Reliability Calculations

Electronic Components Consulting Services Inc.
Reliability Calculations
What, 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
• 631 928-2847
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
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 Predictions
The 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 design
Factors to be considered include Duty Cycle
and refined Thermal and Stress Analyses
• 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
» Determine the Effect of each failure mode, and the severity on system
» 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
• 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.
• 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
• 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 we benefit from them?
• No system benefits from a calculation
– Calculation without action is window dressing
• Contributes to good or bad feelings about system
• Could make customer happy… ...or not
– Calculation after design is complete is a waste of time
• Feedback of results into design yields the benefits
Longer predicted life
Fewer field failures
Lower warranty costs
Better customer relations
Famous Flubs
• BART (Bay Area Rapid Transit)
– FMEA not performed or inadequate
• Oscillator Crystal Failure- Open. Oscillator Frequency went up
Train speed increased rapidly
Train overshot last stop
Train rammed barrier at high speed
Very serious accident
• Browns Ferry Nuclear Power Plant Accident
– Results of FMEA waived
• Main & Redundant Instrumentation & Control Wiring
Required to be in separate cable trays in case of fire
Requirement waived as a cost saving
Fire in containment wall insulation during leak check
Fire destroyed main and redundant I & C wiring
• Nuclear Plant on fire and out of control for several days
Reliability Calculations
• Summary
– What: Analysis Toolkit
– Why: Product Improvement
Reduced Cost
– When: Early for Design Feedback
Prior to Completion to Validate Goals
– Benefit: Reduced Field Failures
Reduced Warranty Costs
Better Customer Relations