FAILURE MODE AND EFFECTS ANALYSIS (FMEA)

Background

1. Introduced in the late 1940s with military purposes .

2. Formalized as a failure analysis technique during the space program of the 1960s.

3. In 1970, North American Automotive Operations at Ford Motor Company developed a reliability training program which included two modules on FMEA. (Design and Process).

4. Spread to other industries – automotive, aeronautics, defense, processing, food, plastics, software’s, healthcare, semiconductor and electronics.

5. Toyota has taken this one step further and developed a tool - Design Review Based on Failure Mode (DRBFM) approach to reduce design related problems.

Factors Contributing to the Spread of FMEA

1. Rapid advancement in technology – must achieve reliability target the first time around.

2. Foreign and domestic competition.

3. Trend toward litigation between customer and supplier. For example a severity rating of 9 or 10 is generally reserved for those effects which would cause injury to a user or otherwise result in litigation.

4. Required by manufactures (e.g. the Ford Motor Company, Boeing, Airbus, etc.).

5. Useful tool to improve product / process without capital cost.

Applications of FMEA

1. Design

Use to eliminate defects in design, for example failures due to engineering specifications, inadequate strength, inappropriate

material (composition of material).

2. Process

Use to eliminate process related defects, i.e. thickness, periphery, core edge depression, surface depression, delamination, wrong size, texture, color, hardness, or welding defects, etc.

a. Most effective when applied to new product or process development stage.

b. Also used for existing products or processes that are undergoing a major design change which could affect their reliability.

c. Used in an existing design or process in a new environment, location, or application

d. The scope of the FMEA is the impact (changes of characteristics or effects) of the new environment or location on the existing design or process.

Definition and Purpose

1. Failure Mode Effect Analysis (FMEA) is methodology for analyzing potential reliability problems early in the development cycle of the product where it is easier to take actions to overcome the potential problems.

2. It identifies potential failure modes, determines their effect on the operation of the product, and identifies actions to mitigate the failures.

3. Failure causes are any errors or defects in process, design, or item especially ones that affect the customer, and can be potential or actual. Effects analysis refers to studying the consequences of those failures.

Types of FMEA

There are many types of FMEAs of which five are most important:

i. System FMEA (focuses on global system functions).

ii. Design FMEA (focuses on components and subsystems).

iii. Process FMEA (focuses on manufacturing and assembly processes).

iv. Service FMEA (focuses on service functions).

v. Software FMEA (focuses on software functions).

What is Failure Mode?

Any design flaw, out of specification condition, or any change in

the product which prevents it from functioning properly, normally

occur in two ways:

1. Complete Failure Mode

The assemble part / product failed to function as a result of the failure mode

2. Partial Failure Mode

The assembled part / product functions not properly (stopping and starting at intervals), degrades over time faster than specified or functions beyond the expected performance (over-performance) as a result of the failure mode.

Example of Potential Failure ModeTypical failure modes with reference to composite manufacturing process

are:

o Delamination

o Wrinkle

o Warpage

o Bridging

o Potting voids

o Scratch

o Wrong orientation

o Material contamination

o Panel thickness

o Pinholes

o Voids

Potential Failure Effect (s) Normally, the effects are stated in terms of product, process / operation performance or system performance, such as:

• Poor appearance

• Inoperative

• Unstable

• Scrap

• Rework / Repairs

• Customer dissatisfaction

• Does not match

• Cause excessive tool wear

• Unskilled operator

• Damages equipment

• Cannot mount

FMEA Process Flow

FMEA is a structured procedure for identifying and minimizing defects. It involves the effects of potential process related failure modes. It is used to study of machines, tools, flow of goods, and changes in process variables. Helps to minimize production process failure effect on the system. It

also helps to maximize the system quality, reliability, and productivity. JMC

Some Key FMEA Terms 1) Customer Input

2) Team – Team Selection (cross functional)

3) Ranking – Rating of decision

4) Risk Priority Assessment

5) Design Process

6) Production Process

   

Severity (sev).1) An assessment of how serious the effect of the potential

failure mode is ?. Severity rating (between 1 and 10 with 10 being most severe).

2) Examples of failure modes are: electrical short-circuiting, corrosion or deformation, insufficient current and dim Bulb etc.

3) The higher severity ranking index can be reduced through a

design change to system. Subsystem or component, or a

redesign of the process.

4) The FMEA team should agree on an evaluation criteria and ranking system.

5) FMEA is all about teamwork.

   

Severity Rating Table

AIC / JMC

Occurrence (occ)

1. In this step it is necessary to look at the cause of a failure and how many times it occurs.

2. It is a probability of a particular cause which is likely to be happened and results in a failure of the product / process (also called failure mode).

3. Manufacturing data of the process / product can be used to determine the occurrence rating. Examples of causes are: excessive voltage or improper operating conditions. A failure mode is given a probability number (Occ), again 1-10. This is a very important step and is called the detailed development section of the FMEA process.

Occurrence Rating Table

Failure Rate: Defect Percent / Output x 100

Cpk: indices give a indication of how well the process is actually capable of producing products within

specifications.

Ppk: process performance assessment with Ppk.

JMC

Process Performance

Process capability is used to measure the magnitude of the problem at hand. It is usually measured by Cpk, which indicates the short term capability of a process to meet process specifications. The estimated Cpk is given by:

where is the estimated value of the process average, is the estimation of process standard deviation computed from short term variation, USL is the upper specification limit and LSL is the lower specification limit.

ˆ3

LSLˆ,

ˆ3

ˆUSLminCpk

Process performance (Cont’d)

Process performance is usually measured by Ppk, which relates

the long term capability of a process to meet process

specifications. The estimated Ppk is defined as

ˆ3

LSLˆ,

ˆ3

ˆUSLminPpk

Higher Cpk and Ppk values yield lower fall out rates and, as a result, are preferable. Cpk and Ppk values are usually compared to some target value with typical target values being 1 or 1.33 or 1.5 or 1.67. A Cpk value of less than 1 implies that the process is producing products that do not conform to specifications.

Process Performance (Cont’d)

Higher Cpk and Ppk values yield lower fall out rates and, as a result, are preferable.

Cpk and Ppk values are usually compared to some target value with typical target values being 1 or 1.33 or 1.5 or 1.67.

A Cpk value of less than 1 implies that the process is producing products that do not conform to specifications.

Process Controls

Prevention:

To prevent the cause of failure from occurring, or reduce their rate. For example by implementing preventive maintenance program the machine failure, measuring and test equipment error can be minimized.

Detection (DET)

a) When appropriate actions are determined, it is necessary to test their efficiency. Also a design verification is needed. The proper inspection methods need to be chosen.

b) For example visual inspection of the panel, with sampling plan 5pcs / every hour, confirm the critical dimension by using calibrated micrometer.

c) List down the pre / post controls such as proper set-up, visual inspection, dimensional measurement, casting balancing by using “Receiver Gauge”, environmental requirements etc,.

d) After these 3 basic steps, Risk Priority Numbers (RPN) are calculated. This number represents the ability of planned tests and inspections at removing defects or detecting failure modes.

Process controls can prevent the failure mode from its occurring .

TYPES OF FMEADetection Evaluation Criteria

AIC / JMC

Risk Priority Number (RPN)

A numerical calculation of the relative risk when a

particular failure mode is encountered. In other words it is a mathematical product of the seriousness of a particular effect (Severity), the likelihood or probability that a cause will create the failure associated with that effect (Occurrence), and system ability to detect the failure before it gets to the customer (Detection). This number is then used to prioritize which items need additional quality planning and improvement.

RPN = SEV x OCC x DET

This value should be used to rank order the concerns in the process (e.g., in Pareto fashion).

AIC / JMC

Recommended ActionsList down the recommended actions which are most effective to

reduce the RPN. For example to reduce severity rating: change in product design can reduce severity rating.

To reduce occurrence rating: process / design revisions can reduce the probability of the potential cause (s) to occur as a result of failure mode.

An action oriented study of the process using statistical methods could be employed for the continuous process improvement and defect prevention.

To reduce detection rating: process / product design revisions are required to reduce the probability of detection of a current process

Implement process controls to prevent the potential causes of failure

Actions TakenIndicate a brief status of the recommended actions and its

completion with effective date

Actions ResultsCompare the previous severity ratings with the new severity

ratings in case the product design is changed.

Calculate RPN and assess whether occurrence is reduced in case the process or product design is reviewed.

Document the FMEA risks, actions taken and results which will serve as the guidelines to handle such kind of process or product defects in the future.

FMEA Success Factors

o Top Management commitment.

o Support from the review team.

o Maintain continuity by keeping the team together.

o Provide team members with as much information about the product or process as possible.

o Team should consists of process technician, engineer, manager rather all the stake holders of the process.

o Implement engineering process controls (EPC).

o Monitor the process

FMEA Constraints

• FMEA is too subjective.

• FMEA is based on intuition, logic and working

experience of the team members rather than data.

• FMEA is dependent of review team which examines

product failures, it is limited by their experience

Despite of constraints FMEA is still the most effective

well-known methodology because all the stakeholders of

the company contributes and pool their skills, talents and

experience in making this technique efficient for process

or product improvement.

FMEA Advantages • Improve the quality, reliability and safety of a

product/process • Improve company image and competitiveness • Increase user satisfaction • Reduce system development timing and cost • Collect information to reduce future failures, capture

engineering knowledge • Reduce the potential for warranty concerns • Early identification and elimination of potential failure

modes • Emphasis problem prevention • Minimize late changes and associated cost • Catalyst for teamwork and idea exchange between

functions

Lecture Summary

•Failure Mode Effect Analysis (FMEA) some time used as the alternative method when a statistically designed experiment or other statistical tool is not feasible.

•In general, the failure mode will be the defect to be eliminated.

•The use of FMEA proceeds by identifying risk factors related to the severity of the failure mode effect, probability of occurrence of the failure mode cause and the ability of a control system to detect the failure mode or its cause.

•The risk is assessed by means of a Risk Priority Number or RPN. The RPN is equal to the product of a severity rating (between 1 and 10 with 10 being most severe), an occurrence rating (between 1 and 10 with 10 being the most frequent) and a detection rating (between 1 and 10 with 10 being the worst detection capability).

Lecture Summary (cont’d)

Actions can then be developed to reduce the stated high risk. In some cases certain experimental actions may be necessary as actions to reduce this risk. After the FMEA actions have been completed a re-assessment of risk is made and the new RPN’s examined to verify that the risk has been reduced to a more acceptable level.

Lecture Summary (cont’d)

•After completion of the FMEA certain targeted small experiments to reduce specific sources of process variation are completed.

•Next, controls are designed and implemented so that process variation can be controlled or further reduced.

•This will enable the gains made from the process changes to be maintained long term. These process changes and control are then implemented and monitored. Part of this monitoring includes a capability assessment with Cpk and process performance assessment with Ppk.

Lecture Summary (cont’d)

This methodology is a comprehensive way to achieve defect reduction and is flexible enough to allow an alternative method to be used when SDE or other problem solving tool is not feasible or practical.