Failure Mode and Effect Analysis (FMEA) is a methodology for analyzing potential problems early in the product development cycle where it is easier to take action to overcome potential issues, thereby enhancing reliability through design. The tool is used to identify relationships between process and product requirements and the potential for unacceptable outputs and their effects.
FMEA was initially developed in 1949 by the US Armed Forces to classify failures "according to their impact on mission success and personnel/equipment safety." It was later adopted in the Apollo space program to mitigate risk. In the 1980s, the automotive industry began implementing FMEA by standardizing the structure and methods through the Automotive Industry Action Group. Although developed by the military, the FMEA methodology is now extensively used in a variety of industries including semiconductor processing, foodservice, plastics, software, and healthcare to name a few.
FMEA takes time to perform, but if done accurately it provides a valuable tool to plan, detect, and react to ensure a successful product life cycle. Continuing to use the tool throughout the product life cycle will significantly improve safety, quality, delivery and cost. Additional benefits from the tool can be gained through cause chain analysis and mistake-proofing (i.e. poke yoke) to reduce Risk Priority Numbers (RPN).
The FMEA is developed prior to the launch of a new job or process and is maintained throughout its life. As a tool embedded in Six Sigma methodologies, FMEA also helps identify the controls (through development or production) that must be implemented to ensure that the product can be produced continuously within specification. During the life of a product/process, the tool — considered a living document — should be modified whenever an existing design, product or process is changed, or even when a derivative is in development. Furthermore, the tools should be used when a new design, product or process is in the discovery process.
There are several applications for FMEA, including:
- design, which focuses on components and subsystems;
- process, for manufacturing and assembly processes;
- system, which orients on global system functions;
- service functions;
- software functions. This article will focus on two primary formats: DFMEA (design) and PFMEA (process).
DFMEAs are initiated during the conceptual phase of new product development. A DFMEA provides an analytical analysis of the potential failure modes and associated causes. By considering the failures associated with a design — including safety, quality, cost, performance and reliability — the processes associated with development or manufacture will be significantly reduced. Additionally, the development of specifications associated with the offering will ensure a product capable of meeting defined requirements.
The example below shows the use of DFMEA during the development of a new sensor. Note the use of critical characteristics under the "Class" column. Critical characteristics are identified during the design of a product to call out aspects that must be given special attention. Critical characteristics are often defined as a product requirement (i.e. dimension, feature, performance aspect) of such significance that if defective or inadequately produced, it would cause personnel injury, loss of station or loss of mission (such as critical bolt torquing specified by drawings and/or procedures). Critical characteristics are identified on applicable drawings/specifications for the hardware/software under surveillance. Companies use various symbols for identifying critical characteristics. At Watlow, for example, a black diamond (♦) is used in the DFMEA to call out a requirement needing special attention.
Once the DFMEA is completed, the PFMEA can be developed to control the production process. At Watlow, PFMEA documents are developed based on part numbers. This means the PFMEA lists the steps for the build of a part from receipt of materials through to shipping. The disadvantage of this approach is that when a change is made to a process common to more than one part number, all PFMEA documents containing references to that process must be changed. Using a process structure — versus the part number structure — it becomes clear as to the benefits gained by focusing on the process. Take Watlow's induction braze process as an example. If a PFMEA is developed for induction brazing, it can be applied to all part numbers that utilize the induction braze process. When improvements are made to the induction braze process, updates are made to the PFMEA specific to this process. The benefit is that only one change must be made on one document to cover all part numbers that use this process. Additionally, the PFMEA can be used to prioritize improvement using a RPN for a process that relates to many part numbers.
What is a Risk Priority Number?
Using FMEA, you can derive a RPN by determining the severity of the potential failure mode, the possibility of occurrence, and the likelihood that a defect will or will not be detected. Once the FMEA is understood, it is easy to determine which area has the greatest concern and, therefore, take action to prevent the problem before it arises. Let's take a look at how the RPN is calculated. In the example above, the process function/ requirement that has been identified is the "joining of similar metals via solder process." Moving to the right, the potential failure mode is "incomplete braze joint." The effect that each potential failure mode may have on the product is listed in the potential effect(s) of failure column (intermittent function and inoperable function), and is ranked with a severity (SEV) number. The severity number is ranked 1-10 with 10 being the most severe and 1 having no effect.
We can now assess what might be the potential cause(s)/mechanism(s) of failure. For example: improper setup (i.e. power setting, coil size, time setting, etc.), improper coil condition and improper flux application (as shown in the PFMEA below) can all determine the failure occurrence (OCCUR). This number is also ranked from 1-10 with 10 having an almost inevitable chance of failure and 1 having a very remote chance of failure. We can then proceed to list the current process control and project the possibility of detection (DETEC) that these controls will prevent. We then rank each item 1-10. However, the ranking is reversed where 10 is almost impossible to detect and 1 almost certain. This is then repeated down the PFMEA form.
After ranking the severity, occurrence and detection modes, we can calculate our risk by multiplying the three numbers and placing the result under the RPN column. Once this is done for the entire design and/or process, the areas of greatest concern are readily identified by simply comparing the RPN numbers. The process(es) that have the highest RPN should be given the highest priority for corrective action. In the example, the greatest area of concern would be documented work instructions to prevent intermittent function as a result of an improper setup (RPN = 512).
Often we want to address situations that are the most severe (inoperable function with a SEV ranking of 9). However, this may not always be true. For instance, the intermittent function (SEV ranking of 8) is identified as having the greatest risk (RPN of 512) because it is more likely to occur (OCCUR ranking of 8) and less detectable (DETEC ranking of 8). In conjunction, we want to ensure that the PFMEA focus is on process controls (i.e. poke yoke, statistical control of critical equipment parameters, etc.) that prevent potential failure modes.
To complete the PFMEA, recommended actions with targets and dates of implementation are noted. Once the actions have been implemented, the severity, occurrence and detection are again ranked in the far right of the form and actions prioritized accordingly. These numbers can easily be put into a graph, providing a visual representation of activities that should be prioritized to mitigate risk.
The use of the DFMEA to effectively develop a product design from concept to production further allows for an effective PFMEA to be created and processes deployed. Using the two documents interactively has helped companies like Watlow meet customer specifications while reducing design changes, reducing scrap, improving delivery and controlling costs.