PFMEA

PFMEA or Process Failure Mode and Effects Analysis is a method that assigns quality risk levels to each task in a process. It is a prevention tool that helps eliminate potential defects before they occur. It is a modification of Failure Mode and Effects Analysis. PFMEA was designed for Six sigma, however it can be used in all quality improvement programs.

Steps of PFMEA

1. Setting scope of analysis

The scope of analysis is essential for the results. Probably there is no need to analyse parts that are well known from earlier products, unless they can be incompatible with new solutions. Also level of decomposition should be considered. Each analysis should be performed in well defined environment.

2. Functional decomposition

The object under investigation should be decomposed into elements. In case of process, it should be decomposed into tasks. For each task data should be collected, including:

• number of failures,
• types of failures,
• known problems (e.g. analysis of literature),
• limits of use (e.g. temperature, humidity).

3. Qualitative analysis

Qualitative analysis concerns possible failures, their causes and effects:

• failure - what wrong can happen to the element under investigation,
• cause - what has to happen to make the failure occur; there can be several causes and each should be analysed separately,
• effect - what will happen if failure occur (not cause! - common mistake); there may be several short-term as well as long-term effects. Each should be analysed separately.

Set of failure + cause + effect is a failure mode.

4. Quantitative analysis

Quantitative analysis refers to three variables:

• (P) Probability of occurrence (related to cause of failure),
• (D) Detection difficulty (related to the failure itself),
• (S) Severity (related to effect).

Product of those three variables gives overall rating of the analysed failure mode.

$R=P\cdot D\cdot S$

Each variable is rated in scale from 1 to 10, where 10 is maximum. The enterprise using FMEA should define own evaluation tables that help in assigning ranks.

5. Corrective and preventive actions in FMEA

There are three different ways of repair:

• Reducing the probability,
• Improve detection,
• Reduce nuisance of failures.

The choice depends on the type and complexity of the failure and the product.

Probability, Detection and Severity scales

Probability scale <0.01 per 1000 0.1 per 1000 0.5 per 1000 1 per 1000 2 per 1000 5 per 1000 10 per 1000 20 per 1000 50 per 1000 >100 per 1000

Detection scale Certain to detect Almost certain High Moderately high Moderate Low Very low Remote Very remote Impossible to detect

Severity scale None Very Minor Minor Very Low Low Moderate High Very High Hazardous, with warning Hazardous, without warning

Do not mistake detection with probability!

PFMEA Rules

1. PFMEA is a teamwork, not one person.
2. Analyse only critical processes. Do not waste resources on non-important ones.
3. Include people with extensive experience, but also few less experienced employees. Younger employees don't know that something is impossible yet, and thus they can achieve it.
4. PFMEA should be done at the place where the process occurs (Gemba).
5. Collect data before PFMEA session
6. Each change of process requires change in PFMEA. Decide when do you want to perform PFMEA session.
7. Set limited time for analysis. Otherwise it can go too deep.

Examples of PFMEA

• Design PFMEA: This is when a PFMEA is conducted on a new product design. It involves analyzing the design to identify potential risks and assigning a quality risk level to each element of the design.
• Process PFMEA: This type of PFMEA is conducted on an existing process. It involves analyzing the process to identify potential risks and assigning a quality risk level to each task.
• Service PFMEA: This type of PFMEA is conducted on a service-based process. It involves analyzing the service process to identify potential risks and assigning a quality risk level to each task.
• Measurement System Analysis (MSA): MSA is a type of PFMEA that specifically looks at the accuracy of a measurement system. It involves analyzing the system to identify potential risks and assigning a quality risk level to each measurement.

Advantages of PFMEA

PFMEA is a powerful tool for quality improvement programs that can help prevent potential defects before they occur. Its advantages include:

• Improved reliability and safety of products and processes: By identifying potential failure modes and their associated risks, PFMEA can help reduce the likelihood of failure or harm.
• Increased efficiency: By identifying process steps that are prone to errors, PFMEA can help identify areas of improvement in a process, resulting in increased efficiency.
• Enhanced customer satisfaction: By identifying and eliminating defects, PFMEA can help ensure that customers receive quality products and services, leading to higher customer satisfaction.
• Cost savings: By reducing the number of defective products, PFMEA can help reduce the cost of rework and scrap, as well as the costs associated with customer complaints and other quality-related issues.

Limitations of PFMEA

PFMEA has several limitations, which can hinder the effectiveness of the analysis:

• The analysis can be time consuming and tedious, as it requires a detailed look at all steps in a process.
• It can be difficult to anticipate all potential failure modes and the effects of each.
• It does not account for external factors, such as environmental or regulatory changes, which can affect the process.
• It can be costly to implement, as it requires resources and funds to be allocated for the analysis.
• It does not consider human error or other non-technical factors, which can be a major source of errors and defects.
• It does not provide specific solutions for eliminating or minimizing process failure, but instead provides a general understanding of the potential areas of risk.

Other approaches related to PFMEA

• Process Flow Charting: This is the process of visually mapping out the steps of a process to identify any potential risks or inefficiencies.
• Failure Modes and Effects Analysis (FMEA): This method is used to identify potential failure modes in a process, the effects of each failure, and the probability of occurrence.
• Process Control Charting: This method is used to monitor and control the performance of a process over time and to identify any potential problems.
• Root Cause Analysis (RCA): This method is used to identify and address the root causes of process failures.
• Design of Experiments (DOE): This method is used to test and evaluate the impact of various process parameters on the output of a process.

In summary, the other approaches related to PFMEA are Process Flow Charting, Failure Modes and Effects Analysis (FMEA), Process Control Charting, Root Cause Analysis (RCA), and Design of Experiments (DOE). All of these approaches are designed to identify and address any potential risks or inefficiencies in a process.

PFMEA — recommended articles

Statistical process control — Failure Mode and Effects Analysis — Maintenance strategy — Quality loss function — Quality control — Assignable cause — Quality inspection — Line balancing — Quality control plan

References

• Mikosa, W. L., & Ferreira, J. C. E. (2007). Knowledge Sharing and Reuse in Potential Failure Mode and Effects Analysis in the Manufacturing and Assembly Processes (PFMEA) Domain. In Complex Systems Concurrent Engineering (pp. 461-468). Springer London.
• Johnson, K. G., & Khan, M. K. (2003). A study into the use of the process failure mode and effects analysis (PFMEA) in the automotive industry in the UK. Journal of Materials Processing Technology, 139(1), 348-356.
• Dawei, G. (2013). Research on Application of PFMEA in Flexible Welding Line. Automobile Technology, 5, 009.

Author: Slawomir Wawak