Environmental stress screening

Hilaire Ananda Perera ( http://linkedin.com/hilaireperera ) Long Term Quality Assurance

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 Screening

ENG

N/A

S

G

G

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.


The Adaptive ESS Process

SUMMARY TO OTHER SCREENING PROGRAMS

SUMMARY TO OTHER PROGRAM MANAGERS

IDENTIFY SCREENING

GOALS

DEVELOP SCREENING

PLANS

INDUSTRY EXPERIENCE

OTHER PROGRAM

EXPERIENCE

ASSESS IMPACT

FAVOURABLE IMPACT?

SUMMARY TO PROGRAM MANAGER

SUMMARY TO QUALITY / RELIABILITY MANAGERS

CONTINUE OR REDUCE SCREENING

SCREENING GOAL MET?

EFFECTIVENESS REVIEW

DEVELOP FACILITIES

SPECIFICATIONS

ACQUIRE OR MODIFY TEST

FACILITIES

SPECIAL TESTS FOR FACILITIES

OR TOOLS

DEVELOP DETAILED

PROCEDURES

INCORPORATE IN ATPs

DEFINE DATA REQUIREMENTS

ORGANIZE DATA COLLECTION RESOURCES

COLLECT DATA FOR

EFFECTIVENESS REVIEW

IMPLEMENT SCREEN

PHASE 1

PHASE 2

PHASE 3

Yes

No

Yes

No

Figure 1


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.


RELIABILITY

ENGINEERING


Procedure (Continued)

Upon approval of the proposed ESS procedures and implementation plan, a detailed 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 )


RELIABILITY

ENGINEERING


Associated Documents

MIL-STD-785: Reliability Program For Systems And Equipment, 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

Education

Environmental stress screening