||NAVSO P-3641A: More Power For The Dollar
3.3.2 Environmental Stress Screening
Environmental Stress Screening (ESS) is a process which involves the application of one or more specific types of environmental stresses for the purpose of precipitating to failure, any latent, intermittent, or incipient defects or flaws which would cause product failure in the use environment. The stress may be applied in combination or in sequence on an accelerated basis but within product design limits. ESS detects manufacturing problems caused by poor workmanship or by faulty and/or marginal parts. It also identifies design problems if the design is inherently marginal and if qualification and engineering tests were too benign. The
most common stimuli used in ESS are temperature cycling and random vibration.
ESS is a process rather than a test in the normal accept/reject sense. Those participating in the effort, including the contractor, should never be led to believe that a "failure" is bad and would be held against them. ESS is intended to stimulate defects, not to simulate the operating environment, and therefore, factory "failures" are encouraged. The root causes of ESS failures need to be found and corrected before the ESS process is complete.
Initially, ESS should be applied to 100% of the units manufactured, including repaired or spare units. By
using a closed loop feedback system, data can be analyzed to determine whether the screening program should be modified. A viable ESS program must be
dynamic - the screening program must be actively managed and tailored to the particular characteristics of the equipment being screened. This includes conducting a survey to determine the mechanical and thermal characteristics of the equipment and refining the vibration and temperature screening profiles as more information becomes available and/or designs, processes, and circumstances evolve.
Baseline ESS Procedures
Tri-Service Technical Brief 002-93-08 "Environmental Stress Screening Guidelines " contains guidance for development and performance of ESS. Baseline vibration and temperature profiles from the Tri-Service Technical Brief are shown below in Tables 3.3-1 and 3.3-2 and represent agreement of the three military Services on minimum ESS levels to ensure effectiveness. They are derived from experimental and analytical stress screening studies plus surveys of screens used in industry. These baseline profiles for random vibration and temperature cycling are not recommended stress levels, and should be refined after conducting surveys to determine the mechanical and thermal characteristics of the particular equipment being developed.
Vibration - The Table 3.3-1 random vibration profile shows the baseline values for response levels, frequencies, axes, duration and monitoring. The excitation must be tailored to the response experienced by the components of the unit under test. The selection of stress levels must be based on available survey data and structural design due to the potential for highly resonant members, as well as the existence of vibration sensitive devices. To avoid potential fatigue or peak level damage, some reduction of the input spectrum may be preformed at points of resonant frequencies which result in amplification of the applied stress by a factor of 6 dB or more. These resonances would be identified from data accumulated during development tests, or by conducting a low-level sine sweep.
Table 3.3-1. Based Vibration Profile
LEVEL OF ASSEMBLY
|Overall Response Level2
(unless an end item or space)
Pure random vibration or quasi-random vibration are considered
acceptable forms of vibration for the purpose of
stress screening. The
objective is to achieve a broadband excitation of vibration.
1. For random vibration the unit level, it may not be cost effective at
the PWA level. However, PWAs
manufactured as end items or spares may
require screening using random vibration as a stimulus. Additionally,
the system level, when a response survey indicates that the most
sensitive PWA is driving the profile in a manner
that causes some PWAs
to experience a relatively benign screen, that PWA should be screened
Each PWA screened separately should have its own profile
determined from a vibration response survey.
2. The preferred power spectral density for 6grms consistws
of 0.04 g2/Hz from 80 to 350 Hz with a 3dB/octave
from 80 to 20 Hz and a 3dB/octave rolloff from 350 to 2000Hz.
3. Vibration input profiles for each specific application should be
determined by vibration response surveys
which identify the correlation
between input and structural responses. higher frequencies are
significantly attenuated at higher levels of assembly.
4. Single axis to two axis vibration may be acceptable if data shows
minimal flaw detection in the other axes.
Notching of damaging resonant frequencies should not be permitted without customer concurrence, and should be the exception, not the general rule. Where warranted, temporary stiffening of the unit should also be considered to prevent overstressing during the stress screen. The contractor may find that the most economic approach is a minor design change to provide permanent stiffening. Whether temporary or permanent, the stiffening should be done in a manner that achieves the desired flat response throughout the unit being screened.
Temperature - The Table 3.3-2
temperature cycling baseline profile shows a range of values for the
temperature extremes, the temperature rate of change and the number of
cycles. The temperature cycling screens also requires tailoring, based
on survey results, to each specific equipment. Differences in
components, materials and heat dissipation lead to variations in the thermal
stresses throughout the item.
Baseline Tempurature Cycling Profile
LEVEL OF ASSEMBLY
|-50°C To +75°C
||-40°C To +70°C
||-40°C To +60°C|
|Temperature Rate Of
when the temperature of the slowest|
responding element in the product
being screened is within 15% of
the specified high and low temperature
extremes. Large magnetic
parts should be avoided when determining
that stabilization has
|Soak Time Of Product
Long enough to
perform functional testing.
||20 to 40
||12 to 20
||12 to 20|
1. All temperature parameters pertain to the
temperature of the unit being screened and not the chamber
temperature. The temperature parameters of the unit being screened
are usually determined by thermocouples
placed at various points on the unit
2. PWA guidelines apply to individual PWAs and to modules, such as
flow-through electronic modules
consisting of one or two PWAs bonded to
a heat exchanger.
3. Unit guidelines apply to electronic boxes and to complete modules
consiting of more than one smaller
4. The designer of the screening profile should decide which elements
of the hardware (e.g., parts, solder joints,
PWAs, or connectors) must
be subjected to extreme temperatures in the thermal cycle. The temperature
of the various elements in the hardware being screened are
determined by means of a thermal survey.
5. Power is applied during the low to high temperature excursion and
remains on until the temperature has
stabilized at the high
temperature. Power is turned off on the high to low temperature excursion
at the low temperature. Power is also turned on and
off a minimum of three times at the teperature extremes on
After the baseline temperature cycle profile
is tailored for a particular equipment, based on survey results, the actual
profile is developed and applied. Each output of a power supply should
have a static (characterized) load equal to the maximum rated load. This
load should be applied to the power supply continuously. The power
supply should be turned on (energized) under full load (characterized)
conditions at the start of the temperature transition from low to high
temperature; defined as the cold-start condition. The power supply
should be turned off for no less than one minute, at least four times during
the thermal cycle, i.e., twice when the chamber ambient is between low
temperature and OOC, and twice at high temperature after thermal stability has
been reached. The input power should be applied and removed in an abrupt
manner, as by a relay or mechanical switch. Except for such planned
on/off cycling, the power supply should then be operated continuously until
thermal stability is reached. During the transition from high to low
temperature, the power supply should be turned off and remain de-energized
until the start of the transition from low to high temperature. The
power supplies should be turned on at the end of the cold soak period of the
thermal cycle. This provides the maximum thermal shock and stimulates
the failure of weak components and elements. It has been observed that
most failures occur during the transition, with power on, from low to high
temperature. The power supply would not be expected to operate
within its performance criteria until its temperature rises within
that specified for power-on operating conditions by the customer specification. An
example temperature cycling profile is provided in Figure
Tri-Service Technical Brief 002-93-08, Environmental Stress Screening Guidelines, should be used to develop and implement an optimum ESS program.
Highly Accelerated Stress
HASS is a form of ESS that can also be used to
precipitate and detect manufacturing defects. HASS stress levels and
profiles are normally determined using step stress testing (e.g., HALT) to
ensure an effective non-destructive screen within the design