detected due to an inadequate
functional test, then the HASS effort is wasted and defective products willreach the customer. It is very important that the functional test be ascomplete and thorough as possible. Functional tests should be able todetect failures that might result from known possible process upsets, or thathave been seen already in the field.Because HASS is typically using
stresses that are outside of the product’s specifications or intended use
environments, the failures found can be indications of the tails of failure
distributions that have not yet reached a sufficient population or severity to
be seen in an ambient environment. Consequently, HASS failures serve asan ‘early warning system’ of a process or product issue that could become
more serious. To get the most value from this early warning, it is very
important that failures found in HASS are always taken seriously, with rapid
failure analysis and corrective action implementation. In HASA, this evenmore critical. Immediate, high urgency responses to failures in HASA are
always appropriate to minimize field exposure.

6.3.

Product specific stresses are stresses that are specific to the producttested. If these stresses were used in HALT, then they should also be
considered for use during the HASS profile to enhance additional failure
precipitation and detection. These stresses may include power cycling, line
voltage margining, line frequency margining, DC supply voltage margining,
output loading, and any others that are applicable. These product specificstresses are incorporated into the functional testing and should be
performed during the HASS profile.

6.4.

It is highly desirable that parametric data (gain, noise, common mode
rejection, etc.) be gathered on the product throughout HASS Development.
Preferably, this data should be gathered through an automated process,i.e., under software control. Parametric data will be valuable as productlimitations are encountered to assist in the determination of what theircause may be.

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7.

Fixture Design and Qualification

7.1.

Design and Construction

7.1.1.

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Proper HASS fixturing must meet several key requirements. These
include:

Applying equal stresses (within a specified variation) to each product

Applying these stresses consistently every time new product is loaded

Applying these stresses consistently over time as the fixture ages



Incorporation of cable management components

Meeting required human factors considerations

Maximizing capacity and minimizing load/unload times



Minimizing any limiting effect of the fixturing on the table vibration
characteristics



Maximizing air flow rate and uniformity of flow through the products

7.1.2.

A primary HASS Fixture design goal is meeting the desired uniformity
specifications for product-to-product stress variation. Typically,
temperature deviations of +/- 5°C or less at the end of a thermal dwelland gRMS vibration deviations of +/-15% or less are specified. These
are deviations from the median readings rather than deviations from
setpoint or system control values, and are measured on a single
reference product that is placed in each location in the fixture. Custom
air flow ducting may be needed. The best solution is often a modular
fixture design, where several, identical fixtures, each holding a smaller
quantity of product, are used simultaneously. In this case, the fixtures
should be mounted in locations in the vibration system that have similar
vibration characteristics. For example, on a square table, the vibration
tends to be symmetric in quadrants of the table, so four fixtures, one in
the center of each quadrant, are often used.

7.1.3.

Loading and unloading a fixture can affect its transmission
characteristics. The fixture must be designed so that these changes areminimized. If necessary, torque values can be specified for the hold-thQualmark Corporation | 10390 East 48 Avenue | Denver, CO 80238 | phone 303-254-8800 | fax 303-254-8343

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down hardware being used, but often it is difficult to use torque wrenches
in a manufacturing setting. Pneumatic fixturing that exerts the sameclamping force regardless of variations in product size and placement
can be a solution. Clamps can also work, although they often include
padded clamping surfaces that introduce variability and damping.
7.1.4.

The vibration tolerance of the fixture itself is important to consider indesign. If the fixture characteristics change due to fasteners coming
loose, deformation of clamping members, etc, then the screen
effectiveness will suffer. All connecting components used in the fixture
should be evaluated for vibration tolerance.

7.1.5.

When many products are tested simultaneously, the number of cables
going in and out of the system can become difficult to manage. Also, theintegrity of plug connections inside the chamber must be maintained, as
connectors tend to wiggle loose during vibration. Finally, minimizing any
wear and fretting on connector pins should be considered. A good fixture
design will minimize the relative motion between the mating components
and so reduce this potential for wear.

7.1.6.

Human Factors considerations for fixture design include lifting limits,bending and reaching limits, pinch and cut risks, etc.

7.1.7.

In any production environment, capacity and speed are important.
Minimizing cable connections, using simple methods to secure productinto the fixture and keeping all of these activities as ‘foolproof’ as
possible will all help meet these needs. It is often feasible to create
fixture modules that can be loaded with product outside of the chamber,thus increasing throughput further.

7.1.8.

The fixture can have a significant effect on the spectral characteristicsof the vibration as well as the air flow. These effects should be
minimized for the best screen results. Avoid heavy, large, rigid fixture
designs that will stiffen large areas of the table. Also, orient product sothat the air flows above and below the product without obstruction. Highthermal change rates can only be achieved if the product can see highair flow rates. This can be difficult when the chamber is full of product,since the fixturing and product in one quadrant will often disrupt the air
flow across the adjacent one. Staggering the product placement slightly,
or using custom air flow components can help meet these goals.

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7.1.9.

7.2.

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The product may have design characteristics that directly interfere withthe goals of HASS. For example, the chassis may be designed tominimize vibration transmission to the interior of the product. The
product may be air tight, making fast thermal change rates inside of the
product impossible. In these cases, consider doing HASS with covers
removed, or perhaps performing HASS at a lower assembly level. For
HASS to be successful, it is imperative that the assemblies andsubassemblies in the product are stressed sufficiently to meet the HASS
screen requirements.
Fixture Design and Integration Qualification

Two levels of qualification are recommended before implementing a HASS fixture– Design and Integration Qualification.


The Design Qualification is done first. Its purpose is to demonstrate thevibration and thermal uniformity and repeatability of the design.



The Integration Qualification is intended to demonstrate the fullfunctionality of the fixturing in the final production environment.

7.2.1.

Fixture Design Qualification

7.2.1.1. The Fixture Design Qualification does not need to be done using thefinal HASS system. If the fixture is being designed and built by a third
party, the Design Qualification is typically done by the third party, using
non-functional or dummy products.
7.2.1.2. It is important that the fixture be completely loaded with products (notnecessarily functional products) during Design Qualification.
7.2.1.3. The key output of the Design Qualification testing is the demonstrationof the uniformity of the vibration and thermal stresses across the
fixture. The best way to do this is to fully instrument a single reference
product with accelerometers and thermocouples in all key locations.
Then, mount this reference product into each of the locations in the
fixture, taking readings at each location. When the readings aretaken, the rest of the fixture should be filled with product, so that theeffect on air flow and vibration is accurately simulated. It is better tomove the reference product, rather than moving the sensors todifferent products, to reduce variables in the qualification. All
measurements should be taken at the same input stress levels.

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7.2.1.4. During the Design Qualification, it is often necessary to make changes
to the fixturing to reach the desired levels of uniformity.
7.2.2.

Integration Qualification

7.2.2.1. Integration Qualification is done in the actual production screening
environment. Its purpose is to confirm the fixture functionality when allof the additional influences of this final environment are included.
7.2.2.2. For Integration Qualification, the fixture is fully populated withfunctional units so that any cabling issues can be identified andresolved. The fixture should be loaded and unloaded by personnel
who will be using it. The functionality of all test equipment will also be
evaluated during this qualification.
7.2.2.3. A part of the Integration Qualification is the repetition of the uniformity
evaluation that was done during Design Qualification. The addition of
cabling, changes in the system used and even the changes in liquid
nitrogen delivery system can affect uniformity, so this second check for
uniformity is important.
7.2.2.4. At the end of the Integration Qualification, the fixturing should be readyfor production use.
7.2.3.

.Fixture Maintenance7.2.3.1. As with the HASS System, Preventative Maintenance for the fixture isimportant. A P.M. procedure should be defined that includes verifying
thermal and vibration transmission to the product, torque levels on all
fasteners, evaluating electrical connectors for wear, etc.
8.

Profile Development
The goal of the HASS profile is to provide sufficient stresses to force latent
defects to become detectable, without inducing new stress related failure modes.
This goal is met by starting with stress levels based on the HALT results, properly
guardbanded, then doing a Proof of Screen to evaluate the screen effectivenessand safety.

8.1.

To create the initial HASS profile, vibration and thermal limits arechosen based on the following guidelines:thQualmark Corporation | 10390 East 48 Avenue | Denver, CO 80238 | phone 303-254-8800 | fax 303-254-8343

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8.1.1.

Vibration setpoint levels should be established to obtain a productresponse equal to 50% of the DL product response, but within the OL
product response, found during HALT, as measured on the product. If
the 50% value exceeds the OL, then use approximately 80% of the OL
as the setpoint.

8.1.2.

The thermal levels are chosen to result in a thermal range that is 80%of the range between the upper and lower operating limits found in HALT.
This can be calculated as:
X = ((LOL - UOL)* .20) / 2
Where X equals the margin temperature subtracted from the upper
operating limit and added to the lower operating limit.
Remember to include any product specific stresses, such as power
cycling, input voltage variation, load variation, frequency variation, etc. as
appropriate.

9.

Profile Type
There are two basic types of HASS profiles. One is a Detection Profile, where
the stress levels are determined as described above, and are within thefunctional limits for the product. The product is powered and functionally tested
throughout the profile. The purpose of the profile is to immediately detect any
induced failures. The second is the Precipitation-Detection Profile. In this profile,the product is first stressed beyond its functional limits, without any functional
testing, then the stresses are reduced to levels within the functional limits of the
product, and the product is functionally tested. The purpose of this two step
profile is to first precipitate failures, using higher levels of stresses, then detect
the failures while stressing at the lower levels If a product has a significant delta
between the operating and destruct limits found in HALT (For example, greaterthan 30%) then the Precipitation-Detection profile will be more effective. If,
however, the thermal and destruct limits are close to each other, then the
Detection Profile alone should be used.

9.1.

The Detection Profile

9.1.1.

In the Detection Profile, the temperature is cycles between the upperand lower limits as identified above, while the vibration is applied. The
unit is powered and is functionally tested throughout the profile.

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9.1.2.

The vibration should be modulated throughout the Detection Profile,
beginning at 5-10 Grms and ramping to the maximum level followed by aminimum dwell time of 5 minutes. This process is then repeated inreverse, from the maximum level to the minimum level.

9.1.3.

The temperature cycle is set so that the ramps go as fast as possible.
The temperature dwells at the extremes should be at least 5 minutes,
longer if necessary to bring the entire product to the temperature
setpoint.

9.1.4.

The Thermal and Vibration profiles are set so that the product sees
maximum vibration during the ramps and during a portion of the dwell.

9.1.5.

It is important that a complete functional test be done during eachthermal dwell. If necessary, the length of the dwells should be adjusted
accordingly.

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Below is an example of a Detection HASS profile using 2 thermal cycles.

9.2.

The Precipitation-Detection Profile

9.2.1.

9.3.

In this profile, the product is first subjected to a Precipitation Cycle,
where the stresses used are beyond those defined for the Detection
Profile. Then, the product is subjected to a Detection Cycle, where thestresses are reduced to the Detection Profile levels. The Precipitation
Cycle seeks to precipitate the dormant or latent defects as quickly as
possible. The units are non-functional during the Precipitation Cycle, but
may be powered. The Detection Cycle seeks to detect the precipitateddefects. The product(s) are powered and functionally tested during the
Detection Cycle. This profile will typically consist of two cycles, one
Precipitation Cycle followed by one Detection cycle.

Thermal stress levels during the Precipitation Cycle are between the
OL’s and DL’s. A good starting level is equal to half the difference between
the OL and DL (example: for UDL of 120C and UOL of 60C, use 90C).

th

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9.4.

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Vibration levels for the Precipitation Cycle should be 50% of the DL.
Below is an example of a Precipitation / Detection HASS profile.

10.

Proof-of-Screen
Proof-of-Screen is a two step process. The first step (Screen Effectiveness) is todetermine how effective the screen is in detecting manufacturing flaws. This isdone by precipitating and detecting latent defects. The second step (SOS,
Safety Of Screen)) is to prove that the screen is safe, and will not removed
significant life from the screened products.

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10.1.

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Screen Effectiveness10.1.1.

The screen’s effectiveness is measured by its ability to precipitate
latent defects. To demonstrate this effectiveness, it is necessary to
screen products that would be expected to fail in the HASS screen due tolatent defects. Units classified as No Trouble Found (NTF) are good forthis purpose (ideally, production floor NTF’s should be used versus field
return NTF’s). Other candidate units would be those that are determined
to be marginal from parametric functionality testing. . If necessary,
known good units can be “seeded” with defects. These defects should be
representative of the manufacturing process going out of control. These
defects attempt to replicate defects that could occur if there was a
problem in the manufacturing or sourcing processes, i.e. poor solder
process, damaged or incorrect component insertion, etc. The defects
should not be detectable by the production testing that is normally done.
They should include marginal but acceptable performance defects,
component value errors and any other valid defects that would
demonstrate the effectiveness of the screen.

10.1.2.

All fixture locations should be populated. If there are not enough units
with latent defects available to fill the fixture, then known good products
should be used in the remaining fixture locations.

10.1.3.

The products shall be subjected to the initial HASS screen. If any
failures occur on the screened unit(s), these failures shall have a root
cause analysis (RCA) performed to determine if the failure(s) were theresult of overstress or wear-out, or due to a manufacturing process flaw.
If a failure occurs at any point during the profile execution, and there is aperceived risk that the failed unit may be damaged further if the screen is
continued, possibly masking the original failure, then the test should bepaused and the failing unit removed for RCA and replaced with another
unit. Then, the testing should resume where it left off.

10.1.4.

The RCA results will determine whether the profile levels are correct, or
require modification. If the failure(s) were the result of overstress or
wear-out, the profile levels shall be reduced, and the profile re-executedusing previously non-tested unit(s). On the other hand, if the defects are
not detected, the screen severity is increased and the process repeated.
The severity shall be increased by either expanding the profile levels, orincreasing the number of cycles performed. The levels will typically bethQualmark Corporation | 10390 East 48 Avenue | Denver, CO 80238 | phone 303-254-8800 | fax 303-254-8343

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increased in 10C increments, but the increment value can be higher orlower, depending on product variables. Increasing the cycle countextends the total Production HASS duration, and so is less desirable.
10.2.

Safety of Screen

10.2.1.

This process shall demonstrate that the HASS screen profile is safe,
and will not remove appreciable life from a product.. In this portion of thescreen, the fixture should be loaded with fully tested and known good
units. The profile shall be repeated a minimum of 20 times, without
failure occurrences. This shows that production units exposed to only
one pass of the profile, will still have 95%, minimum, of the useful life
remaining, or 5%, maximum, of life removed. For example, if the HASS
profile were 2 cycles, then these units would be subjected to 40 cycles.
For greater confidence, it is recommended that of the screen profile berepeated 30 to 50 times.

10.2.2.

If any failures occur on the screened unit(s), these failures shall have aroot cause analysis (RCA) performed to determine if the failure(s) were
the result of overstress or wear-out, or due to a manufacturing processflaw. If a failure occurs, at any point during the profile execution, theprofile should be stopped, and RCA performed.

10.2.3.

After passing through these multiple screen cycles without failing, more
rigorous evaluations of the products can be done. For example, theproducts can be subjected to accelerated life tests, or put through the
Design Qualification Testing that was done during product development.
The purpose of this would be to give even greater confidence that the
products have not been degraded significantly and that minimal life has
been removed.

10.2.4.

The RCA results will determine whether the profile levels are correct, or
require modification. If the failure(s) were the result of overstress or
wear-out, the profile levels shall be reduced, and the profile re-executedusing previously non-tested unit(s).

11.
11.1.

HASS Profile Evolution
It is essential that the screen profile be audited on a continual basis
throughout the life of the product. This ensures that the screen remains
effective, and that subtle changes in the product are not causing the screen
levels to move into the damage area. If failures are occurring in use, thenthQualmark Corporation | 10390 East 48 Avenue | Denver, CO 80238 | phone 303-254-8800 | fax 303-254-8343

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screen severity levels may need to be increased if the failure are process
related, or the screen severity may need to be reduced if the failures are
due to overstress or wear-out.
12.
12.1.

13.

Failure Analysis in HASS
Because the failures found in HASS are almost always found at stresslevels far outside of the normal use environment, there can be a tendency
to discount the validity of the failures, particularly in the face of production
schedules and limited engineering resources. It is very important that this is
not allowed to happen. A clearly defined closed loop failure analysis andcorrective action process should be implemented on every failure found in
HASS.
Variations on HASS

13.1.

As with HALT implementation, the implementation of HASS often does
not follow a specific, repeatable and unchanged process. Careful thought
must be given to each product’s characteristics, limitations and sensitivities,
and the final HASS procedure that is implemented could be very different
from what has been described in this document. The important part is toensure that the core points of HASS – controlled overstressing of a product,
based on known limitations found in HALT, and proved to be safe andeffective in Proof of Screen – are adhered to. Following are some common
variations to consider.

13.2.

HASS on subassemblies: In many cases, the final product is not an
ideal HASS candidate. Key components may be isolated from thermal andvibration stresses by the outside case. The shape of the product may notlend itself to consistent fixturing. Plastics or other soft, flexible materials
that are part of the outside case may deform or experience cosmetic
damage under HASS stresses. In these cases, it can be much morevaluable to HASS the major subassemblies rather than the final, full-up
assembly. This decision must be made by carefully considering theadvantages and disadvantages of each process, including test coverage,
complexity of test equipment, complexity of fixturing, cost and throughput.

13.3.

Sometimes it is difficult, or even impossible, to find products that would
be expected to fail during the HASS Screen Effectiveness Evaluation. In
this case, a screen can be safely implemented by starting with screen levels
that are somewhat higher than normal, then doing an aggressive Safety
Evaluation (50 cycles or more) to ensure that the screen levels are safe.

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Then, implement the screen in production and monitor the failures closely
for first several weeks. If failures are occurring in the first few cycles of thescreen, and failure analysis shows that the failures are not due tooverstress, then the screen can be considered to be effective. However, if
valid failures are consistently occurring near the end of the screen, or failure
analysis shows that the failures are due to overstress, then the screen
should be adjusted.
14.
14.1.

Use of HASS in contract manufacturing
When a product is manufactured by a contract manufacturer, there canbe communications delays and discontinuities between the designers of the
product and the manufacturer. The impact of production issues orcomponent changes may not be fully understood until field failures reach an
unacceptable level. If the product is being manufactured off-shore, these
problems are compounded. If HASS is used, it can provide a ‘safety net’because it will detect potential problems early, and, if the designers are kept
aware of HASS failures and failure analysis results, it will allow corrective
action to be done quickly and efficiently. A properly implemented HASS
process can dramatically reduce the risk of loss of control that is inherent inremote manufacturing.

th

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