Adaptive ESS

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Short description of the principles of Adaptive ESS

Short description of the principles of Adaptive ESS

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  • 1. Adaptive Environmental Stress Screening Hilaire Ananda Perera
  • 2. Environmental Stress Screening (ESS) Environmental stress screening of a product is a process which involves the application of one or more specific t ifi types of environmental stresses f f i tl t for the purpose of precipitating to hard failure, 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 q basis but within product design capabilities. Hilaire Ananda Perera AP – 13 Sep 2006 . 2
  • 3. Should the ESS Chosen Remain the Same The form of ESS chosen by the equipment manufacturer is dependant on the failure mechanisms for the relevant field failures The ESS process is dynamic, and must change as product failure behavior changes For this reason, it is not appropriate to “Spec” an ESS regimen and leave it unchanged throughout product life Hilaire Ananda Perera AP – 13 Sep 2006 . 3
  • 4. What Required from the ESS Process The ESS Process should detect defective devices without consuming an unacceptable portion of the other “ th “good” d i ’ useful lif d” device’s f l life The more flaws an equipment has the higher is its has, failure rate Large Flaws Early Failures (Infant Mortality) Small Flaws End of Life Failures (Durability) ESS should remove Large Flaws without effecting much of the Small Flaw population Hilaire Ananda Perera AP – 13 Sep 2006 . 4
  • 5. Adaptive Environmental Stress Screening ESS is a process rather than a test in the normal p accept/reject sense. Adaptive ESS is based on the adjustment of screens in response to previously observed screening results to minimize Outgoing Defects. With no firm failure mechanism/mode information, Random Vibration followed by Thermal Cycling with few Power On/Off cycles is a good default condition. y g Screening should not stress the equipment such that fatigue failures are precipitated Hilaire Ananda Perera AP – 13 Sep 2006 . 5
  • 6. ESS Strength Estimation with no Failure/Time Data The Screening Strength of a given stress screen profile is defined as the probability that the stress screen will precipitate a latent defect into a detectable failure, given that a defect is present. Screening provide assurance on the Outgoing Reliability Screening Strength for Temperature Cycling (STn) is a function of Temperature Range =T; Temp. Rate of Change =R; Number of Cycles =n = 1 − e[−[0.0017⋅(T + 0.6) ⋅ln(e + R) ⋅n]] 0.6 3 ( STn Screening Strength for Random Vibration (SVt) is a function of G = gRMS; Vibration Duration = t = 1 − e[−(0.0046⋅G ⋅t )] 1.71 SVt When T = 111oC; R = 10oC/Min n =16 Cyc; G = 5gRMS; Combined Screen Strength (SS) = 1 - (1-STn).(1-SVt) t = 15 Min SS = 0.999826 The Screening Strength equations were developed by Hughes Aircraft Company, and modified by Rome Air Development Centre (RADC) based on the data from McDonnel Aircraft Co. and Grumman Aerospace Corporation Hilaire Ananda Perera AP – 13 Sep 2006 . 6
  • 7. Chance Defective Exponential (CDE) Model CDE Model is based on the assumption that the population of components within like equipments is comprised of two subpopulations i.e. A main subpopulation of “good” components and a much smaller subpopulation of defectives defectives. The defectives contain major flaws which degrade with stress and time and are manifested as early-life early life (infant mortality) failures. The failure rate of a defective part is several orders of magnitude greater than the failure rate of a “good” part g p More Information Available in MIL-HDBK-344A Hilaire Ananda Perera AP – 13 Sep 2006 . 7
  • 8. Equation Selection from SigmaPlot Chance Defective Exponential (CDE) Model Hilaire Ananda Perera AP – 13 Sep 2006 . 8
  • 9. Outgoing Reliability Assurance C = Average rate of defect precipitation under a given set of stress conditions B/C = Incoming Defect Density (Din) A / Normal Failure Rate = Acceleration Factor (AF) Accelerated Life Tests - Environmental Stress Screening Screening Strength (SS) = 1 – e -Ct C ESS Test Strength (TS) = SS * Detection Efficiency (DE) Outgoing Defect Density (Dout) = Din * (1 - TS) X -Ct Time To Remove 99.999% Defects = (-1/C)*ln(0.00001) Rate (FR) FR(t) = A + Be X X re ( Failur Chance Defective Exponential Model X X Time t1 (variable) Gather failure data and establish failure rate/time distribution functions • Starting t1 is based on equipment parts count. t1 is varied depending on the effectiveness of the Stress Screen ( Adaptive ESS ). • Proper screening levels assure that there are no manufacturing process related failures and latent defects (i.e. ESS + FFT). FFT = Failure Free Time • Performance of FFT is an accurate precursor to the kind of reliability to be expected in the field Hilaire Ananda Perera AP – 13 Sep 2006 . 10
  • 10. Benefits of Adaptive ESS • Able to produce High Confidence Reliability Numerics for Reliability Guarantees • Minimize Warranty Failures (move failures from field to factory) Using Optimum Outgoing Defect Densities • Able to apply Failure Rate/Time Distributions to Predict Reliability of New Similar Electronic Controllers • Possibility to Determine Cost Optimized ESS Times • Improved Overall Quality of the Process and the Product • Reduced Loading on Screening Facilities g g Hilaire Ananda Perera AP – 13 Sep 2006 . 15