ISO 9001
section 7.5.2 addresses the control of production processes where the resulting
output cannot be verified by subsequent monitoring or measurement. In these cases, defects only become apparent
during the use of the product. Examples
of processes that fall within this section are:
- Welding
- Brazing
- Wave Soldering
There are many others.
Control of processes like these is accomplished by
developing a well-defined process, validating that the process is effective,
and controlling process variables using techniques such as statistical process
control.
One tool for validating that the process is effective is the
process FMEA (pFMEA). Process FMEAs are
interesting because they have wide applicability to a broad range of processes beyond
those covered by 7.5.2.
FMEA stands for Failure Modes and Effects Analysis. The technique has been around for a long
time, having been first developed by the US Military (MIL-P-1629). Many industries, such as automotive and
aerospace have embraced the FMEA approach to both design and process
validation.
In performing a process FMEA, the various steps in the
process are presented in a spreadsheet.
Each process step has some likely failure mode(s). Each failure mode has one or more root cause(s).
For each root cause there is some effect on the
product. This is known as a failure
effect. To each failure effect is
assigned a value for Severity (1-10).
The root causes of the failure effect are assigned frequency of
occurrence (1-10), and a likelihood of detection (10-1). These three values are
multiplied together to come up with a single Risk Priority Number (RPN).
The RPN is used to prioritize actions to improve the
process. As action is taken and process
steps are modified, the RPN of the modified process is calculated to show the
relative improvement.
There are 10 steps to conducting a pFMEA:- Review the process. Create a process flow chart. Liest each process step in a pFMEA spreadsheet.
- Brainstorm the potential failure modes of each process step.
- List the potential failure effects of each failure mode.
- To each failure effect assign Severity (S) rankings (1 means not very severe, 10 means very severe). Record the highest value among the failure effects identified as the severity ranking for the process step.
- Identify the potential root causes of each failure mode. To each root cause assign Occurence rankings (1 means occurence s are rare, 10 means the root cause occurs frequently). When there is more than one root cause, reaord the highest value among the root causes.
- To each root cause assign a Detection (D) ranking (10 means the root cause is unlikely to be dteected, 1 means it is very likely to be detected). When there is more than one root cause, record the highest value among the root causes.
- Calculate the RPN = S x O x D
- Develop an action plan to improve the process, prioritizing on the highest RPNs.
- Take the actions in the plan.
- Recalculate the resulting RPNs after the actions are taken.
The FMEA method is not perfect. The values assigned for Severity, Occurrence
and Detection are somewhat arbitrary.
Moreover the values assigned are ordinal numbers or rankings, meaning a
number assigned to a level. They could
as easily represent descriptions such as very low, low, medium, etc. A higher value for a factor is more important
than a lower value.
Multiplication of ordinal numbers is not defined (We can’t
come up with a numeric value for ”Low” x “High”). A value of “4” is not necessarily four times
as important as “1”. The value “4” is
just more important than “1”. Still, the
RPN has value, and when used properly the FMEA can direct a team towards
process improvements. In addition, the completed pFMEA provides evidence of how the process was developed.
A spreadsheet that can be used to perform a pFMEA can be
downloaded here. It is based on the
process FMEA promoted by the Automotive Information Action Group (AIAG). An example of a simple FMEA can be found here.
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