可靠度的计算方式

Electronic Components Consulting Services Inc.

Reliability CalculationsWhat, Why, When & How do we benefit from them?

Who am I? Harvey Altstadter– 34 years experience in Component Engineering and Reliability Military Commercial Commercial Space

HR Electronic Components Consulting Services Inc Electronic Components Consulting Services Inc. Consultant to Industry

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What are Reliability Calculations? Methodology for analyzing the expected or actual reliability of a product, process or service, and identifying actions to reduce failures or mitigate their effect.– – – – Stress Analysis Reliability Predictions FMEA (Failure Mode and Effects Analysis) or FMECA (Failure Mode Effects and Criticality Analysis)

Yardstick for comparison of design approaches– Cost-Benefit Trade

Why Do Reliability Calculations? Make the product more reliable– – – – – Selling feature Reduce returns Lower costs Enhance or maintain company reputation Comparisons with competition

Customer request– Design goal – Hard Requirement

Stress Analysis Establishes the presence of a safety margin– Good engineering practice – Enhances system life – Provides input data for Reliability Prediction Describes operating condition as a percentage of rating

– Customer requirement – Validates compliance with Derating Criteria

Reliability Predictions (MTBF)

If you don’t like the numbers......give me five minutes, I will make up a better one

Reliability Predictions (MTBF) Form the basis of Reliability Analyses– Compute predicted system failure rate or Mean Time Between Failures Failure Rate is usually expressed in Failures per 106 or 109 hours MTBF is usually expressed in terms of hours– Example: for a system with a predicted MTBF of 1000 hours, on average the system experiences one failure in 1000 hours of operation or a Failure Rate of 1000 per 106 hours

– Methodology Use accepted standards– Model failure rates of components – Analyze system – Calculate the system predicted failure rate or MTBF

Evaluate prediction vs target or required MTBF– Evaluate stress or temperature reduction design changes – Evaluate practicality of design change especially when MTBF is self imposed

Reliability Predictions (Continued) Common Standards– MIL-HDBK-217 Generally associated with military systems– Models are very detailed – Provides for many environments – Provides multiple quality levels

– Bellcore (Telcordia) Telecommunications Industry standard– – – – – Seems to have supplanted French CNET and British Telcom standards Models patterned after MIL-HDBK-217, but simplified Provides multiple quality levels Can incorporate current laboratory test data Can incorporate current field performance data

– Other Standards Auto Industry

– Resources Software packages cover both MIL-HDBK-217 and Bellcore models

– RELEX is widely available

Reliability Predictions (Continued) General approach to Prediction– Model for a single part consists of a number of factors multiplied together ss = G * Q * S * T– – – – – – ss = Steady State Failure rate G = Generic or Base Failure Rate Q = Quality Factor S = Stress Factor T = Temperature Factor Other factors: First Year Multiplier or Experience Factor

– Model for a unit Consists of the sum of all of the individual part failure rates multiplied by an Environmental Factor E

– Source of Factor Information- varies with method used Lookup table Calculation based upon complexity

Reliability PredictionsThe first cut is made with little analysis to get a rough idea where the design is relative to the desired outcome

Better numbers come from better insight into the designFactors to be considered include Duty Cycle and refined Thermal and Stress Analyses

FMEA or FMECA Design FMEA FMEA is a bottoms up method of analyzing and improving a design– Heavily used by US automotive industry » Chrysler, Ford, GM require this type of analysis » Many different company and industry standards » Most widely used is the AIAG (Automotive Industry Action Group) standard – Analytic Process » Consider each component or functional block and how it can fail (Failure Modes) » Determine the Effect of each failure mode, and the severity on system function » Determine the likelihood of occurrence and of detecting the failure. » Calculate the Risk Priority Number, or RPN, using the formula as follows:RPN = Severity x Occurrence x Detection

» Consider corrective actions (may reduce severity of occurrence, or increase probably of detection) » Start with the higher RPN values (most severe problems) and work down » Recalculate RPN after the corrective actions have been determined, the aim is to minimize RPN

FMEA or FMECA (Continued) Process FMEA Similar to a Design FMEA but is applied to a manufacturing process or service. The object is to use this methodology to optimize processes.

FMECA A FMECA is similar to a FMEA,– Criticality is computed in place of RPN . – FMECAs are used extensively in military, aerospace and medical equipment fields, for both design and process reliability analysis. – MIL-HDBK-1629 is a widely accepted standard for FMECAs.

When Stress Analysis– Prior to release of design to production – Prior to implementation of design changes

Reliability predictions should be done at all stages of design– Early design stage- Reliability Prediction may a rough estimate – Late design stage- Reliability Prediction is refined – Fielded system- revised prediction can incorporate field data for future use

Design FMEA or FMECA– As design matures, impact of failure needs to be addressed

Process FMEA– During process design – Prior to implementing new or updated processes

How do

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