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TESTING ELECTRONIC PRODUCTS AND
STRESS SCREENING
1
STRESS SCREENING
6.1
STRESS SCREENING
2
STRESS SCREENING
ENVIRONMENTAL STRESS SCREENING
The intent ofenvironmental stress screening (ESS)is
“ toaccelerate the defect failure mechanisms of the units in the factory, to avoid
earlyfailuresin thefieldorwheninusewiththecustomers”.
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STRESS SCREENING
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earlyfailuresin thefieldorwheninusewiththecustomers”.
To achieve this
“we apply in the factoryexcessive stresseswithout damaging and shortening
the useful life of the units” .
Defects areweaknesses or flaws in a product.
They can be of 2 types :
• inherentor
• induced
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STRESS SCREENING
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They happen generally because of
• substandard materials or
• faulty processes.
Patent defect:
A patent defectcan be defined as
“a condition which does not meet specificationbut is readilydetectableby an
inspection ortestprocedurewithouttheneedforstressscreens.
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STRESS SCREENING
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inspection ortestprocedurewithouttheneedforstressscreens.
Patent defectsrepresent the majority of the defectpopulation in electronic
equipment.
Latent defects:
Latent defectsare those thatcannot be detectedimmediately by conventional
means.
If undetected, latent defects will appear as early failuresin the operating
environment.
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STRESS SCREENING
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Latent defectscannot be detected until they are transformed into patent
defectsby environmental stress applied over time.
Environmental Stress Screening (ESS) accelerate latent defects that would
otherwise occur during a product's early months in the field.
Screeningprocessmustbeappliedtoallproducts,ratherthansamples.
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Screeningprocessmustbeappliedtoallproducts,ratherthansamples.
ESS have been especially used inmilitary electronicapplications, but recently it
is becoming increasingly utilised in commercial electronicstoo.
Utilizing ESS to complement the conventional system of test and repairincreases
production cost.
However, this increase in production costs can be small in comparison to the
reduction in
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STRESS SCREENING
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–warranty
and
–maintenance
In most cases, the support costsof not screening is much morethan the cost of
screening.
Investment in environmental(thermalandvibration)testingmayresultin upto75
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STRESS SCREENING
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Investment in environmental(thermalandvibration)testingmayresultin upto75
percent of reduction in warranty expenses.
Theeffectiveness of a screen is measured in terms of screening strength,
which indicates the fraction of defects revealed as follows:
Defects Revealed
Screening Strength = ---------------Total Defects
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STRESS SCREENING
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Without damaging the product,
– the greater the strength of the screen,
– the better the product reliability.
Figure given contains twoHazard Functions curvesfor the product :
• Hazard function (b) is without ESS.
• Hazard function (a) is with ESS.
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STRESS SCREENING
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Failure
T (Hoursx10)
20 15 10 5 65 70
a. With ESS
b. Without ESS
Hazard function (b) hasabnormally high hazard ratein the range of lifetimes
from60 to 120 hours.
These early failures are due tolatent defects that were not detectable in the
factory but emerged underenvironmental stresses in field use.
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Failure
T (Hoursx10)
20 15 10 5 65 70
a. With ESS
b. Without ESS
When properly applied, the screens willprecipitate failures while not
degradingordamagingthe product life.
All failures in ESSshould bereworked in the factory, and the repaired
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STRESS SCREENING
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All failures in ESSshould bereworked in the factory, and the repaired
units should be brougth to the same qualityas the defect-free units.
With ESS,MTTF (Mean Time To Failure)is improved to a much better value.
Perhaps more important, the early-life field reliability, wherein customer opinions
areoftenformed, issubstantiallyimproved.
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STRESS SCREENING
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areoftenformed, issubstantiallyimproved.
Typically ESS techniques are
–random vibration with a multiaxis vibration sourcelike electrodynamicor
pneumaticshaker tables
and/or
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STRESS SCREENING
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and/or
–thermal cyclingwith a temperature cycling chamber
Vibration and thermal screens are conducted either simultaneously or
sequentially.
Thermal stress levels are typically at temperatures between
50°C and + 100°C.
Vibrationscreensvibratethe test articleatawiderange offrequencies withan
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Vibrationscreensvibratethe test articleatawiderange offrequencies withan
average acceleration of up to6 g.
In general, vibration screens are more effectivethan are thermal screens for
detecting :
– loose contacts,
– debris(çِkme)
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STRESS SCREENING
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– debris(çِkme)
– mounting problems,
– inadequate strain relief,
and
– loose hardware.
Thermal screens are generally more effective for detecting
– partparameter drift,
– contamination,
– poor bonding,
– defectiveconformalcoating,
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STRESS SCREENING
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– defectiveconformalcoating,
and
– improper crimp or mating defects.
Vibration Stress Screening
Screening vibration levels may besignificantly higherthan the field vibration
level, but usually for a much shorter periodof time.
For example, a typical military screen requires an input force of
6gramsfor10minutesin eachofthethreeaxes.
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STRESS SCREENING
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6gramsfor10minutesin eachofthethreeaxes.
Commercial standards have not been established for random vibration screens.
Thermal Stress Screening
Thermal stress screening (TSS) is an
assembly-level electronics manufacturing processthat stimulates latent defects
bysubjectingtheproducttotemperaturecycling.
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STRESS SCREENING
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bysubjectingtheproducttotemperaturecycling.
The materials used to fabricate an electronic device have different thermal
coefficientsof expansion.
Byexpanding and contracting materials at different rates with temperature
changes,thermal stresses are imposed on circuit materials and stimulation of
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STRESS SCREENING
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defects is achieved.
The induced mechanical stresses, activate fatigue mechanisms in the
microstructures of metals, polymers, and dielectrics.
Many defects are related tosoldering problems, and solder defects are
susceptible to thetemperature changesseen in thermal screening.
Thermalcyclingwithpowerappliedto the productwillhaveincreasing effecton
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Thermalcyclingwithpowerappliedto the productwillhaveincreasing effecton
the stress at conductor cracks.
Some defects are revealed by acombination of thermal and vibrationsereens.
Thetransitionanddwelltimes should be determined by analysis, depending
upon thethermal mass properties of the product.
Therecommendedrateoftemperaturechangeisbetween
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STRESS SCREENING
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Therecommendedrateoftemperaturechangeisbetween
½°C and 20°C per minutewith
dwell timesbetween 1 and 10 minutes.
To provide the best thermal-cycling operation, the handbooks recommend
“setting temperature extremes as far apart as possible within the product's
tolerance limits, starting with differences of at least 100°C”.
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STRESS SCREENING
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Equipment should provide atemperature rate of change of
at least 5°C/minute.
To permit the product's temperature profile to closely follow the air temperature in
the chamber,airflow velocityshould exceed 750 fpm(feet/minute).
Power on/off cyclingduring the screen is recommended as it adds to the stress
on defects.
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STRESS SCREENING
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Power should be off during hot to cold transition to increase the rate of
temperature changeand decrease cycle time.
The assemblyhigh-temperature limitis set by the
“part which is the first to be damaged by high temperature”.
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STRESS SCREENING
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A similar approach defines thelow-temperature limit.
Other ESS techniques are
– High temperature burn-in,
– Cold soak,
– Power on-off cycling.
– ThermalShock
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STRESS SCREENING
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– ThermalShock
– Hermeticity
– Radiation hardness
– Corrosion
Burn-in:
Themost common ESS techniqueis classical burn-in.
Burn-in is a usuallystatic screenthat holds devices, boards, or systems at a
constanthightemperature for apredeterminedtime, from afew hoursupto a
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STRESS SCREENING
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constanthightemperature for apredeterminedtime, from afew hoursupto a
week.
Burn-in temperature must not exceed themaximum safe operating temperature.
Some burn-in procedures, especially for military and other critical applications,
raisehumiditylevels aswellastemperatures.
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STRESS SCREENING
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raisehumiditylevels aswellastemperatures.
Certain military specifications, for example, require a soak at85°C and 85%
relative humidity.
These methods are generally referred to as
highlyacceleratedstresstests,orRAST.
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STRESS SCREENING
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highlyacceleratedstresstests,orRAST.
Recent evidence indicates that hot-to-cold and cold-to-hot temperature
transitions aremore effectiveat revealing problems than maintaining a constant
temperature.
Cold soak:
This is low-temperature burn-in which is similar to its high-temperature
counterpart, except that it occurs substantiallybelow ambient.
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STRESS SCREENING
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Power on-off cycling:
it consists of
– power cycling, which turns product power on and off, and
– voltage margining, which varies board voltages above and below nominal
limits.
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STRESS SCREENING
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A functional test, for example, may run
– 5 V power supplies at 5.2 V
and
– 12- V supplies at 12.5 V
to increase fault coverage.
The approach isinexpensive and easy to control but does not uncover many
defects.
Thermal Shock:
Theoretically, thermal shock is thermal cycling with a zero transition time
between temperature extremes.
Thermal shock uses ahot boxand acold box, with the product moving between
them manually or on a conveyor.
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STRESS SCREENING
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The technique can be a cost-effective way to screen forcomponent Ievel defects,
especially onICs, where only a high rate of temperature change can expose latent
defects.
Hermeticity:
Hermeticity screeningdetectsleakage of gases into and out of
–device packages,
–boards and systems containing conformal coatings.
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Hermeticity screening also verifies
–behavior at high altitudesand
–susceptibility to sand and dust.
Radiation hardness:
It looks for ;
–failures from individual radioactive particles.
–circuit reliability in radio-active environments.
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STRESS SCREENING
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Somemilitary productsmust undergo this screen before certification.
Corrosion:
Corrosion screeningverifies resistance to chemical contamination, including salt
sprays and other hostile conditions.
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STRESS SCREENING
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Again, military applications and especiallynaval shipboard requirementsmay
require this screen.

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