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Failure Modes and Effects Analysis (FMEA) Procedural Guide See also: Failure Modes and Effects Analysis (FMEA) Overview

1

Describe the product or process. A clear and specific description of the product or process undergoing FMEAmust first be articulated. The creation of this description ensures that theresponsible engineer fully understands the 'form, fit, and function' of theproduct or process.

2

Draw a block diagram of the product or process. A block diagram of the product/process needs to be developed to show thelogical relationships between the components of the product or thesteps/stages of the process. A block diagram may be in the form of boxesconnected by lines, with each box corresponding to a major component of theproduct or a major step of the process. The lines correspond to how theproduct components or process steps are related to each other.

3

Complete the header of the FMEA Table (Table 1). FMEA Table headers vary from one to the next, since they are supposed to becustomized according to the requirements of the companies using them.Generally the header requires, among others that you may wish to add, thefollowing information: Product/Process/System Name, Component/StepName; Product Designer or Process Engineer, Name of the Person whoprepared the FMEA form; FMEA Date; Revision Level (letter or number); andRevision Date.

Enumerate the items (components, functions, steps, etc.) that make

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4

up the product or process. Break down the product or process being subjected to FMEA into its majorcomponents or steps. List down each of these components or steps in Column1 of the FMEA table. The items must be listed down in a logical manner.

5

Identify all potential Failure Modes associated with the product orprocess. A failure mode is defined as how a system, product, or process is failing.

Now here arises some confusion in the semiconductor industry, which usuallymeasures its failure modes in terms of how the product or process is deviatingfrom its specifications. A product or process can have hundreds of differentfailure modes based on this definition, most of which are highly correlated toeach other because of a common failure mechanism behind them. A failure mechanism is defined as the physical phenomenon behind the failuremode(s) observed, e.g., die cracking, corrosion, electromigration, etc. To simplify the use of FMEA in the semiconductor industry, therefore, theengineer may choose whether to construct the FMEA table in terms of failuremodes or in terms of failure mechanisms. For convenience of discussion, theterm 'failure mode' shall refer to either failure mode or failure mechanismwhen used in this web page (this web page only!).

An example of a semiconductor process where failure mechanisms may bemore effective to use is Wirebonding, whose failure mechanisms include balllifting, wedge lifting, wire breaking, bond-to-bond shorting, etc.

6

List down each Failure Mode using its technical term. Using an official technical term for listing the failure mode prevents confusion. All potential failure modes should be listed down for each item (productcomponent or process step). Column 2 of the FMEA Table shall be used forthis purpose.

7

Describe the effects of each of the failure modes listed and assessthe severity of each of these effects.

For each of the failure modes in Column 2, a corresponding effect (or effects)must be identified and listed in Column 3 of the FMEA Table. A failure effectis what the customer will experience or perceive once the failure occurs. Acustomer may either be internal or external, so effects to both must beincluded. Examples of effects include: inoperability or performancedegradation of the product or process, injury to the user, damage toequipment, etc.

Assign a severity rating to each effect. Each company may develop its own

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severity rating system, depending on the nature of its business. A commonindustry standard is to use a 1-to-10 scale system, with the '1' correspondingto 'no effect' and the '10' corresponding to maximum severity, such as theoccurrence of personal injury or death with no warning or a very costlybreakdown of an enormous system.

Column 4 of the FMEA Table is used for the severity rating (SEV) of thefailure mode.

8

Identify the possible cause(s) of each failure mode.

Aside from its effect(s), the potential cause(s) of every listed failure modemust also be enumerated. A potential cause should be something that canactually trigger the failure to occur. Examples of failure causes include:improper equipment set-up, operator error, use of worn-out tools, use ofincorrect software revision, contamination, etc. The potential causes are listed in Column 5 of the FMEA Table.

9

Quantify the probability of occurrence (Probability Factor or PF) ofeach of the failure mode causes. The likelihood of each of the potential failure cause occurring must bequantified. Every failure cause will then be assigned a number (PF) indicatingthis likelihood or probability of occurrence. A common industry standard forthis is to assign a '1' to a cause that is very unlikely to occur and a '10' to acause that is frequently encountered.

PF values for each of the failure causes are indicated in Column 6 of the FMEATable.

10

Identify all existing controls (Current Controls) that contribute to theprevention of the occurrence of each of these failure mode causes. Existing controls that prevent the cause of the failure mode from occurring ordetect the failure before it reaches the customer must be identified andevaluated for its effectiveness in performing its intended function. Each of thecontrols must be listed in Column 7 of the FMEA Table.

11

Determine the ability of each control in preventing or detecting thefailure mode or its cause. The effectiveness of each of the listed controls must then be assessed in termsof its likelihood of preventing or detecting the occurrence of the failure modeor its failure cause. As usual, a number must be assigned to indicate thedetection effectiveness (DET) of each control. DET numbers are shown inColumn 8 of the FMEA Table.

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12

Calculate the Risk Priority Numbers (RPN). The Risk Priority Number (RPN) is simply the product the Failure ModeSeverity (SEV), Failure Cause Probability (PF), and Control DetectionEffectiveness (DET) ratings. Thus, RPN = (SEV) x (PF) x (DET). The RPN, which is listed in Column 9 of the FMEA Table, is used in prioritizingwhich items require additional quality planning or action.

13

Identify action(s) to address potential failure modes that have ahigh RPN. A high RPN needs the immediate attention of the engineer since it indicatesthat the failure mode can result in an enormous negative effect, its failurecause has a high likelihood of occurring, and there are insufficient controls tocatch it. Thus, action items must be defined to address failure modes thathave high RPN's.

These actions include but should not be limited to the following: inspection,testing, monitoring, redesign, de-rating, conduct of preventative maintenance,redundancy, process evaluation/optimization, etc. Column 10 of the FMEA Tables is used to list down applicable action items.

14

Implement the defined actions. Assign a responsible owner and a target date of completion for each of theactions defined. This makes ownership of the actions clear-cut and facilitatestracking of the actions' progress. The responsible owner and targetcompletion dates must be indicated in Column 11 of the FMEA Table. The status or outcome of each action item must also be indicated in Column12 of the FMEA Table.

15

Review the results of the actions taken and reassess the RPN's. After the defined actions have been completed, their over-all effect on thefailure mode they're supposed to address must be reassessed. The engineermust update the SEV, PF, and DET numbers accordingly. The new RPN mustthen be recalculated once the new SEV, PF, and DET numbers have beenestablished. The new RPN should help the engineer decide if more actions areneeded or if the actions are sufficient.

Columns 13, 14, 15, and 16 of the FMEA Table are used to indicate the newSEV, PF, DET, and RPN, respectively.

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16Keep the FMEA Table updated. Update the FMEA table every time the product design or process changes ornew actions or information cause the SEV, PF, or DET to change.

Table 1. Example of a Simplified FMEA Table

See also: Failure Modes and Effects Analysis (FMEA) Overview. HOME Copyright © 2005 SiliconFarEast.com. All Rights Reserved.