Environmental Stress Screening Handbook

Preface

This book describes the analytical approach to solving the development / comparison of Environmental Stress Screens, It is modeled from DOD-HNBK-344 and 344A which was developed by Rome Air Development Center (RADC). The initial Technical Reports (TR) were released in 1981, and the handbook was released in 1986. The handbook is divided into five procedures that cover defect density (defect numbers assigned to parts, PWA’s and connections). Screen selection and placement (calculations for thermal & vibration screens). Failure Free Acceptance test (calculates the failure free time during ESS from the MBTF/yield requirements). Cost Effective Analysis (allows the user to evaluate a total screen using real dollars). Monitoring, Evaluation and Control (control charts to monitor the dropout/failures at each screen level ).

Section 1 is entitled "An Introduction of How DOD-HNBK-344" can influence new and existing Environmental Stress Screening Programs. It describes the math models developed to calculate thermal and vibration screens, explains the use of a defect density worksheet, compares two existing screens and calculates a failure Free Acceptance Test for a sample screen. The benefits of an ESS Program are also described. The Precipitation Efficiency equations are also contained in this section.

Section 2 describes an Environmental Stress Screen Model. This ESS Model can be used as a foundation to develop new product screens for development and production. The model was developed to existing Programs with the lowest level screen applied to circuit cards and the highest level screen applied to section/Forebody.

Section 3 uses the information in the previous sections and shows how this information can be used to develop a computer spreadsheet program such as EXCEL to develop this ESS model. It describes how this spreadsheet program automatically performs the math equations required for test strength, fallout and defects. This section also includes cost analysis and control worksheet examples.

Section 4 entitled “ Lesson Plan/Study Guide “ describes a baseline & introductory Environmental Stress Screening Course. All viewgraphs are included in this section.

Section 5 is an example of the use of a spread sheet to develop a screening program. The topics discussed are temperature cycling, random vibration, screen composites (a value for more than one screen) and damage index (how much of the unit’s life has been used due to screening).

Section 6 contains Excel worksheets for ESS calculations: temperature cycling, constant temperature (burn in), random vibration, swept sine vibration and single frequency sine vibration.

Appendix A is an explanation of EXCEL worksheets to calculate Screening Strengths, Screening Composites and “G rms” to calculate Vibration equations.

Appendix B contains definitions of common ESS terms.

TEMP vs. VIB

It should be noted that, in practical terms, as many additional temperature cycles as are necessary may be applied without affecting the equipment’s useful life.

Unlike thermal cycling, the maximum time that a unit can be exposed to the specified spectrum of random vibration, without significantly affecting its useful life, is severely limited.

MIL-STD-2164

TEST WHILE PERFORMING ESS

ESS that does not employ testing during stress application is relatively ineffective. It is also the reason why Random Vibration stress should be followed by Thermal Cycling.

DOD-HNBK-344

SHAKE THEN BAKE

The best screen for electronic hardware is to perform a printed wiring assembly (PWA) level thermal cycle stress screen before random vibration and a higher indenture level thermal screen after random vibration. The thermal screen prior to random vibration pre-stresses potential defects which can then be surfaced more efficiently by random vibration. Random vibration can also condition some defects just to the point of failure and a subsequent thermal screen with performance monitoring complies the defect identification.

TE 000-AB-GTP-020

VIBRATION TIME

The requirement for ten minutes in each axis is derived from studies that demonstrate maximum screening effectiveness is obtained in eight to ten minutes.

TE 000-AB-GTP-020

THERMAL & VIB COMBINED

When thermal cycling and random vibration are combined, random vibration shall be required towards the end of thermal cycling if combined at the printed wiring assembly level, or towards the beginning of thermal cycling if combined at a higher indenture level.

TE 000-AB-GTP-020

HARD FAILURES

If all of the failures that occurred were “hard” failures, performance monitoring might not be necessary. Unfortunately, many failures that occur in electronic hardware are “conditional” failures (intermittent). That is, they can only be detected while the thermal or vibration stress is being applied. Once the environmental stress is removed, the failure disappears.

TE 000-AB-GTP-020