Design FMEA and Process FMEA

[Vishwadeep Khatri 265]

Q. 170  What are the key differences between PFMEA and DFMEA? How does Design FMEA feed in to Process FMEA? What kind of feedback is expected from Process FMEA to Design FMEA?

 

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[Natwar Lal 35]

Design Failure Modes and Effects Analysis (DFMEA) and Process Failure Modes and Effects Analysis (PFMEA) are the two types in which FMEA can be done. Both are tools for risk assessment, however there are differences between the two which I have highlighted below

 

 

The DFMEA and PFMEA are closely linked to each other as one identifies the risk in the product and second identifies the risk in the manufacturing (or enabling) process. 

The outputs expected from a DFMEA are

1. Design Risks and corresponding recommendations

2. Identification of Critical Items (CIs) and Key Characteristics (KCs) for the product

 

Both of the above outputs feed into the PFMEA.

1. Design risks and CIs could help identify the potential failure modes in a PFMEA

2. KCs help us identify the control parameters for the process

 

The feedback expected from PFMEA is the process capability in terms of producing the product. Sometimes due to technology or knowledge limitations, the process might not be able to produce a desired product in which case the product needs to be modified and hence a need to re-do the DFMEA. 

 

 

[Mohamed Asif 15]

Major difference between DFMEA and PFMEA:

DFMEA (Design Failure Mode Effect Analysis) detects potential failure modes with regards to a Product or a Service. Whereas, PFMEA (Process Failure Mode Effect Analysis) detects potential failure modes with regards to a Process (usually Manufacturing and Assembly processes). 

DFMEA - Emphasis on Product Function

PFMEA - Emphasis on Process Input Variable 

Another major difference would be in DFMEA, it is used to detect potential deficiencies in Products/Services before they are being released to production. Preferably In PFMEA, needs to be performed before starting with new process. However, most of the time it is used for analysing deficiencies in existing processes. 

Irrespective of the difference, similar kind of steps is followed in both DFMEA and PFMEA. 
Mostly DFMEA is seen from a perspective of identifying deficiencies on Product life or from an Product malfunction view on the contrary PFMEA is seen from a perspective of identifying deficiencies on Product Quality or from improving reliability of the Process standpoint. 

In a typical Product Manufacturing Flow, DFMEA activity is performed before PFMEA.
I would like to explain this with an example in Software and Manufacturing sector. [Refer Enclosed Image]

 

PFMEA is developed to ensure effective process control and any abnormality/feedback is sent back to DFMEA for effective design changes (if any) for optimized flow and if it is effectively used, it can result in good improvement in Quality, Cost, Delivery and Reliability. 

 

Proactive design changes which will prevent process/product failure which can go as a feedback from PFMEA to DFMEA

 

 

 

[IFTAKHAR 0]

PFMEA stands for Process Failure Mode Effect Analysis , which industries use to design a process for a business unit  & DFEMA Design Failure Mode Effect Analysis which industries use to design a product .

[Venugopal R 59]

Benchmark Six Sigma Expert View by Venugopal R

 

The two broad classifications of FMEA methodology are DFMEA and PFMEA, though there are more classifications possible such as ‘Concept FMEA’, ‘Proto FMEA’ and so on.

 

Design FMEA is an exercise that has to be performed before the design of a product is finalized. Design FMEA may be done for a part design, sub-assembly design or for an assembly design. It starts with the item under consideration, describes its expected function and then list of the potential failure modes that could be anticipated for that function of the item. Some of the areas that the failure modes in a DFMEA will cover includes performance, safety, compliance to standards, user friendliness, and manufacturability.  The potential causes will assess the inputs that get into the design and their possible inadequacies, mistakes and variations, while evolving the input requirements. Inadequate or incomplete assessment of the performance requirements of the item is another area for potential cause. The detection controls will look at existence and effectiveness of design verification and validation methods, any ‘fail-safeness’ applied etc.

Study of historical data relating to performance, past failures, and / or reviewing the DFMEA for similar products are certain practices adopted during a DFMEA.

 

Once the design of the product is evolved, the design of the process that needs to create the product will be taken up. Before finalizing the process(es), the PFMEA is performed.  PFMEA begins with the Operation sequence and Process description. The requirements for each process step will include Product characteristics that are dependent on the process and the requisite process controls are identified. The process (or process step) not fulfilling any of these requirements will be identified as potential failure mode. The potential cause(s) for the failure mode could typically cover the process incapability, process sequencing, choice of equipment, skill etc. The current detection controls will look for Mistake Proofing, verification methods including MSA effectiveness.

 

Process Control plan is a document that emerges based on the PFMEA findings.

 

As we saw earlier, one of the potential failure modes for DFMEA is ‘Manufacturability’ or how well a process can be expected to have the capability to fulfil the design requirements. This will be an input from process (and PFMEA) to DFMEA. The Product characteristics that are dependent on the Process controls becomes and input from DFMEA to PFMEA

 

The severity ratings applied by DFMEA for part characteristics could qualify some of them as CTQs or Special characteristics. Such characteristics if they have dependency on Process controls, the associated process characteristics need to be also identified as CTQs.

 

There could be certain potential failure modes in PFMEA for which the detection control systems could be weak. For example, reliability related failures, as a result of Process limitations, may not be easy to detect as part of a day to day control system. Such inputs need to go back to DFMEA and the Design team to inbuilt greater assurance through Design and reduce dependency on Process.

 

It is recommended to begin the PFMEA exercise, even as the DFMEA is evolving, so that both exercises can benefit from the mutual exchange of inputs. The FMEAs will then have to undergo multiple iterations of refinement.

 

Even after finalizing the Design and the Process, the FMEAs will continue to remain as a living document that needs to be referred and updated from time to time.

[Rupinder N 31]

Natwar Lal has provided the best answer - covering all aspects of the question. Mohamed Asif's answer should be read to get more insights, too. For Benchmark expert view, refer to Venugopal's answer.