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Failure Modes and Effects Analysis _FMEA_

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					Failure Modes and Effects Analysis (FMEA)

Failure Modes and Effects Analysis, or FMEA, is a methodology for identifying the potential
failure modes that a product or process may encounter, assessing the risks associated with these
failure modes, prioritization of these failure modes according to their urgency, and prevention of
the more urgent failure modes, i.e., the ones that are most likely to cause serious harm to the
company.

The output of an FMEA cycle is the FMEA Table, which documents how vulnerable a product or
process is to its potential failure modes. The FMEA table also shows the level of risk attached to
each potential failure mode, and the corrective actions needed (or already completed) to make
the product or process more robust. The FMEA Table generally consists of 16 to 17 columns,
with each column corresponding to a piece of information required by FMEA.

The FMEA is a proactive analysis tool, allowing engineers to anticipate failure modes even before
they happen, or even before a new product or process is released. It also helps the engineer to
prevent the negative effects of these failure modes from reaching the customer, primarily by
eliminating their causes and increasing the chances of detecting them before they can do any
damage. The actions generated by a good FMEA cycle will also translate to better yield, quality,
reliability, and of course, greater customer satisfaction.

There are many types of FMEA, but the most widely used are probably the following: 1) System
FMEA, which is used for global systems; 2) Design or Product FMEA, which is used for
components or subsystems; 3) Process FMEA, which is used for manufacturing and assembly
processes; 4) Service FMEA, which is used for services; and 5) Software FMEA, which is used
for software. In the semiconductor industry, the Design or Product FMEA and the Process
FMEA are the most frequently-encountered FMEA versions.

Despite the existence of many types of FMEA today, the basic structure and process for
executing them remains the same. Any FMEA process must include the following steps,
information details of which are documented in the FMEA Table:

1) Assembly of the team;
2) Understanding of the Product or Process to be subjected to FMEA;
3) Breaking down of the product or process into its components or steps (components and steps
are also known as items);
4) Identification and assessment of the following for every item listed: function(s), potential failure
mode(s), failure mode effect(s), failure mode cause(s), and controls for detecting or preventing
the failure mode(s);
5) Evaluation of the risks associated with the failures modes and prioritizing them according to
importance;
6) Implementation of corrective actions to minimize the occurrence of the more significant failure
modes;
7) Reassessment of the product or process by another cycle of FMEA after the actions have
been completed; and
8) Regular updating of the FMEA Table.




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Failure Modes and Effects Analysis (FMEA) Procedural Guide

       Describe the product or process.

 1     A clear and specific description of the product or process undergoing FMEA must first be
       articulated. The creation of this description ensures that the responsible engineer fully
       understands the 'form, fit, and function' of the product or process.


       Draw a block diagram of the product or process.

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


       Complete the header of the FMEA Table (Table 1).

       FMEA Table headers vary from one to the next, since they are supposed to be customized
       according to the requirements of the companies using them. Generally the header requires,
 3     among others that you may wish to add, the following
       information: Product/Process/System Name, Component/Step Name; Product Designer or
       Process Engineer, Name of the Person who prepared the FMEA form; FMEA Date; Revision
       Level (letter or number); and Revision Date.


       Enumerate the items (components, functions, steps, etc.) that make up the
       product or process.
 4     Break down the product or process being subjected to FMEA into its major components or
       steps. List down each of these components or steps in Column 1 of the FMEA table. The
       items must be listed down in a logical manner.


       Identify all potential Failure Modes associated with the product or process.

       A failure mode is defined as how a system, product, or process is failing.

       Now here arises some confusion in the semiconductor industry, which usually measures its
       failure modes in terms of how the product or process is deviating from its specifications. A
       product or process can have hundreds of different failure modes based on this definition,
       most of which are highly correlated to each other because of a common failure mechanism
       behind them.

 5     A failure mechanism is defined as the physical phenomenon behind the failure mode(s)
       observed, e.g., die cracking, corrosion, electromigration, etc.

       To simplify the use of FMEA in the semiconductor industry, therefore, the engineer may
       choose whether to construct the FMEA table in terms of failure modes or in terms of failure
       mechanisms. For convenience of discussion, the term 'failure mode' shall refer to either
       failure mode or failure mechanism when used in this web page (this web page only!).

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


       List down each Failure Mode using its technical term.

       Using an official technical term for listing the failure mode prevents confusion. All potential
 6     failure modes should be listed down for each item (product component or process step).
       Column 2 of the FMEA Table shall be used for this purpose.




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     Describe the effects of each of the failure modes listed and assess the 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 effect is what the customer
     will experience or perceive once the failure occurs. A customer may either be internal or
     external, so effects to both must be included. Examples of effects include: inoperability or
     performance degradation of the product or process, injury to the user, damage to
7    equipment, etc.

     Assign a severity rating to each effect. Each company may develop its own severity rating
     system, depending on the nature of its business. A common industry standard is to use a
     1-to-10 scale system, with the '1' corresponding to 'no effect' and the '10' corresponding to
     maximum severity, such as the occurrence of personal injury or death with no warning or a
     very costly breakdown of an enormous system.

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


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

     Aside from its effect(s), the potential cause(s) of every listed failure mode must also be
8    enumerated. A potential cause should be something that can actually trigger the failure to
     occur. Examples of failure causes include: improper equipment set-up, operator error, use
     of worn-out tools, use of incorrect software revision, contamination, etc.

     The potential causes are listed in Column 5 of the FMEA Table.


     Quantify the probability of occurrence (Probability Factor or PF) of each of the
     failure mode causes.

     The likelihood of each of the potential failure cause occurring must be quantified. Every
9    failure cause will then be assigned a number (PF) indicating this likelihood or probability of
     occurrence. A common industry standard for this is to assign a '1' to a cause that is very
     unlikely to occur and a '10' to a cause that is frequently encountered.

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


     Identify all existing controls (Current Controls) that contribute to the prevention
     of the occurrence of each of these failure mode causes.

10   Existing controls that prevent the cause of the failure mode from occurring or detect the
     failure before it reaches the customer must be identified and evaluated for its effectiveness
     in performing its intended function. Each of the controls must be listed in Column 7 of the
     FMEA Table.


     Determine the ability of each control in preventing or detecting the failure mode
     or its cause.

11   The effectiveness of each of the listed controls must then be assessed in terms of its
     likelihood of preventing or detecting the occurrence of the failure mode or its failure
     cause. As usual, a number must be assigned to indicate the detection effectiveness (DET)
     of each control. DET numbers are shown in Column 8 of the FMEA Table.


     Calculate the Risk Priority Numbers (RPN).

     The Risk Priority Number (RPN) is simply the product the Failure Mode Severity (SEV),
     Failure Cause Probability (PF), and Control Detection Effectiveness (DET)
12   ratings. Thus, RPN = (SEV) x (PF) x (DET).

     The RPN, which is listed in Column 9 of the FMEA Table, is used in prioritizing which items
     require additional quality planning or action.


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        Identify action(s) to address potential failure modes that have a high RPN.

        A high RPN needs the immediate attention of the engineer since it indicates that the failure
        mode can result in an enormous negative effect, its failure cause has a high likelihood of
        occurring, and there are insufficient controls to catch it. Thus, action items must be defined
        to address failure modes that have high RPN's.
13
        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.


        Implement the defined actions.

        Assign a responsible owner and a target date of completion for each of the actions defined.
        This makes ownership of the actions clear-cut and facilitates tracking of the actions'
14      progress. The responsible owner and target completion dates must be indicated in Column
        11 of the FMEA Table.

        The status or outcome of each action item must also be indicated in Column 12 of the
        FMEA Table.


        Review the results of the actions taken and reassess the RPN's.

        After the defined actions have been completed, their over-all effect on the failure mode
        they're supposed to address must be reassessed. The engineer must update the SEV, PF,
        and DET numbers accordingly. The new RPN must then be recalculated once the new SEV,
15      PF, and DET numbers have been established. The new RPN should help the engineer
        decide if more actions are needed or if the actions are sufficient.

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


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




Table 1. Example of a Simplified FMEA Table




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