The fourth edition FMEA document great leap forward in the FMEA understanding and should be acquired and reviewed in detail. The Webpage is not intended to be a substitute to the AIAG FMEA Fourth Edition, but is intended to quickly convey the very important changes of the Fourth Edition FMEA guideline document.
Changes compared to the third edition:
§ Addition of prework activity
§ Boundary Diagram
§ P or Parameter Diagram
§ Top Down Approach (system, subsystem, and component)
§ De-emphasis on the RPN
§ New formats for FMEA
§ New Ranking Tables
§ Tools Linkage
The addition of the Boundary Diagram as a prework document for Design FMEA brings the emphasis of interfaces of systems/components that are not directly under the influence of the FMEA team’s scope. Interfaces that are not defined and/or managed can become cause or contributors of causes.
Typically the scope of the FMEA is found inside the boundary. QAI has used the Boundary Diagram for 10 years when executing robust engineering and FMEA development. Our process uses 4 types of arrows depicting 4 types of interfaces.
Ø energy transfer
Ø material exchange
Ø data exchange
The Parameter Diagram is a tool first used for electronics design looking to understand the signal to noise ratio “S/N”. In FMEA, the Parameter Diagram is used to understand the inputs (signal) and outputs (ideal functions or responses) and to list other inputs that may have an impact on the design. Other inputs and outputs are as follows:
§ Control Factors – things the design engineers can control or manipulate to obtain the desired output.
§ Error States – outputs that are undesirable either technically (failure modes) or customer focused (effects of failure).
§ Noise Factors – noises are things that may be present but are not controllably by the engineer. Therefore he/she must consider them and their impact on the design output.
Ø Design change may be considered to protect a design against one or more noise factors (Robust Design).
§ Noise Factors Include:
PC to PC Variation – variation where change in output can be measured (within blue print expectations). Hint: these may be Special Characteristics
System Interface – other systems which may impact performance of the design
Degradation Over Time – where at first it is ok but eventually performance degrades due to exposure of wear
Customer Misuse – unique ways customers use the design which is not one of our expected functions or ideal functions, but failure to recognize the use may lead to failure
Environment – factors of environment like salt, temperature, and humidity which may also affect the design performance
Top Down Approach to FMEA
A top down approach is recommended starting at system where failure mode causes become indications of what subsystems or components need to be addressed (causes at system level). This infers that Design FMEA for every component may not be necessary. Risk based direction makes FMEA more efficient.
De-Emphasis on “RPN” Risk Priority Number
The Risk Priority Number has been used to determine actions required. This has always been a problem as it drives “bad behaviour”. Clearly actions should be directed at:
Severity First (9, 10)
Occurrence Second (Criticality)
Detection Third (High Detection)
The statement is made that clearly removed RPN as a threshold action based number. Page 57 clearly states the RPN’s importance in actions. QAI has paraphrased this statement many years ago. Our statement, which is compatible to the AIAG statement is: “There is no RPN threshold. There is no number above which a team must take an action or below which a team is excused from taking an action”.
New format for FMEA
AIAG has provided several new formats for FMEA Development. The older formats are still acceptable, as software that is used currently many be very expensive. More interesting is a format which moves the Design or Process controls “prevention” to the left of the Occurrence Ranking. This is a very good format, as prevention affects the occurrence ranking which had previously been to the left of the column.
New Ranking Tables
The ranking tables have been clarified especially the detection tables for both design and process FMEA.
The Fourth Edition FMEA guidelines define links between tools such as DFMEA to PFMEA and DFMEA, PFMEA and DVP&R Design Verification Plan & Report to the Control Plan (CP). It is left to the reader to determine the specific link between these documents. QAI is very prescriptive as to these links. Our methodology is in line with the guidelines:
§ DFMEA effects and severity must be used in PFMEA development if there are Special Characteristics. We recommend a characteristics Matrix to assist in linkage between the DFMEA and PFMEA.
The process flow diagram and process parameters may be listed on the vertical where they are matrixed against design parameters or characteristics and the severity from the DMEA is list on the horizontal.
Links between characteristics and process steps/parameters are transferred to the PFMEA form. The characteristics become a failure mode and the parameters at that process step become causes of failure when they contribute to not making the characteristic correctly.
Source: Quality One