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NAVSO P-3641: Navy Power Supply Reliability Design & Mfg
Guidelines (NAVMAT P4855-1)
Complete
List of Guideline Documents | Table
of Contents

4.6 ENVIRONMENTAL STRESS SCREENING

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There is a very large database which clearly indicates that
subjecting electronic hardware to thermal cycling and random
vibration is a very effective method of eliminating
workmanship and component defects and providing the
ultimate user with very reliable equipment. See
Reference 30. This process has been defined as
Environmental Stress Screening (ESS). A case history of the thermal
cycle screen and resultant measured reliability of a
high- reliability, low-voltage power supply is presented
in "Thermal Cycle Screening of Militarized Power
Supplies." See Reference 25.

RECOMMENDED ESS PROCEDURE

The minimal requirements for performing ESS on power supplies
should be implemented in the following sequence:

(1) Parametric testing

(2) Random vibration with continuous monitoring

(3) Thermal cycling with continuous monitoring

(4) 100% parametric testing per the customer Acceptance Test
Procedure (ATP).

The above ESS process applies to all low-voltage power supplies
and high-voltage power supplies. High-voltage power
supplies should be subjected to the additional ESS
detailed in Section 5.5 of this publication.

RANDOM VIBRATION

There are two general types of random vibration recognized for
the screening process; i.e., pure random and pseudo or
quasi-random. Either type is acceptable. Random
vibration should be done prior to performing the thermal
cycle portion of ESS. The power supply should be
operational, fully loaded (resistive) and continuously monitored.
The profile and levels of the random vibration are
approximately 6 grams (guidelines are provided in NAVMAT
P-9492). The power spectral density versus frequency is
illustrated in that publication. The vibration should be
applied to a minimum of two axes for 10 minutes per
axis. At least one of the axes should be perpendicular to the
plane of any PWB/MIBs in the power supply.

The data gathered during random vibration screening of
militarized digital avionics, and used in the development of these
requirements, was examined in "Random Vibration Screening
of Six Militarized Avionic Computers." See Reference 26.
A comprehensive guide to ESS is provided by
"Environmental Stress Screening for Assemblies." See
Reference 27.

THERMAL CYCLING

Twelve thermal cycles are required in accordance with the profile
in Figure 4-1. The last two cycles must be failure-free.
The temperature limits are:

(1) High Temperature

The chamber ambient conditions should be such that the average
temperature measured at the surface of the components of
the power supply is a maximum temperature of +55ºC
during fully operational (power-on) conditions.

(2) Low Temperature

The chamber ambient conditions should be such that the average
temperature measured at the surface of the components of
the power supply is a minimum temperature of -55ºC with
the power off.

(3) Thermal Rate of Change

The average rate of change of temperature at the surface of the
components should be at least 5ºC per minute, with 15ºC
per minute providing optimum results.

(4) Thermal Stability

The power supply should be considered to have reached thermal
stability at the high- or low-temperature portions of the
thermal cycle when the component with the greatest
thermal inertia has a rate of change of less than 5ºC
per hour, or is within 5ºC of the ambient temperature of
the chamber. To achieve this stability in as short a
time as possible, the cooling fans should operate
continuously during the cool-down part of the cycle.

In order to validate that the above thermal conditions are
achieved, a thermal survey should be performed under the
required temperature and duty cycle to identify the
component of greatest thermal inertia and establish the
time/temperature relationship between it and the chamber
air.

The 3-hour thermal cycle shown in Figure 4-1 is a typical example
for power supplies. 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; this is defined as the cold-start
condition. Except for planned on/off cycling lasting no
more than a few minutes, 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.

Provision for ESS temperature ranges should be incorporated in
the design specification. For the purpose of ESS, it is
required that the power supplies 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 failure mechanisms 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 above
that specified for power-on operating conditions by the
customer specification.

The power supply should be turned off for no less than one
minute, at least four times during the thermal cycle: twice
when the chamber ambient is between low temperature and
0ºC, 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.

POWER SUPPLY LOADING

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 during all ESS.

SPARES

Power supplies and/or their assemblies that are delivered as
spares should be subjected to the same ESS as the original
assemblies.

FAILURE DATA AND CORRECTIVE ACTION

A reliability program should be implemented to collect and
analyze the data from ESS, determine the failure mechanism and
implement corrective
action.

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Last Modified on :
Friday, 07-Mar-03
14:20:12