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  • Fundamentalsof Accelerated Stress Testing

    A C C E L E R AT E D S T R E S S T E S T I N G

    Fundamentalsof Accelerated Stress Testing

  • TABLE OF CONTENTS

    1

    Introduction . . . . . . . . . . . . . . . . . . . . . . 2

    The Changes . . . . . . . . . . . . . . . . . . . . . . 2

    Definition of AST . . . . . . . . . . . . . . . . . 3

    Defect Precipitation . . . . . . . . . . . . . . . . 4

    Accelerated Stress TestingSimulation or Stimulation? . . . . . . . . . . 5

    Implementation of AST. . . . . . . . . . . . 10

    Where Do We Implement AST?. . . . . 11

    Step-Stressing . . . . . . . . . . . . . . . . . . . . 12

    HALT Testing . . . . . . . . . . . . . . . . . . . . 13

    Types of StressesUsed in AST. . . . . . . . . . . . . . . . . . . . . 14

    Temperature Cycling . . . . . . . . . . . . . . 15

    Vibration . . . . . . . . . . . . . . . . . . . . . . . . 16

    Comparison of Electrodynamic and Repetitive Shock (RS) VibrationSystem Characteristics . . . . . . . . . . . . . 17

    Thermal Shock . . . . . . . . . . . . . . . . . . . 18

    Humidity . . . . . . . . . . . . . . . . . . . . . . . . 18

    Electrical Stress . . . . . . . . . . . . . . . . . . . 19

    Combined Environments . . . . . . . . . . . 19

    Environmental Stresses, Effects and Potential Reliability ImprovementTechniques . . . . . . . . . . . . . . . . . . . . . . 20

    Failure Identification . . . . . . . . . . . . . . 22

    Tools for Continuous Monitoring . . . . . . . . . . . . . . . . . . . . . . 23

    Reliability Growth Monitoring . . . . . . 24

    Summary . . . . . . . . . . . . . . . . . . . . . . . . 24

    Definitions. . . . . . . . . . . . . . . . . . . . . . . 25

    References . . . . . . . . . . . . . . . . . . . . . . . 30

    Recommended Information Sources. . . . . . . . . . . . . . . . . . . . . . . . . . 31

  • 2

    Manufacturing techniques and procedureshave changed dramatically over the lasttwenty years. Products have becomemuch more complex, customers productexpectations have grown, and competitionfor customers has increased. Additionallynational and international quality andreliability standards are continually beingpublished. Because of these changes, mostproducts must now undergo testing toachieve acceptable levels of quality andreliability improvements, and these testingmethods can vary greatly. On one hand,there are pragmatic methods based uponsound management and engineering, andthe wide use of statistical techniques. On the other hand is a range of methodsbased largely upon a systems approach inquantification of factors such as yield andfailure rates1.

    The requirements for product reliabilityand testing have undergone major changesin the last few years. Customers are startingto recognize the importance of buildingreliability into their products. To build inreliability, todays manufacturers need toknow as much about how things fail asthey know how things work2. The adventof cost-effective computer technologycoupled with low testing and test personnelbudgets have caused a renewed interest inAccelerated Stress Testing. The number oftools and techniques used to test productshas also changed dramatically and willcontinue to evolve as technology evolves.

    AST is a product testing technique thateveryone from the management team,to the marketing people, to the productmanagers, to the product designers, to themanufacturing personnel, to the qualityand reliability personnel need to knowabout. AST can be used quite simply tomake better, more reliable product.

    THE CHANGES

    This handbook is written to show howgreater control can be gained over totalproduct reliability by the utilization ofAccelerated Stress Testing techniques(AST). It will examine the concept ofAST; how Accelerated Stress Testingcompares to Accelerated Life Testing;the technologies used in Accelerated StressTesting; and the issues involved in properlyimplementing Accelerated Stress Testing.

    General questions relating to the purposeand intent of Accelerated Stress Testingare also addressed in this booklet. However,because Accelerated Stress Testing isproduct specific, and each product has itsown set of variables, it is not possible tocover individual product applications indetail. Sources for assistance of this typeare referenced in the Bibliography Section.

    NOTE ON TERMINOLOGYAs it is true throughout various industries,terms used to define or describe things ina particular discipline can vary widely. Inunderstanding Accelerated Stress Testing,there are a number of areas where thesevariations cause confusion. In interestof clarity, this book makes some neededdistinctions. It is recommended thatreaders refer to the Definitions Section forclarification of the terms used in this book.

    INTRODUCTION

  • Accelerated Stress Testing can be simplydefined as: applying high levels of stressfor a short period of time to a device undertest assuming it will exhibit the samefailure mechanisms as it would in a longeramount of time at lower stress levels. Incontrast to many types of testing AST istesting to fail versus that of testing topass a product. The key here is to under-

    stand the failures and their relationship to the applied stresses.

    The main purpose of AST is to insure product reliability and accelerate the reliability growth for a given product. The intent is to attain product reliabilitymaturity early in the design phase (see Fig. 1 & 2).

    DEFINITION OF ASTPr

    oduc

    t Rel

    iabi

    lity

    Gro

    wth

    Time

    TYPICAL RELIABILITY GROWTH

    FOR A TYPICAL PRODUCT

    Fig. 1

    Prod

    uct R

    elia

    bilit

    y G

    row

    th

    Time

    RELIABILITY GROWTH FOR A TYPICAL

    PRODUCT USING AST TECHNIQUES

    Fig. 2

    3

  • It is very important to understand thatspecific types of defects are precipitatedor caused to surface by certain stresses orcombinations of stresses. Remember, eachproduct is unique and there is no onetool or machine that will precipitate allrelevant flaws for all products. This is notonly due to various product dimensionsand functions, but more importantly thephysics of the product response to variousstimuli. Mathematical models used inpredicting or counting failures has its place,but when concerned with accelerating thefatigue of products, we must first rememberwhat causes product failures. The maincauses of failure, whether in productionor in use are1:

    Over-stress In which the stress(volts, torque, etc.) exceeds the strengthavailable to withstand it.

    Wear-out Caused by fatigue wear,corrosion, etc.

    Performance Such as resistance value,drift, dimensional change, etc.

    Variation Variation of parametervalues (dimensions, environments, etc.)can cause failure to meet specifications;or can lead to failure due to over-stress orwear-out. Variation in the product is theresult of manufacturing processes that arenot exactly repeatable.

    By understanding the product, we canfocus on how to accelerate the life of theproduct and identify potential weaknessesor failures. To accomplish this we mustidentify what stresses the product cansustain and what stresses precipitatedefects. This has long been practiced by skilled Environmental Engineers byapplying TAAF (Test, Analyze, And Fix)Procedures.

    DEFECT PRECIPITATION

    One test is worth a thousand

    expert opinions

    Anonymous

    4

  • A great deal of confusion in the areaof AST testing can be eliminated if thesimple question is asked: Am I trying tosimulate the products life or am I tryingto stimulate any failure that might occurduring the products life? Although thismay sound like the same thing, it is not.In product life simulation tests, you aretrying to simulate a lifetime of actualstresses the product will be exposed to.In product stimulation you apply stressesto the product that are typically moresubstantial than what the product wouldsee in its use environment in an effort toinduce fatigue, which precipitates failuresand reveals product weakness. Bothtechniques have uses in a well-plannedproduct development program, and whenused improperly, both techniques lead tofalse conclusions and can have costlyimplications.

    It is worth noting that a test orientedtowards evaluating long term effects(fatigue or durability) will almost alwaystake more time than a test orientedtowards evaluating short term (limit loador step-stress) effects. This does not meanthat one is better than the other; findingone type of problem often requires anapproach that is not suited to findinganother type of problem. More importantly,it does not mean that substituting a shortertype of test for a longer test will produceequivalent results, regardless of what thetest is called. The cheaper test may notalways be the right test3.

    SIMULATION TESTINGThe intent of product life simulation testsis to identify relevant failure mechanismsthat would occur, and correlate these with the

    point in the products life the failure wouldoccur. We want to understand the productsperformance under the conditions that customers would expect the product tooperate.12 This can be very simple for sometypes of products and very complex forother types of products.

    For example, a keyboard could expect tobe used for 5 years. Assuming an operatorcould type 80 WPM (350 characters perminute), 7 hours a day, 280 days a year,and the highest usage keys were vowels,and the vowels were each used an averageof once every five letters typed, then41.16 million strokes would occur on thehighest usage keys. So by striking a key41.16 million times we would simulate5 years of operation. This could be doneusing an X-Y Cartesian robot (so you couldstrike off center as well), with a set ofsolenoids that could strike

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