Environmental Stress Screening (ESS) is a test designed to uncover weak parts and workmanship defects. It subjects components, subassemblies, or full systems to environmental stresses like thermal cycling and vibration to induce early failures during manufacturing rather than in the field. This improves reliability and maintainability. The adaptive ESS process dynamically adjusts stress levels and times based on failure data to efficiently screen parts at minimum cost. ESS is generally applied during full-scale development and production and selectively during validation to improve outgoing quality and reliability.

1. RELIABILITY
ENGINEERING
Environmental Stress Screening (ESS)
Purpose ESS is a test or series of tests specifically designed to disclose weak parts and
workmanship defects requiring correction. It may be applied to components,
subassemblies, assemblies, or equipment (as appropriate and cost-effective). The
intent is to remove defects which would otherwise cause failure during later testing
or field service. By moving early failures from the field into the factory, effective
ESS programs have demonstrated two important payoffs. First, ESS has helped
fielded systems achieve their inherent Reliability and Maintainability (R&M), which
translates directly into improved operational performance over time. ESS has
significant potential return on investment during both development, production and
usage.
The Adaptive ESS process is dynamic. That is the starting ESS parameters (Stress
Levels, Time) are based on a similar equipment and changed depending on the
effectiveness of the screening process (See Figure 1). The ESS failure distribution
should be analyzed using the Chance Defective Exponential Model (CDE) described
in MIL-HDBK-344A to obtain the best screening parameters that will give the
highest outgoing reliability at minimum cost.
Scope The following Table reproduced from MIL-STD-785B (Reliability Program For
Systems And Equipment, Development And Production) contains identification to
show when the Environmental Stress Screening (Task 301) is used in different
Program Phases. In the Contract Data Requirements List (CDRL), the Task 301 is
referred as DI-RELI-80249.
TASK TITLE TASK PROGRAM PHASE
TYPE Concept Validation FSED Production
301 Environmental Stress ENG N/A S G G
Screening
S - Selectively Applicable G - Generally Applicable N/A - Not
Applicable
ENG - Reliability Engineering Tasks focus on the prevention, detection, and
correction of reliability design deficiencies, weak parts, and workmanship defects.
An effective reliability program stresses early investment in reliability engineering
tasks to avoid subsequent additional costs and schedule delays.
Hilaire Ananda Perera ( http://linkedin.com/hilaireperera )
Long Term Quality Assurance

2. The Adaptive ESS Process
OTHER DEVELOP ACQUIRE OR
INDUSTRY
PROGRAM FACILITIES MODIFY TEST
EXPERIENCE
EXPERIENCE SPECIFICATIONS FACILITIES
IDENTIFY DEVELOP PHASE 2 SPECIAL TESTS
ASSESS
SCREENING SCREENING FOR FACILITIES
IMPACT
GOALS PLANS OR TOOLS
DEVELOP
DETAILED
No
PROCEDURES
FAVOURABLE
PHASE 1 IMPACT?
Yes INCORPORATE
IN ATPs
SUMMARY TO PROGRAM
MANAGER DEFINE DATA
REQUIREMENTS
SUMMARY TO QUALITY /
RELIABILITY MANAGERS
No ORGANIZE DATA
COLLECTION
RESOURCES
CONTINUE
SCREENING EFFECTIVENESS
OR REDUCE
GOAL MET? REVIEW
SCREENING
Yes
SUMMARY TO OTHER SCREENING
PROGRAMS COLLECT DATA
FOR IMPLEMENT
SUMMARY TO OTHER PROGRAM EFFECTIVENESS SCREEN
PHASE 3 MANAGERS REVIEW
Figure 1
Hilaire Ananda Perera ( http://linkedin.com/hilaireperera )
Long Term Quality Assurance

3. RELIABILITY
ENGINEERING
Environmental Stress Screening (ESS), Continued
Definitions CDRL = Contract Data Requirements List
FSED = Full Scale Engineering Development
CDE = Chance Defective Exponential
Latent Defect = An inherent or induced weakness or flaw which will manifest itself
as a failure in the operational environment
Responsibilities It is the responsibility of the reliability engineer assigned to the particular project
together with a test engineer to prepare the ESS Test Plan and Procedure. These
documents should be reviewed and approved by the Reliability Group Leader and
the Project Manager
Procedure Environmental Stress Screening (also known as Burn-In) should be conducted at
those levels of assembly (component, circuit card assembly, subassembly, assembly)
needed to detect and remove early failures due to weak parts, workmanship defects
and other nonconformance anomalies.
During development, ESS procedures, taking into consideration the equipment
design, component technology, and production fabrication techniques, should be
formulated. ESS procedures should be designed for the end item and for all lower
level items which will be procured separately as spare or repair parts. A plan for
implementing these procedures should also be prepared, indicating the proposed
application of ESS during development and production. The proposed ESS
procedures and implementation plan should be subject to approval by the Procuring
Authority (PA).
ESS should be designed to stimulate relevant failures by stressing the item. The
stressing need not simulate the precise operational environment the item will see.
Environmental stress types (The most effective stress types for electronic equipment
are Thermal Cycling and Random Vibration) may be applied in sequence. During
ESS, the item should be cycled through its operational modes while simultaneously
being subjected to the required environmental stresses.
Hilaire Ananda Perera ( http://linkedin.com/hilaireperera )
Long Term Quality Assurance

4. RELIABILITY
ENGINEERING
Environmental Stress Screening (ESS), Continued
Procedure Upon approval of the proposed ESS procedures and implementation plan, a detailed
(Continued) environmental stress screening process plan should be prepared and included as part
of the reliability test plan. The ESS detailed process plan should include the
following, subject to PA approval prior to initiation of ESS:
• Description of environmental stress types, levels, profiles, and exposure times to
be applied.
• Identification of level (part/component, board, subassembly, assembly) at which
ESS will be accomplished
• Identification of item performance and stress parameters to be monitored during
ESS.
• Proposed ESS durations (best ESS exposure, failure-free interval and allowable
cumulative ESS time per item) based on CDE Model application are:
• Let frD be the average failure rate of a defect under a given set of stress
conditions. (frD is determined from the ESS curve)
• Acceleration Factor is defined as the ratio of the Constant Failure Rate of the
ESS Curve to the Predicted Failure Rate or Field Failure Rate. Let Acceleration
Factor = A
• Best ESS Exposure [Time to remove 99.999% of the Latent Defects] =
(-1/frD)*ln(0.00001)
• Failure-Free Time (FFT) for 99.99% ESS Yield @ 90% Lower Confidence
Level = 7/frD [ This is for A=60 or more ]
• If 5% of Operational Life (OL) should not be consumed during ESS when
multiple failures and repairs are encountered, Allowable Cumulative ESS time =
OL*0.05/A
• Method of incorporating ESS results into FRACAS to attain the needed
operational reliability (MIL-STD-785B Task 104 )
Hilaire Ananda Perera ( http://linkedin.com/hilaireperera )
Long Term Quality Assurance

5. RELIABILITY
ENGINEERING
Environmental Stress Screening (ESS), Continued
Associated MIL-STD-785: Reliability Program For Systems And Equipment,
Documents Development And Production
MIL-STD-721: Definitions Of Terms For Reliability And Maintainability
MIL-HDBK- 344A: Environmental Stress Screening (ESS) Of Electronic
Equipment
Customer provided Statement Of Work and the Reliability Program Plan
ISO 9001 Level II Document: Design Control - The Product Assurance
Process
Hilaire Ananda Perera ( http://linkedin.com/hilaireperera )
Long Term Quality Assurance