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Reliability Plus - Product reliability terms Category: General technical and industrial > Product reliability Date & country: 16/12/2007, UK Words: 64

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

g RMS
Gravity root mean square â€` 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

Vibration machine (or shaker)
A device which produces controlled and reproducible mechanical vibration for the vibration testing of mechanical systems, components and structures

Vibration
Mechanical oscillation or motion about a reference point of equilibrium

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

Tri-axial
(Vibration in) three linear orthogonal axes

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

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

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

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

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

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

Stress Margin Evaluation (SME)
An alternative term used to describe step stressing a UUT with thermal and random vibration stressors

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

MTTF
Abbreviation for mean (or average) time to failure

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

MTBF
Abbreviation for mean (or average) time between failures

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

LN2
Liquid nitrogen, often used for rapid cooling of environmental test chambers. LN2 boils at â€`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

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

hertz
(Abbreviated Hz) The unit of frequency

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

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

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

HALT
Highly accelerated life test. See accelerated life test

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

Fundamental mode of vibration
That mode having the lowest natural frequency

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

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

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

DoF
See Degrees of freedom

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

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

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

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

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

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

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

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