Fundamentals of 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
Fundamentals of Accelerated Stress Testing
TABLE OF CONTENTS
1
Introduction . . . . . . . . . . . . . . . . . . . . . . 2
The Changes . . . . . . . . . . . . . . . . . . . . . . 2
Definition of AST . . . . . . . . . . . . . . . . . 3
Defect Precipitation . . . . . . . . . . . . . . . . 4
Accelerated Stress Testing–Simulation 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, today’s manufacturers need toknow as much about “how things fail” asthey know “how things work”2. 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 is“testing 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 product’s life or am I tryingto stimulate any failure that might occurduring the product’s 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 product’s life the failure wouldoccur. We want to understand the product’sperformance 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 900 strokes/minute to simulate 5 years of life in31.7 days. This is an example of timecompression using actual simulated lifeconditions. This can also be done withthe dynamic and climactic conditions aproduct would see. This type of acceleratedlife testing is very easy to correlate to theactual life of the product.
Unfortunately, not all product simulationsare quite this simple. The steps to aproduct life simulation are as follows:
1. Determine what life your customerexpects from the product.
The most important thing to know forany life test, or any test for that matter,is what your customer expects from yourproduct. Each market segment has its own
ACCELERATED STRESS TESTING–SIMULATION OR STIMULATION?
5
expectation level for what is acceptable.The expectation level varies based on theproduct’s type and price. If the productdoes not meet the expectation level of itsmarket segment, it will most probably bea failure.
An example of this is comparing a Rolex™watch with a watch that that you get freewhen you fill your gas tank. The marketsegment that the Rolex™ was designed for expects precision, and lifetime opera-tion. If it does not last the customers’ life-time, then it will not meet the customersexpectation level, and the customer willbe dissatisfied. On the other hand, if thegiveaway watch keeps reasonable time fortwo years, the customer will most likelybe very satisfied. It is obvious that thecost to manufacture a product such as theRolex™is much greater than the cost ofthe giveaway watch. If we tried to make awatch for the giveaway market with toohigh a cost, it would also be a failure.
By first determining what the customerexpects from your product, you canintelligently put together a life test thathelps you meet or exceed the customerexpectations.
2. Determine what operating andnon-operating environment(s) the productwill experience.
The first environments to consider arethose the product will experienceduring transportation (all the modes oftransportation used as well the regions the product will be shipped to). The nextconsideration is storage environments.
The final set of environments are those the product will encounter during its primary use (all the situations where a customer may use the product). After allthese conditions are identified, you willhave determined the standard and extreme environments the product will see during its life.
3. Determine the frequency and durationof operation the product will experience.
Identifying how often the product willbe used and the duration of its usesrequires an understanding of statisticsand probability, as not all users will usethe product the same number of timesor for the same length of time.11 It is bestto assume worst case use of the productfor life test purposes.
Once the number of uses and the durationis determined, we can look for ways tocompress the time. This is done by cyclingthe product on and off the total numbertimes the product would see during its life.As for duration of use, it is applicable forsome products but not others. For example,for most electronic equipment durationapplies to the amount of time it takes tobring the product to temperature. Afterthat point, the stress incurred is minimal.However, for some mechanical devices,the length of operation does apply andmust be taken into consideration.
4. Determine the frequency the productwill see particular use environments.
A. Determine the extreme stress levelsand how many times the product willexperience these.
6
We now need to consider the extremesthat were decided earlier to determine howmany times the product will see these inits life, assuming the worst case (again,this will vary from user to user). Oncethe frequency of cycles is determined, wecan look at compressing the time period.We do need to differentiate the numberof cycles and extremes the product willsee during transportation, storage, andoperation. It has been shown that once theproduct reaches a temperature condition,the majority of the fatigue has been incurred.This means that typically once the productreaches a targeted temperature a dwellperiod is not required and we can continueto the next target temperature condition.Humidity extremes are affected by durationdue to breakdowns that can occur in coat-ings and seals as well as corrosion growth.In vibration testing, how often a productexperiences extreme vibration levelsdirectly relates to the total amount of stress that the product is subjected to, and needs to be addressed.
B. Determine transitions and howmany times the product will see these.
Chances are, there will be cases in aproduct’s life where an extreme changein temperature will occur. Even officeequipment that typically resides inair-conditioned comfort will mostlikely experience extreme changes intemperature, humidity, and vibrationlevels during transportation. We needto identify the maximum transitions andincorporate the worst case number intoour accelerated life test.11
5. Put together the life test.
Now that we know the stress conditions,the frequency of use and exposure, andthe maximum transitions, we can create aprofile that represents a particular product’slife. One of the easiest ways to formulatethis test is to break the product’s life downinto two sections.
The first section represents the producttransportation and storage environments,and the second section represents theproduct use environment. During thetransportation and storage section, becausethe product experiences vibration extremesbefore it experiences climactic extremes,we apply the vibration conditions first.That is, the greater stresses will be appliedfirst. A good reference for the transporta-tion vibration profiles is MIL-STD-810 or SAE J-1211. These specifications detail environmental profiles for different modesof transportation. After the vibrationextremes are tested, the climactic extremeswith transition rates are applied. Finallythe use environment tests are applied;we have already determined the extremesand frequency of variation.
While applying the environmental anddynamic stress conditions, rememberto exercise and continually monitor theproduct. It is extremely important todocument failures and at what conditionthey occur. This information is criticalto identify the failure mechanisms so theproper corrective action can be taken.
7
STIMULATION TESTINGThe intent of environmental stimulationis to induce product fatigue by applyinghigher levels of stress than the productwould experience in normal use. This typeof testing is based on the concept thathigher stresses applied to a productaccelerate the rate at which relevantfailures would become evident in theproduct’s life. The key is that the failurescaused during stimulation must be relevantto failures that the product would experi-ence sometime during its expected life. Predicting the life of parts stressed abovethe endurance limit is at best a roughprocedure4. For many mechanical andelectronic parts subjected to randomlyvarying stress cycle intensity, the predictionof fatigue life is further complicated. Theprocedure referenced here for addressingthis situation was proposed by Palmgrenof Sweden in 1924 and, independently,by Miner of the United States in 1945.The procedure is often called the linearcumulative-damage rule. Palmgren andMiner very logically proposed the simpleconcept that if a part cyclically loaded at a stress level causing failure in 105 cycles,then each cycle of this loading consumesone part in 105 of the life of the part. If other stress cycles are interposed corresponding to a life of 104 cycles, eachof these consumes one part in 104 the life,and so on. When, on this basis, 100 percent of the life is consumed, fatiguefailure is predicted4. Miner’s equation fordamage at failure is:
Where n1, n2...nk represent
the number of cycles at specific
overstress levels, and N1, N2...Nk
represent the life (in cycles) at
these overstress levels, as taken
from the appropriate S-N curve.
A point to be taken into consideration isthat once a product is fatigued, a portionof its life is consumed, and this damageis irreversible. We need to temper thisfatigue accumulation equation with thefact that all products are unique. Thisstatement was proved very conclusivelyby showing the order in which vibrationand temperature cycling is applied doesaffect time to failure5. To calculate aparticular material’s accumulated fatiguedamage in relationship to the stress cyclesat various stress levels, we find a form ofthe Miner’s rule using idealized S-N curvesCrandell and Mark10 suggested. Therelationship can be stated as:
AFD = nσβ
AFD is the accumulated fatigue
damage; n is the number of stress
cycles; σ is the mechanical stress;
and β is the exponential drive from
the SN Diagram for the material
this is typically between 6 and 25.
This illustrates that by increasing theamount of stress, you can reduce thenumber of cycles. This principle appliesfor a single type of material and does notwork blindly on products that containmany different types of materials and theinteractions that occur when variousstresses are applied.
8
n1 n2 nk
——— + ——— + . . . + ——— = 1Nf1 Nf2 Nfk
ACCELERATED SCREEN TESTING IS NOT ENVIRONMENTAL STRESS SCREENING (ESS)There is sometimes confusion betweenAST and ESS. This is because both tech-nologies use stimulation as a means of pre-cipitating latent defects. AST, however, isa test applied to a sample of the produc-tion, while ESS is a process applied toevery unit that is produced. AST is typically used in the product develop-ment phase to identify design weaknesses,while ESS is used primarily to identifyweaknesses in materials and processes.
It has been proven that relevant latent(hidden) defects can be precipitated byexaggerated stress levels utilizing a smallernumber of cycles. The reasons for stimula-tion testing are quite simple. First, it allowsyou to quickly gauge the overall ruggednessof a product early in the design phase byidentifying operation failure limits. Next,it produces failures which can be judged asrelevant or irrelevant by the design team,and the corresponding corrective measurescan be implemented quickly with minimalrework. Finally, it provides invaluableinformation regarding how the systemreacts to various stress environments andoperating conditions, and this informationcan help close the design feedback loop.
ACCELERATED STRESS TESTING(AST) VS. ACCELERATED LIFETESTING (ALT)As discussed earlier there is quite adifference between simulation testing andstimulation testing. This is the differencebetween AST and ALT. Even if youare using accelerated models with higherlevels of stress for time compression, the intent of life testing is to be able todetermine not only that a failure will occur during the products expected life-time but also when in the product’s life the failure will occur. AST on the otherhand, is based on the assumption thatan item will exhibit the same failuremechanisms in a short time under highstress conditions that it will exhibit in alonger time under low stress conditions2.With AST you must examine and reviewall failures, determine relevance, anddecide what, if any, corrective actionis required.
9
The most important thing to recognizewhen implementing AST is that yourproduct determines the test. The firstconsideration when implementing ASTis the type of stresses to apply. This canbe determined by reviewing typical fieldfailure data on related products (if possible)to gauge expected early failure mechanisms.The concept is to apply stresses thatprecipitate all the latent flaws that would occur during a product’s lifetime.14
There are many sources to familiarize yourself with the various screens and thetypes of defects that they precipitate. SeeTable 2 (on pages 20 and 21) for examples.If you have no past related field data, aStep Stress or a HALT test needs to beperformed to identify a product’s potentialweaknesses. Ideally, when considering what types of stresses to apply to the product, you should use as many relevantstressing environments as possible. By incorporating multiple stressing environments, a synergistic stress effect is produced.
The next consideration is the properlevels of stress to apply. This can bedetermined by applying a step stressprocedure to determine the destruct andoperational limits of a product. Stress leveldetermination is very product dependent.While applying the stresses, it is importantto apply TAAF (Test, Analyze And Fix)techniques to identify product weaknessand implement any required correctiveactions. To do this requires that the product be exercised and monitored. This cannot be overstated.
The final major consideration is stressduration. This is determined by the natureof the AST test and the time required tobe properly electrically or mechanicallyoperated and tested.
IMPLEMENTATION OF AST
10
MODES OF STRESS TESTING6
There are several modes of using AST:design qualification, manufacturingqualification, production sampling, andongoing AST. These are presented inorder; from the most effective in increasingproduct reliability to the least.
Design Qualification AST: The productis tested near the end of the design stageto see if it is robust with respect to stresslevels in excess of those likely to beencountered in the use environment.If deficiencies are found, the root causeis determined and corrective actions aretaken to fix the underlying causes of theproblem. This is the most productive modeof doing AST because the benefits arerealized over the whole life of the product.
Manufacturing Qualification AST:A representative sample of product issubjected to AST during manufacturingramp-up to identify deficiencies incomponent quality or manufacturingprocesses. In addition, design margindeficiencies that did not show up indesign qualification AST may be found.The emphasis again is on Failure ModeAnalysis (FMA) and corrective action,so that deficiencies in the product canbe quickly eliminated before productionvolumes become large.
Production Sampling AST: For productsrequiring high reliability, it is useful tocontinue to perform AST on a samplingbasis to monitor the production processfor manufacturing or component qualityvariations, even after satisfactoryqualification has been achieved. Theemphasis continues to be on determiningthe root cause of any problems found andtaking corrective action to fix them.
Ongoing AST: It sometimes occursthat AST is performed with the intentionof doing good FMA and correcting anyproblems found, but because of a lackof sufficient resources, quality problemspersist. In this case, it may still beeconomically feasible to continue doingAST on an ongoing basis, with somedegree of FMA and corrective actionto achieve a high reliability product.However, this mode is certainly lessdesirable than the approaches mentionedabove.
WHERE DO WE IMPLEMENT AST?
11
Step-stressing has been used since the earlydays of the space program. It is basically aprocess of starting at a known stress level,then increasing the “stress” levels incontrolled “steps”. The stress applied canbe temperature, vibration, electrical, orother stresses, and the stresses can beapplied separately or in various combinations (see Fig. 3).
First, the product is tested to determinethe maximum operational limits of stressthe product can withstand. When theproduct reaches the limits of operation(destruct limits), and no longer works,the failures are analyzed and categorized,and the product is repaired, and it istested again. This process is repeated untilsatisfactory results are achieved. To deter-
mine when to terminate the step stresstest, continue the step-stressing until thefollowing occurs5:
■ Stress levels well above those expectedin service are reached,
■ All samples fail, or
■ Irrelevant failures begin to appear
By understanding the main cause offailure, we can then identify and addressthese potential failures. To do this wemust first have an idea of what stressesthe product can survive. The product isthen ruggedized to the level which thecompany has intended.
STEP-STRESSING
Appl
ied
Stre
ss
Time➟
➟
Fig. 3
OPERATIONAL LIMIT
12
TECHNOLOGY LIMIT
Another type of stress stimulationtesting is known as HALT testing. HALTis an acronym for Highly Accelerated LifeTesting. It was coined by Dr. Greg Hobbs8.HALT is really a package of testingtechniques, such as aggressive temperatureand vibration step-stressing. In the strictestdefinition, HALT testing also implementsrepetitive multiple axis vibration, alsoknown as quasi-random, omni-axial, orsix-degree-of-freedom vibration. This wasdeveloped in the 1970’s by Hughes aircraftas a ruggedization tool.
HALT testing offers a convenientstep-by-step process which will by itsvery nature precipitate defects. It inducesfatigue, which in turn causes failuresthat can be analyzed to determine theappropriate corrective action. However,
HALT testing does not correlate thefailures to the point in time in the product’s life that the failure would actuallyoccur. Although HALT testing does stressand fatigue products, the actual amount of time compression is unknown. For this reason, HALT testing should probably becategorized as highly accelerated fatiguetesting instead of inferring that it actuallyaccelerates the lifetime of a product by any measurable amount of time.
HALT TESTING
13
Accelerated Stress Testing consists ofexposing a group of products to one ormore stressing environments. Theseenvironments can include ThermalCycling, Vibration, Electrical Stress,Thermal Shock, Humidity, and others.The actual detailed product profiles, suchas the rates of change, types of stress, stressextremes, etc., must be tailored for eachproduct. In this section we will present
some of the commonly used stressingenvironments, what effects these have onpotential failure mechanisms, and a listingof failure mechanisms precipitated byindividual stresses.
It is important to remember that each type of stress precipitates certain failuremechanisms.
TYPES OF STRESSES USED IN AST
AN EXAMPLE OF THE RELATIONSHIP OF
PRECIPITATIVE FAILURE POPULATIONS
AND STRESSES
Fig. 4
Dust
Vibration
TemperatureCycling
HumidityThermalShock
PowerCycling
ClockVariation
CorrosiveEnvironments
Voltage Margining
14
Temperature Cycling consists of multiplecycles of changing temperature betweenpredetermined extremes. TemperatureCycling can be used in AST for bothsimulation and stimulation testing. As allthe variables in AST screening are productdependent, temperature extremes must befar enough apart to allow for optimumstressing caused by the temperature changebetween the extremes. It is the constantrate of change that provides the expansionand contraction necessary to sufficientlystress the product.
When we talk about temperature stressingof a product, we are talking about productchange rates. Because products havedifferent amounts of surface area, differentspecific heats, and different coefficientsof thermal expansion, we must treat everyproduct as unique. There are three mainvariables to consider when planningtemperature stress tests. These are product
temperature change rates, differencebetween temperature extremes and numberof cycles. Other considerations are:
Airflow: For each product, there is an airvelocity at which maximum heat transferis obtained. Exceeding this air velocitycan be counter-productive. The correctair velocity and direction for a particularproduct can be determined throughexperimentation.
Dwell periods: Once the product hasreached the desired temperature, dwellperiods at temperature extremes typicallydo not significantly contribute to stress.Many manufacturers allow the product toremain at temperature extremes only longenough to allow for a functional test ofthe product.
Thermal Cycling is the most commonlyused, and one of the most effectiveprecipitation stresses. Like any stress, itmust be properly implemented. Failurerates must be analyzed to determine whichlatent defects are causing failures; andexperimentation must be performed todetermine the stress profile best suited totrigger those particular latent defects andtheir failures.
TEMPERATURE CYCLING
15
Vibration involves stimulating a productwith a pre-determined force over afrequency spectrum. There are severaltypes of machines used for vibrationtesting, but in AST, typically two types ofvibration are used: Electrodynamic andRepetitive Shock vibration.
The typical Electrodynamic vibrationsystem frequency range is 5-2000 Hertz.The most common type of vibration stressused is broadband random. This is createdthrough simultaneous excitation of all theresident frequencies within a profile range.Random vibration is transferred to theproduct by fixturing attached to themoving element of the Electrodynamicshaker. The Electrodynamic shaker iscontrolled by a closed loop vibrationcontrol system which is repeatable and willaccurately reproduce real world vibration.It can be used for simulation testing aswell as stimulation testing. Products maybe vibrated on a single axis or multipleaxis concurrently or consecutively.Several categories of vibration can berun on Electrodynamic vibration systemssuch as sine, random, resonance searchand dwell, shock, sine on random, randomon random, and several others. Thesestress environments may or may not beapplicable depending on the productapplication.
Repetitive Shock Vibration (RS Vibration)is generated using one or more pneumaticimpact hammer(s) that strike a plate onwhich the product is attached. Commonly,Repetitive Shock Machines providevibration in three distinct axes and threerotational axes simultaneously. Thefrequency in which most of the energyis generated is in the 2 to 5000 hertzbandwidth. Repetitive shock vibrationmachines cannot simulate real worldvibration conditions, so their use is limitedto stimulation testing only.
Fixturing for either type of vibrationmachine needs to be designed to ensurethat the stress is transmitted to the productand that the process can be repeatablewith reasonable accuracy. Currentlythere are varying opinions as to whichprovides a more stressful environmentover a particular frequency range. Table 1provides a comparison.
VIBRATION
16
COMPARISON OF ELECTRODYNAMIC AND REPETITIVE SHOCK(RS) VIBRATION SYSTEM CHARACTERISTICS
FACTOR ELECTRODYNAMIC REPETITIVE SHOCK (RS)
1. Number of Axes per Shaker One Six
2. Simultaneous Multiple One or more shakers per axis (complex YesAxis Capability control algorithms) or diagonal (skewed)
force vector input
3. Typical Range of Thousands of pounds Possible: Low hundreds of lbsProduct Mass Practical: Less than 100 lbs
4. Energy Source Electrical Pneumatic
5. Typical Frequency Range < 5 to 2000 Hz 10 to >5,000 Hz
6. Controllable Spectrum Yes No
7. Force Limiting Factors Armature mass, field supply, Piston mass, impact velocity,amplifier cooling capacity piston/fixture coupling, distance
from impact point, modal characteristics of plate
8. Low Frequency Energy Yes, controllable Minimal
9. Can be used for Yes NoSimulation Testing
10. Can be used for Yes YesStimulation Testing
11. Input Displacement Range Inches applied to entire fixture and product Microinches, applied locally to fixture impact points
12. Spectral Characteristics Continuous control in each filter band; Uncontrolled (open loop) spectrumsine, random, shock, sine-on-random, with periodic “holes” and spikes,narrowband-on-broadband vibration “quasi-random” vibration only
13. Spectrum Limiting Factors Fixture resonances, number of filters, Open-loop control, piston repetition ED armature/test mass resonance rate, fixture resonances and nodes,(~2500 Hz) GRMs control only
14. Product Characterization Straight forward analysis Analysis complicated by untailorablespectrum
15. Uniformity On Table Very uniform Not uniform
16. Mechanical Shock Pulses Classical waveform, shock spectrum, No control capabilitypyrotechnic
17. Acoustic Noise Varies with spectrum, fixture and product Very loud (could affect performancedesign, and input force level of noise-sensitive products)
18. Electromagnetic Fields Magnetic fields can affect some products; No effectsmay require degauss coil
17
TABLE 1
Humidity testing is typically performedin a chamber that can precisely controlwet and dry bulb temperatures. Exposingproducts to humidity stress precipitatescorrosion and contamination defects inproducts. Humidity can penetrate porousmaterials, cause leakage between electricalconductors, and is also an important stresswhen evaluating coatings and seals.Because oxygen is required for oxidation, it limits the use of nitrogen-cooledchambers, which create an inert environ-ment that inhibits corrosion. Typically,humidity tests are of long duration, andcertain types of corrosion require minimalairflow, which is not typical of mostchambers.
Another type of humidity testing, knownas HAST (Highly Accelerated StressTesting), is done in a pressurized vessel.This type of system is analogous to a pressure cooker or autoclave. The HASTsystem aggressively forces moisture into potential failure sites.
HUMIDITY
Thermal shock testing involves exposingproducts to severe temperature extremes ina rapid fashion. For example, the product istransferred either mechanically or manuallyfrom an extremely hot environment to anextremely cold environment. This processis repeated several times. Thermal shockis generally considered a cost-effectiveway to screen defects at the componentlevel; particularly in integrated circuitswhich require a high degree of stress toexperience the rates of change requiredto force latent defects into failure.
There are two types of Thermal shock testmediums: Air-to-Air and Liquid-to-Liquid.Air-to-Air Thermal shock systems exposethe product to preconditioned hot andcold air. Liquid-to-Liquid Thermal shocksimmerse a product in high and low temperature inert fluids. These tests can be performed on a variety of Thermalshock machines.
When using Thermal shock for AST testing, consider the following: If theproduct is manually transferred, the riskof accidental product damage increases.Additionally, it is difficult, or sometimesimpractical, to apply product power andmonitor the product’s operation while theThermal shock stress environment is beingapplied. This severely limits opportunitiesfor collecting data for product failureanalysis.
THERMAL SHOCK
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Electrical Stress is used to test a productnear or at its electrical limits. Exercisingcircuitry and/or simulating junctiontemperatures on semi-conductors are bothgood examples of electrical stress tests.There are two basic types of electricalstress tests: Power Cycling and VoltageMargining.
Power Cycling consists of turning productpower on and off at specified levels.Voltage Margining involves varying inputpower above and below nominal productpower requirements. A subset of VoltageMargining is frequency margining.
Typically, electrical stress by itself does notexpose the number of defects commonlyfound through thermal cycling and vibra-tion stress. However, because it is typicallynecessary to supply power to productsin order to find the soft or intermittentfailures, it can be relatively inexpensive toimplement electrical stress with the otherstress environments to increase the overallAccelerated Stress Test effectiveness.
ELECTRICAL STRESS
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Depending on product complexity,cost, and other reliability specifications,multiple environmental screens maybe used simultaneously. For example,Thermal Cycling and Vibration are oftencombined in an Accelerated Stress Testprogram. Many other combinations ofstresses can be combined to precipitatelatent defects. Keep in mind that eachspecific stress precipitates specific failuremechanisms, and when used in conjunc-tion with each other, some synergisticeffects may be realized.
The primary consideration should bewhether the additional stress, appliedsimultaneously or consecutively, wouldexpose a significant number of additionaldefects. With all Accelerated StressTesting programs, the most effective profile is product and application dependent. In general, the proper combination of stresses will produce the greatest number of failures in theshortest amount of time.
COMBINED ENVIRONMENTS
ENVIRONMENTAL STRESSES, EFFECTS AND POTENTIALRELIABILITY IMPROVEMENT TECHNIQUES
ENVIRONMENTAL STRESS EFFECTS POTENTIAL RELIABILITYIMPROVEMENT TECHNIQUES
High Temperature Parameters such as resistance, inductance, Thermal Insulation, heat
capacitance, power, dielectric constant will vary; withstanding materials,
insulation may soften; moving parts may jam due cooling systems.
to expansion and finish may blister; thermal aging,
oxidation and other chemical reactions may be
enhanced; viscosity may be reduced and
evaporation of lubricants can arise and structural
overloads may occur due to physical expansions.
Low Temperature Plastics and rubbers lose flexibility and become Thermal Insulation, cold
brittle; electrical constants vary; ice formation withstanding materials,
occurs when moisture is present; lubricants and cooling systems.
gels increase viscosity; finishes may crack;
structures may be overloaded due to physical
contraction.
Thermal Cycling and Shock Materials may be instantaneously over stressed Combination of techniques
causing cracks and mechanical failures; electrical for high and low temperature
properties may be permanently altered. Crazing
delamination, ruptured seals can arise.
Shock Mechanical structures may be over stressed Strengthened structural members,
causing weakening or collapse; items may be reduced inertia and moment,
ripped from their mounts; mechanical functions shock absorbing mounts.
may be impaired.
Vibration Mechanical strength may deteriorate due to fatigue Stiffening control of resonance.
or over stress; electrical signals may be erroneously
modulated; materials and structure may be cracked,
displaced, or shaken loose from mounts;
mechanical functions may be impaired.
TABLE 2
CALCE Electronic Packaging Research Center, University of Maryland, May 15, 1995
20
ENVIRONMENTAL STRESSES, EFFECTS AND POTENTIALRELIABILITY IMPROVEMENT TECHNIQUES
CALCE Electronic Packaging Research Center, University of Maryland, May 15, 1995
ENVIRONMENTAL STRESS EFFECTS POTENTIAL RELIABILITY
IMPROVEMENT TECHNIQUES
Humidity Penetrates porous substances and causes leakage Moisture Resistant materials,
paths between electrical conductors; causes dehumidifiers, protective coatings,
oxidation which may lead to corrosion; moisture hermetic sealing
causes swelling in materials such as gaskets;
excessive loss of humidity can cause embrittlement
Contaminated Atmosphere Many contaminants combined with water provide Non-metal product covers, reduce
Spray good conductor which can lower insulation use of dissimilar metals in contact,
resistance; cause galvanic corrosion of metals of dissimilar metals in contact,
and accelerates chemical corrosion. heretic sealing, dehumidifiers
Electromagnetic Radiation Causes spurious and erroneous signals from Shielding, radiation hardening
electrical and electronic equipment and
components; may cause complete disruption of
normal electrical and electronic equipment such
as communication and measuring systems.
Nuclear/Cosmic Radiation Causes heating and thermal aging; can alter Shielding, radiation hardening
chemical, physical and electrical properties of
materials; can produce gases and secondary
radiation; can cause oxidation and discoloration
of surfaces; damages electrical and electronic
components, especially semi-conductors.
Sand and Dust Finely finished surfaces are scratched and abraded; Air-filtering, wear-proof materials,
friction between surfaces may be increased; sealing
lubricants can be contaminated; clogging of
orifices; materials may be worn, cracked, or
chipped; abrasion, contaminates insulation,
corona paths.
Low Pressure (High Altitude) Structures such as containers and tanks, are over Increased mechanical strength
stressed and can be exploded or fractured; seals of containers, pressurization,
may leak; air bubbles in materials may increase alternate liquids (low volatility),
due to lack of cooling medium; insulation may improved insulation, improved
suffer arcing breakdown; ozone may be formed; heat transfer methods.
outgassing is more likely.
TABLE 2 (CONTINUED)
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There are two distinct areas in ASTthat need to be understood: precipitatingdefects (which we have just discussed), anddetecting failures. These topics are equallyimportant. Most people involved in testinghave a good idea of how to induce failures.Failure detection, on the other hand,usually causes confusion. The mostimportant thing to remember about ASTis that if you precipitate failures withoutdetecting them you have accomplished nothing.
Due to the cost and complexity involved,many companies do not implementoperational Failure Detection Systems(FDS) while testing their products.This can be extremely dangerous and/orcounter productive. Remember, wheneveryou apply any stress you start using upthe products fatigue life. A number ofpublished cases have shown that a highpercentage of the total failures are softfailures. A soft failure means that a failureoccurs at some condition(s) and thennormal operation resumes as the condi-tion(s) change. These are the most difficultfailures to catch, and they will not beidentified unless the product is continuous-ly monitored. Failure detection must alsobe correlated to the stress levels applied.This also requires continuous monitoringof the product under test.
After a failure is identified, Failure ModeAnalysis (FMA) is then performed. Thisrequires knowledge of what failed, whyit failed, when it failed, where it failed,and how it failed. After a complete FMAis done, you can apply what you havelearned to implement the correctiveaction.
The corresponding corrective action isthe key to closing the loop for a properlydesigned AST program. This is wherethe product improvement is obtained. Productimprovement is gained by first precipitat-ing the hidden flaw(s), identifying theflaw(s), and correcting the flaw(s). Noneof the three can stand without the others.
FAILURE IDENTIFICATION
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Continuous monitoring requires severalconsiderations regarding the equipmentrequired to do the actual test. To getsignals to and from products under testcan require fixturing, interconnects, powersupplies, product loads, and productstimulus and monitoring instrumentation.Because typically there are a number ofinterconnects to the product, care must betaken to insure that the interconnect doesnot fail, producing false failures. To insurereliable interconnect and fixturing thefollowing items should be evaluated:
■ Material
■ Structural Integrity
■ Product/Operator Interface
■ Replaceability
■ Reliability
■ Serviceability
■ Adjustability
Remember, everything that is in thestressing environments is subjected to thesame stresses as the product under test.
A quality power supply is also essential,especially when considering electricalstress tests. A poor power supply couldfluctuate incorrectly and damage your testproduct. A good power supply should beaccurate, programmable, flexible, andreliable. Most commercially availablepower supplies today have the featuresthat are required for basic AST testing.There are also some supplies today thatallow specialized testing such as program-mable voltage fluctuations, frequencyvariations, harmonics, flicker and severalother unique functions.
Another very important feature is theoperator interface. The proper operatorinterface makes very complex instrumen-tation easy to use, as well as easy toimplement. Other concerns are the easeof adding additional functions, simplifiedprogramming, and interface with theother AST equipment.
The final component required forimplementing AST product monitoring isthe stimulus and monitoring equipment.Flexibility is the key issue when looking forinstrumentation for AST. This is becauseAST tests are typically done during thedesign phase over a variety of productsand functions.
TOOLS FOR CONTINUOUS MONITORING
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One of the most overlooked areas of product testing is the area of reliabilitymonitoring. Reliability monitoring meanskeeping a record of successes and failuresand creating a database of lessons learned.17
Reliability monitoring allows you to buildon past experiences, which improves overall reliability and helps companiesavoid costly mistakes time and timeagain. This database becomes a referencetool for design engineers to expediteproper design and provides an excellentknowledge-sharing tool.
RELIABILITY GROWTH MONITORING
This booklet is just a quick overview of thearea of AST. We have covered what ASTis, how it compares to life testing, types ofAST, stress environments, and the types ofdefects that are precipitated. We have alsolooked at the importance of continuouslymonitoring the product under test. Theseare just a few topics a person looking toimplement AST should understand. Thereare several articles listed in the additionalreference area that the reader should findhelpful.
SUMMARY
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Accelerated Life Testing – A process,through which a small percentage ofproducts are subjected to stresses at a higher level than they would experienceduring their use environments during theirproduct lives. This compresses test time.By simulating the product’s life, usingeither actual or accelerated stress, we can determine actually where a particularfailure would occur and at exactly whattime in a product’s life.
Accelerated Stress Testing – Applyinghigh levels of stress to a product for shortperiods of time assuming it will exhibit thesame failure mechanisms as it would in alonger amount of time at lower stress levels. Accelerated stress testing employsvarious stresses to precipitate failures whichin turn need to be analyzed and the propercorrective action implemented. AST isstimulation testing.
AFDF (Accumulated Fatigue damageFactor) – The AFDF uses Minor’s Ruleto sum fatigue damage. The well knownMinor’s Rule states that an S-N Curvecan be used to compute total fatiguedamage. It should be noted that S-N Datais taken using Sign Loading. Whenapplying AFDF to a product, it must beremembered that each product is uniqueand complex, and that the product consistsof more than one type of material. It mustalso be remembered that each particularstressing machine, the way that theproduct is placed on the machine, fixturetransmissibility, table response, and actualrates of each individual material, all affectany possible AFDF estimate. Therefore,AFDF’s estimates are far from accurate.
Aging, Life-Aging or Product Maturing –Simultaneously operating and/or stressinga product in order to advance its maturitythrough infancy and to uncover latentdefects.
Board Level – A level at which circuitboards are tested or screened prior to subor final assembly. Components are in placeand the product can be operated withproper interconnects. The only addedcomplexity is usually in packaging.
Burn-in or Steady-State Burn-in – The process of continuously powering aproduct; usually at a constant temperature,in order to accelerate the aging process. Inburn-in, it is very common to elevate thetemperature well beyond normal operationtemperatures, to accelerate the process ofdefect precipitation.
Chamber – A cabinet in which productsare placed to subject them to stress orstresses. This usually consists of an insulatedunit equipped with air circulation andconditioning equipment.
Component Level – A level at whichcomponents are screened or tested beforebeing placed on circuit boards.Components may be screened or tested bythe manufacturer or by the user uponreceipt.
CA (Corrective Action) – The actiontaken, after understanding the root causeof a failure, to eliminate the source of thedefect in future design and productionrevisions.
DEFINITIONS
25
DDT (Device Defect Tracking) – A database which tracks a product’s defects fromtheir discoveries; examines root causes,documents corrective actions, and detailsmethodologies to prevent reoccurrencein the future. Companies can apply thisknowledge to products that are beingcurrently developed, so that the samemistakes do not reoccur.
Defect Precipitation – Changing a latentdefect to a patent defect; or simply makingan undetected failure, detectable.
Design Limits – The operational limit ofa product beyond which point the productwill not operate correctly and is considereddefective.
Design Ruggedization – The processof finding weak links in the design andfixing them so that the design becomesmore robust. Stresses used in DesignRuggedization are much greater than fieldenvironments.
Destruct Limits (Refer to Design Limits)
DT (Destructive Test) – A test orphysical analysis which destroys theproduct after analysis completion.
DUT – Device Under Test
EMI – Electro-magnetic Interference
Environmental Testing – A process inwhich a sample of products is subjected toenvironmental simulation or stimulation.Testing is used in variety of ways. Aprototype can be tested using extremestresses to confirm the range in which itwas designed to operate. A randomlyselected product can be tested at anextreme temperature range to confirmcontinuing design and production processcompliance. A product can be tested todetermine MTBF (Mean Time BetweenFailures). Testing can be used to simulatethe environments in which the productswill encounter in transportation andoperation. In AST, environmental testingtypically applies to the accelerated stressesapplied to the product to acceleratepotential failure mechanisms.
ESS (Environmental Stress Screening) –Unlike Accelerated Stress Testing, ESS isa process in which 100% of the productsare subjected to one or more stresses withthe intent of forcing latent defects toprecipitate failures. ESS may involve anyor all of the following:
■ Thermal Cycling
■ Vibration
■ High Temperature Burn-In
■ Electrical Stress
■ Thermal Shock
■ Humidity
■ Low Temperature
■ Altitude
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Failures – The most common types of failure are:
Critical Failure – A case where theproduct is unable to operate under theconditions it is expected to.
Non-Critical Failure – A failure thatoccurs only outside of the normaloperating range of the product.
Hard Failure – A case where the prod-uct stops and does not resume function-ing. These are always designated asCritical Failures.
Soft Failure – A case where the productstops functioning under certain condi-tions, but then resumes operating underothers. For example, when a product ispowered, various stresses are applied, aparticular stress or a combination ofstresses may be the cause of the productto stop operating. When the stress isremoved, the product may start func-tioning again. This typeof failure may either be Critical (if theproduct fails under the condition inwhich it must operate in the field) orNon-Critical (if the product fails underconditions which the product will neverbe exposed to in the field).
Infancy Failure – Failures that occur inthe early stages of the product life.Generally, most product failures occur atthis stage.
Electrical Failures – An electricalmalfunction caused by electricalcomponents, connections, switches, orrelated devices.
Mechanical Failures – A mechanicalmalfunction caused by cracking, dis-placement, misalignment, loosening ofnutsand bolts, etc.
Process Failures – Failures caused bythe manufacturing process. These fail-ures result mainly from mis-loaded com-ponents;damage caused by improper handling; orinadequate assembly procedures.
Failure Mode Analysis – A procedurewhich is used to identify the cause of thedefect and understand the defect so thatit is not repeated.
Failure Mechanism – The mechanical,physical, chemical or other process thatresults in failure.
Failure Mode – The manifestation of afailure mechanism at a failure site. Theeffect by which a failure is observed.
Failure Site – The location of a failure.
Fatigue Life – The amount of time underdefined operation conditions that aproduct is expected to survive.
Fixturing – A device or series of devicesused to secure a product within a chamberor on a vibration system, in preparation torunning screens or tests.
27
Frequency Margining – The use of varyingfrequencies on product input to determineacceptable operational limits of theproduct.
GRMS – This is acceleration due to gravityin the root mean squared, or the averageintensity of the signal.
HALT – Highly Accelerated Life Test
HASS – Highly Accelerated StressScreens
Latent Defect – A flaw in the part and/orworkmanship that is not immediatelyapparent, but that can result in failures.A defect that is dormant and is not visible,or is undetectable.
Life Cycle Testing – A process throughwhich a small percentage of products aresubjected to stresses similar to those thatthey will experience during their productlives. This process is usually used todetermine a product’s anticipated lifeexpectancy and its MTBF.
LN2 (Liquid Nitrogen) – Used as acooling media in some Thermal CyclingSystems.
MTBF – Mean Time Between Failures
Product Monitoring – While applyingstress to the product, the product is operatedand is monitored to insure it is performingits’ intended design functions. This isrequired to identify many of the failuresfound during Accelerated Stress Testing.In most cases, it is the only way to catchsoft failures.
Power Cycling of Operational Cycling –The process of continuously turning theproduct on and off at predeterminedintervals. Product Cycling adds internalheat and ages the product faster thancontinuous power on and off.
Operational Limit – The upper and lowerlimits where the product will not operatewith its’ intended design functionality.
Parametric Drift – The shift in a knownparameter versus time when stress isapplied.
Patent Defect – An open or evidentdefect. Defect precipitation. Changing alatent defect to a patent defect; or simplymaking an undetected failure detectable.
Product Specification – The specificationsof the product manufacturer as communi-cated to its’ customers.
Proof of Screen – This is a process that isused when determining production (ESS)screens for products. It is used to show thatthe screen is effective in finding defectspresent in the product, but yet does notdamage good hardware.
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Random Vibration – A form of stress inwhich broad band, multiple frequencies arepresent simultaneously at predeterminedpower levels. The product is attached to ashaker and vibrated in one or more axisduring the stress cycle.
RGDT (Reliability Growth DevelopmentTesting) – A test in which normal operat-ing stresses are applied to products. Whendefects are detected, corrective action istaken.
Root Cause – Understanding what hascaused a particular failure. It is importantto determine the root cause when a latentdefect is precipitated.
RS (Repetitive Shock) Vibration –Vibration which is typically generatedusing pneumatic hammers striking a plateupon which the product is attached.Commonly, Repetitive Shock Machinesprovide vibration in three (3) distinct axesand three (3) rotational axis simultaneously.
Screening – The process of stressing products so that defective units may beidentified and repaired or replaced.
Step Stressing – Incrementally increasingstress levels in known increments.
Stress – Intensity at the failure site ofapplied load.
SRS (Shock Response Spectrum) – SRSis a time-domain function sensitive topeaks of excitation. SRS can be a valuableanalysis tool when looking at theexcitation from RS Vibration Machines(Repetitive Shock).
TAAF (Test/Analyze and Fix) – A testwhere normal operating environments arestimulated. The problems are identified,analyzed, and corrected.
Thermal Cycling – A process throughwhich a product is subjected to predeter-mined temperature change rates betweenestablished temperature extremes. InAccelerated Life Testing Thermal-Cycling,the ability to develop fast rates of changewill determine how many cycles arerequired to force the greatest number oflatent defects into failures.
Voltage Margining – Stress applied tothe product by varying voltage, whichdetermines the voltage at which normaloperation can be expected.
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REFERENCES
1 O’Conner, P.D. (1993). Quality and Reliability:
Illusions and Realities. Quality and Reliability
Engineering International, 9:163-168.
2 Dasgupta, A. (April 1996). Cost-Effective Integrated
Product Development. CALCE Accelerated Test
Workshop.
3 Caruso, H. (1996). An Overview of Environmental
Reliability Testing. Proceedings, Reliability and
Maintainability Symposium, 107.
4 Juvinall, R.C., & Marshek, K.M. (1991).
Fundamentals of Machine Component Design,
(2nd ed.). John Wiley & Sons, 288-289.
5 Dasgupta, A. (April 1996). Physics-of-Failure (PoF)
Principles For Accelerated Stress Tests. CALCE
Accelerated Test Workshop.
6 Chan, H.A. (October 1997). Overview of Accelerated
Stress Testing Principle. 3rd IEEE Workshop on
Accelerated Stress Testing.
7 Diekema, J. (1987). The ESS Handbook. Thermotron
Industries.
8 Hobbs, G. (May 23-24, 1996). Advanced HALT/HASS
Seminar.
9 Tustin, W. (October 1997). Electrodynamic versus
Pneumatic Shakers for Stress Testing. 3rd IEEE
Workshop on Accelerated Stress Testing.
10 Crandell, S.H., & Mark, W.D. (1963). Random
Vibration in Mechanical Systems. NY: Academic Press.
11 Head, R. (February 15, 1997). Test Methodology
Interview. Chrysler-Huntsville Electronics.
12 Darby, B. (February 15, 1997). Test Methodology
Interview. Chrysler-Huntsville Electronics.
13 Kececioglu, D. (1993). Reliability and Life Testing
Handbook, (Vol I & II). Englewood Cliffs, NJ: PTR
Prentice-Hall, Inc.
14 Upton, J. (May 3, 1997). AST Implementation.
MA/COM.
15 Barsom, J.M., & Rolfe, S.T. (1987). Fracture and
Fatigue Control in Structures, (2nd ed.). Englewood
Cliffs, NJ: Prentice-Hall.
16 Caruso, H. (1997). Huntsville, AL: TTI Course.
17 Crowe, D., & Feinberr, A. (1998). Stage-Gating
Accelerated Reliability Growth in an Industrial
Environment. IEST Proceedings, 246-254.
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RECOMMENDED INFORMATION SOURCES
– Anon. (1997). SAE Fatigue Design Handbook.
Soc. Auto Eng. Doc. AE-22 (3rd ed.).
– Barsom, J.M., & Rolfe S.T. (1987). Fracture and
Fatigue Control in Structures, (2nd ed.). Englewood
Cliffs, NJ: Prentice-Hall.
– IEEE/CMPT TC7 Annual AST Workshops
Contact IEEE at (800) 677-IEEE.
www.cmpt.org/tc/tc7.html
– Institute of Environmental Sciences
940 E. Northwest Highway, Mt. Prospect, IL 60056;
or fax to (847) 255-1699.
– Kececioglu, D. (1993). Reliability and Life Testing
Handbook, (Vol I & II). Englewood Cliffs, NJ: PTR
Prentice-Hall, Inc.
– Reliability And Maintainability Symposium
www.rams.org
– Wirsching, P.H., Paez, T.L., & Oritz, K. (1995).
Random Vibrations. NY: John Wiley & Sons.
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