## Accelerated Life Testing

An activity during development of a new product. Prototypes are subjected to stress levels (including vibration, usually random) that are much higher than those anticipated in the field. The purpose is to identify failure-prone, marginally-strong elements by causing them to fail. Those elements are strengthened and tests are continued at higher levels.

## Accelerated Stress Testing

A post-production activity on a sampling (initially 100%) of units. The intent is to precipitate hidden or latent failures caused by poor workmanship and to prevent flawed units from reaching the next higher level of assembly or the customer. Intensity is determined from levels achieved in accelerated life testing.

## ANOVA

ANOVA stands for analysis of variance, a method by which the source of variability is identified. This method is widely used in industry to help identify the source of potential problems in the production process, and identify whether variation in measured output values is due to variability between various manufacturing processes, or within them. By varying the factors in a predetermined pattern and analyzing the output, one can use statistical techniques to make an accurate assessment as to the cause of variation in a manufacturing process.

## Arrhenius Model

A model used in accelerated life testing to establish a relationship between absolute temperature and reliability. It was originally developed by Swedish chemist Svante Arrhenius to define the relationship between temperature and the rates of chemical reaction.

## Availability

Availability is the probability that an item will be able to function (i.e. not failed or undergoing repair) when called upon to do so. This measure takes into account an item’s reliability (how quickly it fails) and its maintainability (how quickly it can be repaired). The numerical value of availability is expressed as a probability from 0 to 1. Availability calculations take into account both the failures and the repairs of the system.

## Burn-In

The process of operating devices or equipment often under accelerated voltage, temperature, or load in order to screen out infant mortality failures. Most effective for semiconductor components; relatively ineffective for screening circuit boards and assemblies.

## BX Life

The time at which X% of the units in a population will have failed. For example, if an item has a B10 life of 100 hours, that means that 10% of the population will have failed by 100 hours of operation.

## Contour Plot

A graphical representation of the possible solutions to the likelihood ratio equation. This is employed to determine confidence bounds as well as comparisons between two different data sets.

## Cumulative damage model

An accelerated life testing model used to model accelerated tests where the stress levels vary with time.

## Cumulative Density Function (CDF)

A function obtained by integrating the failure distribution pdf. In life data analysis, the cdf is equivalent to the unreliability function.

## Decomposition Method

A method for determining the reliability of complex systems. The decomposition method is an application of the law of total probability, which involves choosing a “key” component and then calculating the reliability of the system twice

## Degradation Analysis

Analysis involving the measurement and extrapolation of degradation or performance data that can be directly related to the presumed failure of the product in question. Degradation analysis allows the user to extrapolate to an assumed failure time based on the measurements of degradation or performance over time.

## Degrees of Freedom

In mechanics, the total number of directions of motion of an assembly being evaluated. E.g. a ship or aircraft experiencing linear motion up and down, fore-and-aft and left-and-right motions as well as roll, pitch and yaw, is said to have six degrees of freedom.

## Design Limit

The operational limit of a product, beyond which it not required to function properly.

## Down Time

The amount of time a repairable unit is not operating. This can be due to being in a failed state, administrative delay, waiting for replacement parts to be shipped or undergoing active repair.

## Duane Model

A reliability growth model that uses a relationship between cumulative test time and cumulative failures to develop a reliability growth profile.

## DUT

Device under test. The component, sub-assembly or system being tested. See also UUT.

## DV

Design validation. A series of tests performed to verify that a design meets its specification. Will usually include performance verification under particular environmental test conditions. Does not usually include accelerated stress testsing.

## ED

Electro Dynamic, as in ED shaker. A shaker that generates vibration energy electro-magnetically.

## Environmental Stress Screening (ESS)

A post-production activity on 100% or a batch sample of units. The intent is to precipitate latent defects caused by poor workmanship and to prevent flawed units from reaching the next assembly level or the customer.

## Environmental Testing

Subjecting a sample of products to a simulation of anticipated storage, transport and service environments (such as vibration, shock, temperature, altitude, humidity, etc.)

## Event Space Method

A method for determining the reliability of complex systems. With the event space method, all mutually exclusive events are determined. The reliability of the system is simply the probability of the union of all mutually exclusive events that yield a system success (the unreliability is the probability of the union of all mutually exclusive events that yield a system failure).

## Exponential Distribution

A lifetime statistical distribution that assumes a constant failure rate for the units being modeled.

## Eyring model

An accelerated life testing model based on quantum mechanics for use when temperature is the accelerating factor.

## Failure Analysis

After failure, the logical systematic examination of an item, its construction, application, and documentation to identify that failure made and determine the failure mechanism and its consequences.

## Failure Distribution

A mathematical model that describes the probability of failures occurring over time. Also known as the probability density function (PDF), this function is integrated to obtain the probability that the failure time takes a value in a given time interval. This function is the basis for other important reliability functions, including the reliability function, the failure rate function and the mean life.

## Failure Mechanism

The mechanical, chemical, physical or other process that results in failure.

## Failure mode and effects analysis (FMEA)

A procedure by which each potential failure mode in a system is analyzed to determine its cause and the effects thereof, on the overall system, and to classify each potential failure mode according to its severity.

## Failure Rate

A function that describes the number of failures that can be expected to take place over a given unit of time. The failure rate function has the units of failures per unit time among surviving units, i.e. one failure per month.

## Fisher Matrix

A mathematical expression that is used to determine the variability of estimated parameter values based on the variability of the data used to make the parameter estimates. It is used to determine confidence bounds when using maximum likelihood estimation (MLE) techniques.

## Fixture

The intermediate structure that attaches a device under test (DUT) to a shaker or shock test machine. Special fixtures may also be required to correctly mount product(s) inside a thermal cycling chamber during screening.

## Fundamental Mode of Vibration

That mode having the lowest natural frequency.

## General Log-Linear Model

An accelerated life testing model that can account for multiple non-thermal stresses as acceleration factors.

## Generalized Gamma D

istribution

While not as frequently used for modeling life data as other life distributions, the generalized gamma distribution does have the ability to mimic the attributes of other distributions such as the Weibull or lognormal, based on the values of the distribution’s parameters. While the generalized gamma distribution is not often used to model life data by itself, its ability to behave like other more commonly-used life distributions is sometimes used to determine which of those life distributions should be used to model a particular set of data.

## Gompertz Model

A reliability growth model that models reliability values at different stages of development and produces an S-shaped reliability growth curve.

## HALT

Highly accelerated life test. See accelerated life test.

## HASS

Highly accelerated stress screening. See environmental stress screening (ESS).

## Hard Failure

A product under test ceases to work correctly. It does not resume correct operation, even when the stressing environment is eased. Differs from soft failure.

## Hazard Rate

A function that describes the number of failures that can be expected to take place over a given unit of time. The failure rate function has the units of failures per unit time among surviving units, i.e. one failure per month.

## Importance Measure

A measure of the relative contribution of a component’s contribution to the overall system’s reliability. The importance measure of a component is equivalent to the first partial derivative of the component reliability with respect to the system reliability.

## Inverse Power Law

An accelerated life testing model commonly used when the accelerating factor is a single, non-thermal stress.

## Kaplan-Meier Estimator

This is an estimator used as an alternative to the median ranks method for calculating the estimates of the unreliability for probability plotting purposes. It is also used to determine reliability estimates for nonparametric data analysis.

## Latent Defect

A flaw in the design, of a component part or in the manufacturing process of a product, which is not immediately apparent visually or detectable by testing, but will result in a future failure – usually within the warranty period. See Patent Defect.

## Life Cycle Testing

Subjecting products to stresses similar to those anticipated in actual service while collecting engineering data related to life expectancy, reliability, specification compliance, or product improvements. Usually aimed at determining the products’ mean time between failures or MTBF.

## Life Data Analysis

The statistical analysis of failure and usage data performed in order to be able to mathematically model the reliability and failure characteristics of a product.

## Life Distribution

A mathematical model that describes the probability of failures occurring over time. Also known as the probability density function (pdf), this function is integrated to obtain the probability that the failure time takes a value in a given time interval. This function is the basis for other important reliability functions, including the reliability function, the failure rate function and the mean life.

## Likelihood Function

A function that represents the joint probability of all the points in a data set. For complete data, the likelihood function consists of the product of the pdf for each data point; for data sets that also include suspended or censored data, the likelihood function is more complex. Maximum likelihood estimation (MLE) techniques maximize this function in order to determine the best parameter estimates.

## Likelihood Ratio

The ratio of a likelihood function for an unknown parameter vector to the likelihood function calculated at the estimated parameter vector. The relationship of this ratio to the chi-squared distribution can then be used to calculate confidence bounds and confidence regions.

## Lloyd-Lipow Model

A reliability growth model based on the number of trials and successes at each stage of product development.

## Lognormal Distribution

A lifetime statistical distribution that is often used to model products in which physical fatigue is the prominent contributor to the primary failure mode.

## Maintainability

The probability that a failed unit will be repaired within a given amount of time. The term is also used to denote the discipline of studying and improving the maintainability of products, primarily by reducing the amount of time required to diagnose and repair failures.

## Maximum Likelihood Estimation (MLE)

A method of parameter estimation involving the maximization of the likelihood equation. The best parameter estimates are obtained by determining the parameter values that maximize the value of the likelihood equation for a particular data set.

## Mean Life

A reliability measure that represents the expected value of the failure times for a failure distribution, also known as the average or central life value. While this represents a useful representative value of a distribution of failure times, it is often over-used as the sole reliability metric.

## Mechanical Failure

A malfunction consisting of cracking, excessive displacement, misalignment, loosening, etc.

## Median Ranks

Measures used to obtain estimates of the unreliability. Median ranks are the values that the true probability of failure should have at the jth failure out of a sample of N units, at a 50% confidence level, or the best estimate for the unreliability. This estimate is based on a solution of the binomial equation.

## Mixed Weibull Distribution

A variation of the Weibull distribution used to model data with distinct subpopulations that may represent different failure characteristics over the lifetime of a product. Each subpopulation has separate Weibull parameters calculated and the results are combined in a mixed Weibull distribution to represent all of the subpopulations in one function.

## Modified Gompertz model

A reliability growth model that models based on a variation of the Gompertz model.

## Monte Carlo Simulation

A method of generating values from a known distribution for the purposes of experimentation. This is accomplished by generating uniform random variables and using them in an inverse reliability equation to produce failure times that would conform to the desired input distribution.

## MTBF

In the case of repairable systems, MTBF stands for mean time between failures. This average time excludes the time spent waiting for repair, being repaired, being re-qualified, and other downing events such as inspections and preventive maintenances and so on; it is intended to measure only the time a system is available and operating. Whereas, in the case of non-repairable systems, MTBF stands for mean time before failure and is represented by the mean life value for a failure distribution of non-repairable units.

## MTTF

MTTF stands for mean time to failure and is represented by the mean life value for a failure distribution of non-repairable units. The total number of life units of an item divided by the total number of failures within that component lot, during a particular measurement interval under stated conditions.

## MTTR

MTTR stands for mean time to repair and is represented by the mean life value for a distribution of repair times.

## NHPP

NHPP stands for non-homogeneous Poisson process, which is a simple parametric model used to represent events with a non-constant failure recurrence rate. This type of model is often used to model reliability growth and the reliability of repairable units.

## Nonparametric Analysis

A method of analysis that allows the user to characterize failure data without assuming an underlying failure distribution. This avoids the potentially large errors brought about by making incorrect assumptions about the distribution. However, the confidence bounds associated with nonparametric analysis are usually much wider than those calculated via parametric analysis. Additionally, predictions outside the range of the observations are not

possible.

## Normal Distribution

A common lifetime statistical distribution that was developed by mathematician C. F. Gauss. The distribution is a continuous, bell-shaped distribution which is symmetric about its mean and can take on values from negative infinity to positive infinity.

## Operational Environment

The aggregate of all external and internal conditions (such as temperature, humidity, radiation, magnetic and electric fields, shock vibration, etc.) either natural or man made, or self-induced, that influences the form, operational performance, reliability or survival of an item.

## Operational Limit

The extremes beyond which a product is not expected to operate.

## Patent Defect

A flaw in the design, of a component part or in the manufacturing process of a product that has failed under test or screen. See Latent Defect.

## Path-Tracing Method

A method for determining the reliability of complex systems. With this method, every path from a starting point to an ending point is considered. Since system success involves having at least one path available from one end of the reliability block diagram to the other, as long as at least one path is available, the system has not failed. The reliability of the system is simply the probability of the union of these paths.

## Proportional Integral Derivative (PID)

Variable parameters used in a thermal chamber control system.

## Probability

A quantitative description of the possible likelihood of a particular event. Probability is conventionally expressed on a scale from 0 to 1, or 0% to 100%, with an unlikely event having a probability close to 0, and a very common event having a probability close to 1.

## Probability Density Function (PDF)

A mathematical model that describes the probability of events occurring over time. This function is integrated to obtain the probability that the event time takes a value in a given time interval. In life data analysis, the event in question is a failure, and the pdf is the basis for other important reliability functions, including the reliability function, the failure rate function and the mean life.

## Probability Plot

A type of plot that linearizes a distribution’s cdf, allowing the user to manually plot failure time vs. estimated unreliability. Provided that the plotted points fall on a relatively straight line (thus indicating that the chosen distribution is a good fit), the parameter estimates can be obtained from scales on the plot. This is a crude, time-consuming method of fitting a distribution to failure data, but it was practically the only method available prior to the widespread use of computers.

## Probability Plotting Paper

A specially designed type of graph paper that allows the user to plot failure time vs. unreliability as a linear function. Plotting paper constructions varies from distribution to distribution.

## Proportional Hazards Model

An accelerated life testing model that can account for multiple non-thermal stresses as acceleration factors.

## Quality

A common buzzword referring to the non-quantifiable point-level excellence of a product or process. While sometimes used interchangeably with the term reliability, quality refers to the characteristics of a product at one point in time, while reliability refers to the characteristics of a product over its entire lifetime.

## Quasi Random

A form of random vibration energy with a frequency spectrum very similar, but not mathematically identical to, white noise derived random vibration energy. A quasi random vibration response is typically generated by a vibration table that utilises pneumatically actuated impact hammers.

## Random Vibration

Vibration whose instantaneous magnitudes cannot be predicted. May be broad-band, covering a wide, continuous frequency range, or narrow band, covering a relatively narrow frequency range. No periodic or deterministic components.

## Range

A statement of the upper and lower limits over which an instrument works satisfactorily.

## Reliability

The probability of an item operating for a given amount of time without failure. More generally, reliability is the capability of parts, components, equipment, products and systems to perform their required functions for desired periods of time without failure, in specified environments and with a desired confidence.

## Reliability Analysis

The statistical analysis of failure and usage data performed in order to be able to mathematically model the reliability and failure characteristics of a product.

## Reliability Growth

The analysis of the change in reliability over time, usually applied to products under development. Reliability growth analysis provides the means by which the reliability, mean life or failure rate is tracked over time, allowing the user to predict future reliability values based on the current rate of growth of the reliability measurement of interest.

## Reliability Importance

A measure of the relative contribution of a component’s contribution to the overall system’s reliability. The importance measure of a component is equivalent to the first partial derivative of the component reliability with respect to the system reliability.

## Reliability Test Design

The process of designing plans for reliability testing.

## Reliability testing

Testing units to failure in order to obtain raw failure time data for life data analysis.

## Repair

An action that restores a failed part or component to operating condition.

## Repair Distribution

A mathematical model that describes the probability of repairs occurring over time.

## Repairable System

A system that can be restored to operating condition after a failure by the repair or replacement of one or more components.

## Repetitive Shock Machine

A platform to which products (to be tested or screened) are attached. Often this platform forms the bottom surface of a thermal test chamber. Pneumatic vibrators are attached to the bottom of the platform, causing it to vibrate, usually simultaneously in several axes.

## Response

The vibratory motion or force that results from some mechanical input.

## Resonance

Forced vibration of a true single DoF system causes resonance when the forcing frequency equals the natural frequency. More complex systems have many resonances.

## Reliability Enhancement Test (RET)

A term sometimes used to describe a range of design validation and accelerated life testing processes.

## Root Cause Analysis

Determining what actually caused a failure, as opposed to what appears to have been the cause. Detailed knowledge of the UUT is required and the services of a specialized failure analysis lab may also be needed.

## Run-In

Continuously powering a product under ambient conditions in order to accelerate the aging process. Unlikely to introduce significant acceleration. More effective when power cycling an assembly which itself dissipates heat, when it becomes a variant of thermal cycling.

## Screening

The process of stressing products so that defective units can be identified, then repaired or replaced.

## Screen Strength

A term used to describe the likelihood that a particular stress screening process will precipitate latent defects in a UUT. Screen strength is usually expressed as a percentage or as a probability, and is best interpreted as a relative guide to the effectiveness of a process.

## SIVL

Super Insulated Vacuum Line. A vacuum-insulated rigid or semi-flexible pipe used to transfer LN2, with the minimum of losses, from a bulk supply tank to an environmental test chamber. SIVL is obtained from and installed by specialist suppliers.

## Sequential testing

A testing methodology in which test units are tested consecutively instead of simultaneously.

## S-N Diagram

Plot of stress (S) against the number of cycles (N) required to cause failure of similar specimens in a fatigue test. Data for each curve on an S-N diagram are obtained by determining fatigue life of a number of specimens subjected to various amounts of fluctuating stress. The s

tress axis can represent stress amplitude, maximum stress or minimum stress. A log scale is almost always used for the N scale and sometimes for the S scale.

## Soft Failure

A product under test ceases to operate correctly, but resumes correct operation when the stressing environment is eased. Also known as an intermittent failure. See hard failure.

**Note:** All solid sate devices, notably memory storage elements, are subject to random “soft-error” failures. Such failures are the consequence of high-energy alpha particles and cosmic radiation which can, for example, change the data in a storage cell. These are random and transient by nature and cannot be avoided.

## Spares Provisioning

The stocking of spare units or components based on the anticipated number of failures for a given mission or length of operation.

## Spectrum Analyzer

An instrument which displays the frequency spectrum of an input signal, usually amplitude vertical vs. frequency horizontal.

## SPRT

SPRT stands for sequential probability ratio test. This is a type of accept/reject sequential testing in which accept/reject boundaries are defined by the user and units are sequentially tested until either the accept boundary or the reject boundary have been reached, and a decision is made about the suitability of the units.

## Statistics

The branch of mathematics that deals with the collection, organization, analysis and interpretation of data.

## Step Stressing

Increasing stresses in a series of pre-selected increments.

## Stress

Intensity of applied load, usually at the site of a failure.

## Stress Concentration

A stress concentration is a phenomenon found in mechanical engineering where an object under load has higher than average local stresses due to its shape. The types of shape that cause these concentrations are cracks, sharp corners, holes and narrowing of the object. High local stresses can cause the object to fail more easily. A counter intuitive method of reducing one of the worst types of stress concentration, a crack, is to drill a large hole at the end of the crack. The drilled hole, with its relatively large diameter, causes less stress concentration than the sharp end of a crack.

## Stress Concentration Factor

Ratio of the greatest stress in the area of a notch or other stress raiser to the corresponding nominal stress. It is a theoretical indication of the effect of stress concentrators on mechanical behaviour.

## Stress Testing

Testing units at stresses higher than what would be encountered during normal operating conditions, usually to induce failures.

## Stress Margin Evaluation (SME)

An alternative term used to describe step stressing a UUT with thermal and random vibration stressors.

## Stress Screening

A modern electronics production tool for precipitating latent defects (such as poor solder connections). Utilizes random vibration and rapid temperature cycling.

## Stress-strength interference

A method by which the probability of failure of an item is calculated by superimposing the distribution of the item’s strength with the distribution of the stress it will encounter during normal usage.

## Suspended Data

Data in which not all of the data points represent failures, i.e. there may be operation times for units that have not failed. Censoring schemes include right-censoring, left-censoring and interval censoring.

## System Reliability

The reliability of an entire system, as opposed to the reliability of its components. The system reliability is defined by the reliability of the components as well as the way the components are arranged reliability-wise.

## Swept-Sine Testing

Sinewave vibration whose frequency is smoothly and continuously varied. Commonly required for sequentially identifying resonances.

## Temperature-Humidity Model

An accelerated life testing model used when the two accelerating factors are temperature and humidity.

## TAAF

Test, Analyze And Fix.

## THALT

Thermal HALT; the part of the HALT process that uses thermal stress only.

## Temperature-Non-Thermal model

An accelerated life testing model used when the two accelerating factors are temperature and another non-thermal stress factor.

## Thermal Cycling

Subjecting a product to predetermined temperature changes, between hot and cold extremes.

## Thermocouple

An electrical device used for temperature measurement. Two dissimilar metals joined together, making a continuous loop.

## Tri-Axial

(Vibration in) three linear orthogonal axes

## Up Time

The amount of time a repairable unit is operating per design.

## UUT

Unit under test. See also DUT or device under test.

## VHALT

Vibration HALT; the part of the HALT process using random vibration stress under ambient conditions

## Vibration

Mechanical oscillation or motion about a reference point of equilibrium.

## Vibration Machine (or Shaker)

A device which produces controlled and reproducible mechanical vibration for the vibration testing of mechanical systems, components and structures.

## Warranty Analysis

The analysis of warranty and return data for the purpose of determining the reliability characteristics of a product.