DFMEA – design based root causes

The company should adopt these sample ratings to what is appropriate to their industry. This table of a Severity ranking scale provides guidance for assigning criteria values to the failure mode effects on a consistent basis. It is only a sample and should not be used without adaptation to the industry and company. The project team should evaluate each effect and reach a consensus on their ranking values. The idea is to find comparative differences among the effects so that their associated causes can later be prioritized. Failure modes with a severity rank of 1 probably do not need to be analyzed further.
It is an indication of a design weakness, the consequence of which is the failure mode. Consider the possible design mechanisms and/or causes of each failure mode. Analyze what conditions can help bring about the failure modes.


Make sure the list of causes is thorough. This helps point the way toward preventive/corrective actions for all pertinent causes. Consider the difference between effects, contributing causes, and root causes.

Examples of causes:

* Wrong polymer specified → moisture absorption → loss of dielectric
* Excess annealing → malleable base material alloy → bent tab
* Maximum material condition stack-up → worn mating surface
* Insufficient gold plating specified →  loss of signal transmission integrity
* Least material condition stack-up → gap → Tab edge does not engage cam

Here you are looking for the potential design-based root causes of the potential failure modes. This includes causes that occur in production but are due to the design. Do not include causes that are due strictly to errors or flaws in the production process – save these for the PFMEA. Again, refer to the completed FTA to help ensure the list of causes is as thorough as possible, and that the root causes are identified. Other possible causes could include improper material specified for process or end-use operating environment, incorrect algorithm, incorrect software specification, insufficient re-lubrication capability, incorrect cam path, etc. Other possible failure mechanisms could include creep, fatigue, wear, galvanic action, EMI (electromagnetic interference), etc.

* The probability that the cause will happen, and that it will result in the failure mode, on a 1 to 10 scale. A 1 reflects a remote possibility of it happening and a 10 indicates a high probability of the cause and its failure occurring in major proportions.
* Ask “How likely to happen is the cause?” and then “How likely is it to result in the potential failure mode?”
* Generally, to reduce the probability of the failure mode happening, a design change will be required.

Occurrence is the second of the 3 Risk Priority Numbers (RPN) rating criteria. Think of the relative likelihood or probability of the cause actually
happening during the design life of the part and then resulting in the failure mode.

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