FMECA

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

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

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

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 identifyin identifying g all possible failures in a design, a manufacturing or assembly process, or a product or service.

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

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.

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?

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

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.

The results of the FMECA Rank each failure mode.

Highlight single point failures requiring  corrective action  Identify reliability and safety critical  components 

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.

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.

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.

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

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.

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.

Functional Block Diagram

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.

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

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.

10

Very high severity ranking when a potential failure mode affects safe tool operation

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.

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

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.

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

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

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

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

short

Standard Defect

no current limit

Change test procedure

short

Standard Defect

high current draw

U10

FPGA

7

7

4

10

10

4

280

280

Change Component

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

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

Summary

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