1. Failure Modes, Effects and Criticality Analysis
FMECA
Failure Modes, Effects and Criticality Analysis
Mehmet YILMAZ
May 2009

2. FMECA What is FMECA? Why to perform FMECA? How to perform FMECA?
Conclusions

3. FMECA Definition
Failure Modes = Incorrect behavior of a 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 severity of its effect
Failure Modes Effects and Criticality Analysis (FMECA) = a step-by-step approach for identifying all possible failures in a design, a manufacturing or assembly process, or a product or service.

4. 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

5. 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 analyses.

6. 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?

7. 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.
to prioritize and focus on high-risk failures

8. 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
design of effective built-in test techniques.

9. The results of the FMECA
Rank each failure mode.
Highlight single point failures requiring corrective action
Identify reliability and safety critical components

10. 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.

11. 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.

12. 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.

13. FMECA Procedure FMECA pre-requirements
System structure and failure analysis
Preparation of FMECA worksheets
Team review
Corrective actions to remove failure modes

14. 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.

15. 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.

16. Functional Block Diagram

17. 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
Higher RPN = higher priority to be improved.

18. Severity Classification
A qualitative measure of the worst potential consequences resulting from a function failure.
It is rated relatively scaled from 1-10.

19. Severity Classification1
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. 10
Very high severity ranking when a potential failure mode affects safe tool operation and/or involves noncompliance with government regulation without warning.

20. Probability of Occurrence
Probability that an identified potential failure mode will occur over the item operating time.
It is rated relatively scaled from 1-10.

21. Occurrence Classification10
>= 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

22. 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.
It is rated relatively scaled from 1-10.

23. 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. 10
Absolute uncertainty. Design Control will not and /or cannot detect a potential cause/mechanism and subsequent failure mode; or there is no Design Control.

24. FMECA CASE STUDY Component = D1
Function = restricting the direction of current
Failure = short
Cause = Physical Damage
Effect = Reverse current

25. FMECA CASE STUDY Severity = 7 Occurrence = 5 Detection = 9
RPN = 7*5*9 = 315

26. restricts the direction of current
FMECA Worksheet
Component
Function
Severity
Occurrence
detection
RPN
Failure
Cause
Effect
Recommendation D1
restricts the direction of current 7 5 9
315
short
Physical Damage
Reverse current
Change test procedure
R41
Current limit for T1 4 10
280
Standard Defect
no current limit
U10
FPGA
high current draw
Change Component

27. Corrective Actions
RPN reduction: the risk reduction related to a corrective action.

28. FMECA Checklist System description/specification Ground rules
Functional Block Diagram
Identify failure modes
Failure effect analysis
Worksheet (RPN ranking)
Recommendations (Corrective action)
Reporting

29. Summary

30. References
MIL-P-1629 “Procedures for performing a failure mode, effects and criticality analysis