Process Failure Mode Effects Analysis, PFMEA...

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Process Failure Mode Effects Analysis, PFMEA is an analytical technique used to ensure that potential failure modes and their associated causes are considered and addressed.

The PFMEA should be initiated as early in the process development cycle as possible. Early involvement will help a process development team anticipate, resolve or monitor potential process concerns. It will also allow ample time to mitigate risk items through the redesign of components, sub-systems or systems and through the development  of robust processes, through validation and the creation of test systems.

Process Failure Mode Effects Analysis

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What part of the process should be covered during a Process Failure Mode Effects Analysis, PFMEA?
The entire process should be reviewed to determine if any step can cause a business or safety risk and to determine if risk mitigation activities can be performed. This could include activities such as … parts being received into incoming inspection, parts coming from the warehouse to the floor, parts being queued for assembly, parts being assembled, sub-assembly or assembly testing, sub-assembly transfer between units or lines, packaging, shipment to the warehouse, shipment to the customer, etc..

What preparation or prerequisites should be done prior to a PFMEA?
The process owner should gather the information needed to analyze the subject process. This information may include but is not limited to:


Related Design Failure Mode and Effects Analysis (DFMEA)
Related Preliminary Hazard Analysis – Design and Process
Related product or competitive product search
Applicable performance, material process, environmental and safety standards
Related product complaint history
Related product rework data
Related product scrap data
Summaries from related product or process design reviews
Related product Functional and Design Specifications
Related process requirements
Available models, prototypes, drawings, etc.
Similar product warranty return rate data
Process maps, flowchart.In addition, participants need adequate lead-time to prepare the information listed above.


The Potential Failure Mode (How).
This is the manner in which the process could potentially fail to meet the process requirements or design intent as described in the Process Function or Requirements column (part of the FMEA risk matrix). It is the description of the non-conformance of that specific operation.

Ask the question “How can the process or part fail to meet the requirements?”

There are any number of potential failure modes.

Important considerations at this step:
i) The Potential Failure Mode should be described in physical or technical terms, not as a symptom noticed by the customer or a downstream user.
ii) Assume that incoming parts or materials are correct for this exercise. If concerns arise regarding the correctness of parts or materials due to past history or expertise knowledge, this could be identified by a Design Failure Mode and Effects Analysis (DFMEA) or Component Failure Mode and Effects Analysis (CFMEA)
iii) Encourage team brainstorming. Do not get limited due to artificial restraints (e.g. size of sheet).
iv) Address one process at a time.
v) Brainstorm all of the failure modes on a separate page. Then list the failure modes, one at a time on the PFMEA form and discuss cause before moving on.


Process Failure Mode Effects Analysis

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Potential Effect of Failure (What).
This is the effect of the failure mode on the customer, i.e. what the customer might notice. When determining the potential effects, the team should focus on both the external customers (client, patient, user, standards or government regulations) and internal customers (next or downstream assembler, inspector, process).

Important considerations at this step:
i) It should be stated clearly whether the failure mode could impact safety or cause non-compliance to standards or government regulations, customer requirements.
ii) For the external customer, should always be stated in terms of product or system performance.
iii) For the internal customer, should always be stated in terms of process or operation performance.
iv) There can be more than one internal or external effect listed for a Failure Mode, and this is desirable if safety may be a potential issue. For example: Failure Mode – “Nut comes loose on a medical product such as a scanner with a hand-piece”

External Effect:
(1) Nut falls into patient wound site, causing infection
(2) Hand-piece does not work
(3) Cosmetic issue

Internal Effect:

Fails inspection – rework

This will help identify the most severe failure and the most likely failure.


Severity (SEV).
Severity is the rank associated with the failure effect. In general, the Severity rank should be listed as the most serious effect for the given failure mode. However, if there is a chance of the failure mode having a potential safety issue which would have a different probability of occurring than a non-safety issue, then the Severity rank can be different. Also, the SEV rank for Internal and External can be evaluated separately.
The team should refer to some form of pre-defined* rating scales.
Using the previous example: Failure Mode – Nut comes loose

External Effect:
(1) Nut falls into patient wound site, causing infection – SEV=Critical or (say) rated as a 5.
(2) Handpiece does not work – SEV= Major or (say) rated as a 4.
(3) Cosmetic issue – SEV=Minor or (say) rated as a 2.

Internal Effect:

Fails Inspection causing rework, SEV= Minor therefore rated as a 2.

*Predefined – The risk team/risk process will be working to a risk rating scale, such as:
Critical risks = 5 or 6
Major risks = 3 or 4
Minor risks = 1 or 2

Potential Cause(s)/Mechanism of Failure.
Defined as how the failure could occur described in something that can be corrected or controlled. List, to the extent possible, every failure cause assignable to each potential failure mode. Ask the question, “How could a process operation or assembly step be properly performed?”

Typical types of failure causes:
Inaccurate gauging
Improper torque – over, under
Improper heat treat-time, temperature
Inadequate or no lubrication
Improper weld, current, time, pressure
Part missing
Improper setup
Improper mix ratio

Important considerations at this step:
Work one failure mode at a time.
List only specific causes: ambiguous statements such as operator error and machine malfunction should be avoided.
Team should understand cause and effect relationships. Every reasonable effort should focus on ROOT cause. Ask the question “why” five times, or use off-line problem solving tools.


Probability (PROB).
Probability is the likelihood that a specific cause or mechanism will occur that will lead to the failure. When determining the Probability, take into account the chance the cause will occur, along with it causing the effect.

Important considerations at this step:

Detection of the cause of failure should not be considered when determining Probability.

Preventative control measures can be considered when determining Probability.

Possible failure rates should be based on the number of failures that are anticipated during the process execution. If statistical data is available from a similar predicate process, the data should be used to determine the ranking. In other cases, a subjective assessment can be made using word descriptions.

Current Process Controls
Describe the controls that either prevent or minimize to the extent possible the failure mode or cause/mechanism of failure from occurring, or detect the failure mode or cause/mechanism of failure should it occur.
There are two types of Process Controls, Prevention and Detection

Prevent the cause or mechanism of failure or the failure mode from occurring or reduce the rate of Probability.
Typical types of prevention controls could include: ESD mats, Process validation, Mistake proofing, Gloves, Operability checks, Calibration, Preventative maintenance, etc.

Important considerations at this step:
First approach is normally to use prevention controls. If the prevention controls are already integrated as part of the process intent, it will affect the Probability ranking.

Detection (DET).
Important considerations at this step:
Always assume the failure has occurred and then assess the capabilities of all Process Controls to prevent shipment of the product (or continuation of a process) having the failure mode or defect; again not just the one process control but the sum of all the process controls.
Do not assume the Detection is low because the Probability is low, but do assess the ability of the process controls to detect low frequency failure modes or prevent them from going any further in the process.
Random quality and or visual checks are unlikely to detect the existence of an isolated defect and should not influence the Detection ranking. If this is the only control method, the ranking would be high. Sampling done on a statistical basis is a valid detection control.
Validation is not a guarantee that defects will be detected; therefore, it is often assigned a middle-of-the-road ranking.
Error proofing or mistake proofing and a robust test system with 100% inspection will provide the best chance of detection.
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