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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.

AGREE. A type of environmental test chamber designed to implement test procedures defined by the Advisory Group on Reliability of Electronic Equipment.

Ambient conditions. The conditions (e.g. temperature and humidity) characterizing the air or other medium that surrounds the UUT.


Bandwidth. The frequency range in hertz (Hz) within which a measuring system can accurately measure a quantity.

Burn-in. Continuously powering a product at constant elevated temperature, in order to accelerate the aging process. Most effective for semiconductor components; relatively ineffective for screening circuit boards and assemblies. See Run In.

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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.

DoF. See Degrees of freedom.

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.)

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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 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.

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.


g. The acceleration produced by Earth's gravity. By international agreement, the value for 1 gravitational unit is ~9.81 ms-².

g RMS. Gravity root mean square – the unit of vibration acceleration. When specifying the g RMS figure for random vibration energy the frequency range (or bandwidth) of the applied vibration must always be stated for the figure to be meaningful.


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.

Harmonic. A sinusoidal quantity having a frequency that is an integral multiple (x2, x3, etc.) of a fundamental (x1) frequency.

hertz. (Abbreviated Hz) The unit of frequency.

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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.

LN2. Liquid nitrogen, often used for rapid cooling of environmental test chambers. LN2 boils at –196°C and is stored in highly insulated, double-skinned, bulk storage, pressure vessels. Storage vessels must constantly allow excess pressure to be vented to atmosphere. Eventually all stored liquid will evaporate to gaseous nitrogen whether or not it is used for cooling purposes.


Mean-Time-Between-Failures (MTBF).is the average time between failures, the reciprocal of the failure rate in the special case when failure rate is constant. Calculations of MTBF assume that a system is fixed, after each failure, and then returned to service immediately after failure.

Mean-Time-To-Failure (MTTF). A basic measure of reliability for non-repairable items: The total number of life units of an item divided by the total number of failures within that population, during a particular measurement interval under stated conditions.

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

MTBF. Abbreviation for mean (or average) time between failures.

MTTF. Abbreviation for mean (or average) time to failure.

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Natural frequency. The frequency of an undamped system's free vibration; also, the frequency of any of the normal modes of vibration. Natural frequency drops when damping is present.

N2. Nitrogen (gaseous). Not useful as a cooling medium for environmental chambers.

The by-product of allowing LN2 to evaporate. Valuable during thermal screening since it ensures extremely dry ambient conditions within a thermal chamber and greatly reduces the likelihood of condensation forming on sensitive electrical assemblies.

The earth’s atmosphere comprises ~78% nitrogen. Nitrogen itself is not toxic, however, an excess within an enclosed area will substantially reduce the amount of available oxygen which then poses asphyxiation risk. Also, nitrogen gas, as it is vented from an environmental chamber will periodically be very cold, and appropriate precautions must be taken. For detailed information about safety issues please refer to cryogenic gas suppliers.


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.

PID. Proportional Integral Derivative. Variable parameters used in a thermal chamber control system.

Power spectral density or PSD. Describes the power of random vibration intensity, in mean-square acceleration per frequency unit, typically g²/Hz. Also known as Acceleration spectral density or ASD.

Proof of screen. A process aimed at showing that a screen is effective in identifying latent defects in products but does not damage good products.

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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. Reliability is the probability a device or system will NOT fail to perform its intended function(s) during a specified time interval when operated under stated conditions

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

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.

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.

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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.

Theoretical screen strengths for thermal and vibration stress may be calculated using the so-called “Hughes Equations”. The equations are empirical calculations (only) and are based on original experiments and investigations by the Hughes Aircraft Company. Ref: Rome Air Development Centre Technical report TR-82-27 (1982)

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.

Six DOF machine.

(a) Repetitive Shock. One design of random vibration producing machine that uses air driven impact hammers to excite an un-damped solid or rigid vibrating table onto which products are directly attached. The resulting excitation is a combination of impulse and table modal responses.

(b) Compare with a Damped Segmented design of vibrating table which also uses air driven impact hammers to produce modally rich random vibration energy that is well distributed in all axes at all table locations.

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 stress 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.

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

Step stressing. Increasing stresses in a series of pre-selected increments.

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

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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 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.

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


TAAF. Test, Analyze And Fix.

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

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


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.


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