FMECA

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FMECA

Failure Modes, Effects and

Criticality Analysis

Mehmet YILMAZ

May 2009

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FMECA

What is FMECA?

Why to perform FMECA?

How to perform FMECA?

Conclusions

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FMECA

What is FMECA?

Why to perform FMECA?

How to perform FMECA?

Conclusions

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FMECA Definition

Failure Modes = Incorrect behavior of a

Failure Modes = Incorrect behavior of a subsystem or subsystem or

component due to a physical or procedural malfunction.

Effects = Incorrect behavior of the system caused by a

failure.

Criticality = The combined impact of

–

– The probability that a failure will occur The probability that a failure will occur

–

– The severity of its effectThe severity of its effect

Failure Modes Effects and Criticality Analysis (FMECA) =

a step-by-step approach for

a step-by-step approach for identifyinidentifying all g all possiblepossible

failures in a design, a manufacturing or assembly

process, or a product or

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Evolution of FMECA

FMEA was originally developed by NASA to improve and verify the reliability of

space program hardware.

MIL-STD-1629 establishes requirements and procedures for performing FMECA

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Purpose of FMECA

Select the most suitable design with high reliability and high safety potential in the design phases.

List potential failures and identify the severity of their effects in the early design phases.

Develop criteria for test planning and requirements. Provide necessary documentation for future design and consideration of design changes.

Provide a basis for maintenance management. Provide a basis for reliability and availability

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Basic Questions of FMECA

Why failures will happen (Failure mode)? What is the consequence when the failure occurs (Failure effect)?

Is the failure in the safe or danger direction (Failure Criticality)?

How to remove the failure or reduce its frequency?

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Benefits of FMECA

FMECA is one of the most important and most widely used tools of reliability analysis.

The FMECA facilitates identification of potential design reliability problems

– Identify possible failure modes and their effects – Determine severity of each failure effect

FMECA helps

– removing causes of failures

– developing systems that can mitigate the effects of failures.

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Benefits of FMECA

It provides detailed insight about the systems interrelationships and potentials of failures.

Information gained by performing FMECA can be used as a basis for

– troubleshooting activities

– maintenance manual development

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The results of the FMECA

Rank each failure mode.

Highlight single point failures requiring corrective action

Identify reliability and safety critical components

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FMECA Techniques

The FMEA can be implemented using a hardware (bottom-up) or functional (top- down) approach

Due to system complexity, it isperformed as a combination of the two methods.

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FMECA Techniques

Hardware Approach :

– The bottom-up approach is used when a system design has been decided already.

– Each component in the system on the lowest level is studied one-byone.

– Evaluates risks that the component incorrectly implements its functional specification.

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FMECA Techniques

Functional Approach :

– Considers the function of each item. Each function can be classified and described in terms of having any number of associated output failure modes.

– The functional method is used when hardware items cannot uniquely identified

– This method should be applied to when the design process has developed a functional block diagram of the system, but not yet identified specific hardware to be used.

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FMECA Procedure

FMECA pre-requirements

System structure and failure analysis Preparation of FMECA worksheets Team review

Corrective actions to remove failure modes

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FMECA Prerequisites

Define the system to be analyzed

– System boundaries.

– Main system missions and functions.

– Operational or/and environmental conditions.

Collect available information that

describes the system functions to be analyzed.

Collect necessary information about previous and similar designs.

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Functional Block Diagram

Functional block diagram shows how the

different parts of the system interact with each other.

It is recommended

– to break the system down to different levels.

– to review schematics of the system to show how different parts interface with one another by their critical support systems to understand the normal functional flow requirements.

– to list all functions of the equipment before examining the potential failure modes of each of those functions. – to include operating conditions (such as; temperature,

loads, and pressure), and environmental conditions in the components list.

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Rate the Risks Relatively

A systematic methodology is used to rate the risks relative to each other. The Risk Priority Number is the critical indicator for each failure mode.

RPN = Severity rating X Occurrence rating X Detection rating

– The RPN can range from 1 to 1,000

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Severity Classification

A qualitative measure of the worst

potential consequences resulting from a function failure.

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Severity Classification

1 Failure would cause no effect.

2 Boarderline pass but still shippable.

3 Redundant systems failed but tool still works.

4 Would fail manufacturing testing but tool still functions with degraded performance.

5 Tool / item inoperable with loss of primary function. No damage to other

components on board. Failure can be easily fixed (for example, socketed DIP chips). 6 Tool / item inoperable with loss of primary function. No damage to other

components on board. Failure cannot be easily fixed (true if not field repairable). 7 Tool / item inoperable, with loss of primary function. Probably cause damage to

other components on board or system.

8 Tool / item inoperable with loss of primary function. Probably scraping one or more PCBAs.

9 Very high severity ranking. A potential failure mode affecting safe tool operation and/or involves noncompliance with government regulation with warning.

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Probability of Occurrence

Probability that an identified potential failure mode will occur over the item operating time.

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Occurrence Classification

10 >= 50% (1 in two) 9 >= 25% (1 in four) 8 >= 10% (1 in ten) 7 >= 5% (1 in 20) 6 >= 2% (1 in 50) 5 >= 1% (1 in 100) 4 >= 0.1% (1 in 1,000) 3 >= 0.01% (1 in 10,000) 2 >= 0.001% (1 in 100,000) 1 Almost Never

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Detection rating

A numerical ranking based on an

assessment of the probability that the failure mode will be detected given the controls that are in place.

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Detection rating

1 Detected by self test.

2 Easily detected by standard visual inspection or ATE.

3 Symptom can be detected. The technician would know exactly what the source of the failure is.

4 Symptom can be detected at test bench. There are more than 2-4 possible candidates for the technician to find out the sources of failure mode.

5 Symptom can be detected at test bench. There are more than 5-10 possible candidates for the technician to find out the sources of failure mode.

6 Symptom can be detected at test bench. There are more than 10 possible candidates for the technician to find out the sources of failure mode.

7 The symptom can be detected, and it required considerable engineering knowledge/resource to determine the source / cause.

8 The symptom can be detected by the design control, but no way to determine the source / cause of failure mode.

9 Very Remote. Very remote chance the Design Control will detect a potential

cause/mechanism and subsequent failure mode. Theoretically the defect can be detected, but high chance would be ignored by the operators.

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FMECA CASE STUDY

Component = D1

Function = restricting the direction of current

Failure = short

Cause = Physical Damage Effect = Reverse current

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FMECA CASE STUDY

Severity = 7

Occurrence = 5 Detection = 9

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FMECA Worksheet

C o m p o_n e_n t F u_n c t _i o n S e_v e_r i t y O c c u r r e_n c e d e t e c t _i o n R P N F a_i l u r e C a u s e E f f e c t R e c o m m e_n d a t _i o n D1 restricts the direction of current 7 5 9 315 short Physical Damage Reverse current Change test procedure R41 Current limit for T1 7 4 10 280 short Standard Defect no current limit Change test procedure

U10 FPGA 7 10 4 280 short Standard

Defect high current draw Change Component

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Corrective Actions

RPN reduction: the risk reduction related to a corrective action.

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FMECA Checklist

System description/specification Ground rules

Functional Block Diagram Identify failure modes

Failure effect analysis

Worksheet (RPN ranking)

Recommendations (Corrective action) Reporting

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Summary

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