Reliability Services in the Design Phase
Failure Modes Effects and Criticality Analysis (FMECA)
In a FMECA, each failure mode of the product is identified and then evaluated for criticality. This criticality is then translated into a risk, and if this level of risk is not acceptable, corrective action must be taken. Risk Management is a process for identifying hazards associated with a product, estimating and evaluating the associated risks, controlling these risks, and monitoring the effectiveness of the control. The process includes Risk Analysis, Risk Evaluation, and Risk Control. Risk Management uses a Failure Modes, Effects, and Criticality Analysis (FMECA) as a tool when evaluating and controlling risks.
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FMECA's are performed not only on the hardware design but on the software, the manufacturing processes used to build the product, and the user interfaces (everyday use and abuse as well as preventive maintenance tasks).
Whenever a user is involved, we must pay specific attention to the possibility of the user using the product incorrectly, risking either the integrity of the product or, worse, possibly creating an unsafe situation.
The objective of a FMECA is the early identification of all catastrophic and critical failure possibilities so that they can be eliminated or minimized through design correction at the earliest possible time.
VALUE TO YOUR ORGANIZATION
By understanding the critical failure modes of a design and their effects, we can perform a risk assessment and if the risk is deemed too high, we can work to mitigate the failure mode, thereby reducing the risk.
An example of Reliability Integration during FMECA is as follows:
FMECA's can identify failure modes that require extra planning to find during HALT
One of the first steps in planning a HALT is to determine which stresses to apply and what test routines and monitoring techniques to use. A FMECA can identify failure modes that may be difficult to find, requiring special types or sequences of stresses as well as how to exercise the product so that these types of failure modes are being looked for.
When we perform a FMECA, we are identifying all potential failure modes and their associated effects. To make this task more manageable, we must first decide what type of FMECA we want to perform - Design, Process, User, Software, Test, to name a few. We must also decide if we want to perform the FMECA at the piece-part level or functional level. At the piece-part level, we start with each individual chip, process step, software subroutine, etc. At the functional level, we identify the major functions of the product and identify failure modes for each of these functions.
FMECA's are similar to FTA's. The big difference is an FTA starts with one specific failure effect and then identifies only those failure modes that can cause the particular effect, whereas a FMECA is trying to identifying all possible failure modes of a product and the effects of these failure modes.
- How we decide whether to use an FTA or a FMECA
FTA is preferred over FMECA when:
- A small number of top events can be identified
- Product functionality is highly complex
- The product is not repairable once initiated
- FMECA is preferred over FTA when:
- The events cannot be limited to a small number
- Multiple successful functional profiles are possible
- Identification of "all possible" failure modes is important
For each type of FMECA, the steps are as follows:
1) Brainstorming Facilitation
The first phase is the brainstorming session in which we facilitate a series of meetings with key individuals within your organization (design, software, manufacturing engineering, customer services, etc.) and come up with the most critical failure modes in the system and the effects these will have on the customer's equipment as well as safety of personnel.
2) Scoring System Development
Developing the scoring system is a very important piece of the FMECA because this scoring system must be tailored to the specific product being analyzed in order for the process to yield the proper results.
The scores consist of Severity of Failure, Probability of Occurrence, Ease of Detection, and Overall Risk Priority Number.
We shall work with your team to help develop this scoring system, educating you in the process.
3) Facilitation of FMECA Process
Throughout the process, we will facilitate the process and assure that we drive the FMECA following the process identified below:
a) definition of the system and its functional and minimal operating requirements;
b) development of functional and reliability block diagrams and other diagrammatic or mathematical models and descriptions;
c) establishment of basic principles and corresponding documentation in performing analysis;
d) identification of failure modes, their causes and effects, their relative importance, and their sequence;
e) identification of failure detection and isolation provisions and methods;
f) identification of design and operating provisions against particularly undesirable events;
g) determination of event criticality;
h) evaluation of failure probability;
i) search for specific combinations of multiple failures to be considered.
4) Mitigation of Failures
Without a method of mitigating each failure mode, the process is only half complete. We must now take each failure mode with a high risk number and come up with a method of mitigating or reducing the total risk. This may be by reducing the severity when the failure does occur, by reducing the probability of the failure occurring, by increasing the likelihood of detecting the failure before the product is released, or a combination of all three.
5) Criticality Analysis
What makes a FMEA into a FMECA is the criticality analysis. When we talk about FMECA's (Failure Modes, Effects, and Criticality Analysis), we have introduced criticality into the analysis by evaluating each failure mode not only by the effect it will have on the system and the user, but how critical the failure will be.
Criticality is a relative measure of the consequences of a failure mode and its frequency of occurrence.
The following case studies and options provide example approaches. We shall tailor our approach to meet your specific situation.
1) FMECA used to Help Write a HALT Plan
A Semiconductor Manufacturing Equipment company was embarking on a HALT project and wanted to identify non-relevant failure modes before the HALT so that they could design the test so as not to find these issues during HALT. We facilitated a FMECA for them and helped them identify two non-relevant failure modes, and we fed these into the HALT Plan. This saved them a lot of time and money during the HALT process.
2) User FMECA (also known as User Task Analysis)
A Medical Device company had us help them with a User FMECA to determine all instances in which the user will come in contact with the product and then analyze each of these to determine what can go wrong - either due to the user not following instructions, lack of instructions, product failures, etc.
3) Design FMECA
A Power Supply manufacturer needed a Design FMECA at the component level - analyzing all major failure modes for each component in the product. During the FMECA, we paid special attention to those components whose failures could cause safety-related effects.
There are many different companies that sell FMECA software packages and there are many different FMECA methodologies, handbooks and guidelines (IEC 812, Sematech E14, RAC-FMECA, MIL-STD-1629, AIAG EIA/JEP131, RADC-TR-83-72, etc). As consultants, we maintain an objective viewpoint in these areas and first evaluate our clients needs and then make recommendations as to the best sofware package and the best methodology.