Glossary of Reliability Terms

Accelerometer:  A transducer that measures acceleration.  Typically based on piezo-electronic technology, the device provides a voltage output proportional to the acceleration at the location and in the defined direction of the device.  Typically several are attached to the UUT during and are used to monitor the UUT response to the vibration input from the table.

Air Control Thermocouple:  A thermocouple that provides air temperature feedback to the chamber thermal control system.  Depending on the control system settings, the air control thermocouple can act as a temperature safety limit controller, or as the control thermocouple for the system.

Alternating Low Level Vibration,  Also referenced as “Modulated Vibration” or “Tickle Vibration”:  A vibration test procedure that is typically introduced in HALT (Highly Accelerated Life Test) Vibration Step Stressing at input levels of 30 gRMS and higher.  At the end of each vibration dwell and functional test, the vibration level is reduced to 5 gRMS and a complete functional test is conducted, without waiting for a dwell at that level.  Then, the vibration level is increased to the next step level.  Alternating low level vibration will help detect a failure mode that is precipitated at a higher gRMS level, but is not detectable at that level, and is also not detectable when the UUT is at rest.  Low levels of vibration, however, can make the failure mode detectable.  Certain types of cracked solder joints, including through-hole solder issues, can be identified this way.

Broadband Vibration:  Vibration that contains energy over a broad frequency range (i.e. 10 Hz to 5 kHz or greater).

Corrective Action (CA):  A change implemented in a design or process to eliminate a product weakness or flaw.  Corrective actions may include changes in parts or material sources, product design, and production process changes.

Destruct Limit (DL):  The stress level where a failure is identified and UUT functionality is not recovered when the stress is reduced.  This type of failure is also known as a hard failure.  Vibration step stressing will reveal the Vibration Destruct Limit (VDL), while cold thermal step stressing will reveal the Lower Destruct Limit (LDL) and hot thermal step stressing will reveal the Upper Destruct Limit (UDL).

ED (Electrodynamic) Shaker:  A vibration system consisting of a table structure that is connected to a ferrous armature surrounded by a coil.  This coil is separated by an air gap from a second coil in the body of the shaker system.  Controlled current is passed through the coils to create an electromagnetic force between them that moves the table in a single axis.  The UUT is mounted to the table structure.  The spectral and real time content of the driving force is specifically controlled to meet a desired vibration profile.

Fast Fourier Transform (FFT):  An algorithm for describing a real time signal in terms of its component frequencies.  Used in vibration analysis to provide magnitude and frequency information about the frequency components of an acceleration signal. 

Functional Test:  A test of the UUT that measures functionality, UUT operation, and critical parameters to determine if UUT fails to perform to specifications or degradation has occurred within the UUT. This test may include internal diagnostics.  Functional testing is to be performed throughout the thermal and vibration stresses of a HALT (Highly Accelerated Life Test) test.

gRMS:  The RMS (Root Mean Square) level of an acceleration signal, normalized to the value of acceleration due to gravity.   gRMS is measured across a specified frequency range, depending on the cutoff frequency of the filter on the acceleration level.  gRMS can be measured both in the time domain, using analog RMS converter circuitry, and through sampling and FFT analysis, then calculating the square root of the area under the Power Spectral Density curve, cut off at the desired frequency (Figure gRMS is typically used as a measurement of the vibration energy present in a repetitive vibration signal.  For vibration from an RS machine, the frequency cutoff used is typically 5 kHz.

HALT (Highly Accelerated Life Test), also referenced as “HALT test”:  A process that utilizes a step stress approach to subject a UUT to thermal and vibration stresses of types and levels beyond what it may see in actual use, but which will rapidly induce failure modes, allowing them to be detected and corrected.  The stresses applied include thermal extremes, extreme thermal ramp rates, 6DoF (Degrees of Freedom) RS (Repetitive Shock) vibration and combinations of these stresses.

HASS (Highly Accelerated Stress Screen): A production screen using the same accelerated techniques as HALT, but derated.  Its purpose is to monitor the manufacturing process for deviations, by screening production units.

HASS Development: A process used to determine and define the appropriate HASS profile manufacturing test screen

HASS Profile: The temperature, vibration levels, SSR control, and number of cycles performed to provide an effective screen using the HASS chamber. 

Hard Failure:  A non-recoverable failure mode.  A hard failure will not go away when the stress that induced the failure is reduced or removed.  A hard failure will define the Destruct Limit of a UUT.

Isolation; thermal and vibration:  Techniques used to protect portions of a UUT from the stresses of the HALT test system in order to debug failure modes or to work around a known issue and continue testing the rest of the UUT.

NTF, (No Trouble Found), Also referenced as NFF, No Fault Found; NPF, No Problem Found, or  NDF, No Defect Found;  A product failure mechanism that has occurred in use but cannot be reproduced on the bench when returned to manufacturer.

Operational Limit (OL),  Also referenced as “Operating Limit”:  The stress level prior to where a failure is identified but UUT functionality is recovered when the stress is reduced.  This type of failure is also known as a soft failure.  Vibration step stressing will reveal the Vibration Operational Limit (VOL), while cold thermal step stressing will reveal the Lower Operational Limit (LOL) and hot thermal step stressing will reveal the Upper Operational Limit (UOL).

Parametrically Marginal Unit: A unit that has been determined to be marginal based on functional testing results.

Picket Fencing:  A description of the shape of the PSD (Power Spectral Density) of an RS vibration system with actuators that are all striking at the same frequency but at slightly different times.  Such a system will result in a table spectrum with high vibration levels at the actuator strike frequency and its harmonics, with little energy between those harmonics.  The resulting PSD resembles a ‘picket fence’.

Power Spectral Density (PSD):  A measurement of the amplitude and frequency content of a repetitive vibration spectrum, expressed in units of grms2/Hz; also commonly referred to as the Acceleration Spectral Density (ASD).  The shape of the PSD graph provides a visual representation of the relative levels of vibration across the frequency range of interest, allowing analysis of the effectiveness of vibration fixturing and insight into the UUT’s transmissive and absorptive characteristics.

Product Control Thermocouple:  A thermocouple that provides product temperature information to the chamber thermal control system.  Typically the chamber control system is set to use the temperature level of this thermocouple to control the thermal system, although control from the air temperature thermocouple is also possible.

Product Control Accelerometer:  An accelerometer that provides table vibration level information to the chamber vibration control system.  The chamber control system uses the vibration level of this accelerometer to control the vibration system.

Production HASS Test: The process of screening manufactured products using accelerated stress test techniques.

Quasi-Repetitive Vibration:  Non-periodic vibration that possesses all frequencies within a band and having a continuously varying amplitude and phase that is specified as a probability that it will exceed a given value during a time interval.

Return on Investment (ROI):  A calculation used to determine the value of a proposed investment. It is calculated as the ratio of the amount gained (taken as positive), or lost (taken as negative), relative to the amount invested.  When considered specifically for HALT testing, it is the ratio of the amount of money saved by discovering and correcting faults by using HALT testing relative to the cost of performing the HALT test, including capital equipment, infrastructure, personnel, etc.

Repetitive Shock (RS) Vibration:   Vibration originating from a repeated shock impulse excitation to a table supported by springs. Typically created from pneumatic actuators impacting a vibration table to which UUTs are attached.

Root Cause Analysis (RCA):  Identifying the true cause of a weakness or flaw; fully understanding what failed and why.  This process may require use of failure analysis tools like scanning electron microscopes.

Seeded Defects:  Defects that are intentionally inserted into a test unit to determine the effectiveness of the HASS screen.  These defects attempt to replicate a manufacturing process shifting out of control.  Typical seeding would include; cold solder joints, or other types of solder defects, excessive mechanical bend radius of component leads or nicking a component lead, or incorrect or improper component insertions.

Six Degree of Freedom Vibration (6DoF):  Vibration that has simultaneous acceleration energies in three axes (X, Y and Z) and the three rotations (roll, pitch and yaw) around those axes.

Soft Failure:  A recoverable failure mode.  A soft failure will go away when the stress that induced the failure is reduced or removed.  A soft failure will define the Operational Limit of a product.

Step Stressing:  A stress testing process where a stress level is incrementally increased to identify UUT weaknesses and limitations.

Thermocouples:  Temperature sensors that are created when two dissimilar metals are fused, creating a thermoelectric current flow and a resultant voltage potential proportional to temperature.

Thermal Ramp Rate:  The rate of change of temperature in a system.  When used as a thermal system specification, it is the fastest change rate the system can achieve. The measurement point is typically in the air at a centrally located point in the chamber, near the location of a UUT.  It can also be specified and measured on specific areas or components on a UUT as desired.  It can be used to evaluate chamber thermal performance as well as the effectiveness of the transmission of thermal energy to the UUT.  Higher ramp rates result in more stress on the UUT and hence a more effective test.

Triaxial Accelerometer:  Three accelerometers mounted in a single package with three outputs, to monitor x, y and z axes simultaneously.

Unit Under Test (UUT), Also referenced as DUT, Device Under Test: the device being HALT tested..

Acronyms Encountered When Performing HALT and HASS

6DoF Six Degrees of Freedom
ASD Acceleration Spectral Density
CA  Corrective Action
DL  Destruct Limit
DoF  Degrees of Freedom
DUT  Device Under Test
ED Electrodynamic
FFT Fast Fourier Transform 
HALT Highly Accelerated Life Test
HASS Highly Accelerated Stress Screen
LDL Lower Destruct Limit
LOL Lower Operating Limit
NDF No Defect Found
NFF No Fault Found
NPF No Problem Found
NTF No Trouble Found
OL Operating Limit
PSD Power Spectral Density
RCA Root Cause Analysis
RMS Root Mean Square
ROI Return on Investment
RS Repetitive Shock
UDL Upper Destruct Limit
UOL  Upper Operating Limit
UUT Unit Under Test
VDL Vibration Destruct Limit
VOL Vibration Operational Limit

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