Key Takeaways

• What is FMECA?

• What is fault tree analysis?

• A Comparison of FMECA vs fault tree analysis risk management approaches.

• How to apply risk management during PCBA design.

Qualitative vs quantitative analysis.

For millennia there has been a battle over what is the best approach to a solution for many of life’s greatest challenges. On the one hand, there is a more philosophical, qualitative view that espouses the need for a complete understanding of a problem’s impact and effects. Then, there is the more pragmatic, quantitative strategy that is built on dividing a problem into mathematical models that can be discretely processed. This fight certainly makes for interesting debates. However, for many actual problems, the best approach may not be either qualitative or quantitive. Instead, it may be both. 

For PCBA development, the important objectives are for your board design to be manufacturable and that the built board reliably performs as desired.  The importance of the latter is evidenced by the fact that many companies employ reliability engineers whose responsibility is to ensure that boards and electronics meet their performance criteria over their lifecycle. Aside from always designing and building perfect boards, the best way to ensure reliability is by having an effective risk management plan in place. 

Although good risk management plans are comparable in terms of trying to identify the root cause of risks or failures, the approaches differ. There are top-down and bottom-up design architectures and there are qualitative and quantitative strategies. Let’s take a look at two methods that exhibit these varying perspectives--FMECA vs fault tree analysis--and see which is a better solution for risk management of PCBA development.

The FMECA Quantitative Approach to Risk Management

As failure modes and effects analysis (FMEA) is a prelude to failure modes and effects criticality analysis (FMECA), it is informative to explain the former first. For PCBAs, FMEA is a quantitative technique that can be used to improve the reliability of components, the supply chain, board development, and production efficiency. FMEA may be implemented by top-down or bottom-up design. The method depends upon the ability to identify and mathematically represent risks and their impact or effect in the form of a risk priority number (RPN). The RPNs prioritize risks from the most severe to the least and, if necessary, are used to devise controls to be activated to mitigate the impact of the risk, if or when it occurs.  

FMECA supplements FMEA by adding a criticality component, as shown in the figure below.

Relationship between FMECA and FMEA.

Criticality, which can be viewed as refining the FMEA results such that a specific fault mode occurrence can be linked with a definitive effect in terms of its actual impact on the process under evaluation and expressed or measured, is given in the following equation:

The above includes the essential parameters for criticality analysis. These may be customized to include other factors such as operation time. 

Although typically implemented as a quantitative method of risk management, FMECA may be performed qualitatively where criticality is defined as one of a number of general conditions, such as major, minor, severe, or critical. Let’s look at another qualitative method--fault tree analysis.

Managing Risks Qualitatively with Fault Tree Analysis

Fault tree analysis binary structure example.

Another powerful tool for assessing the reliability of a product or process is fault tree analysis (FTA), an example of which is depicted in the figure above. As shown, FTA is a graphical representation of the path from the source through the impacted stages that illustrates the effects of a fault. The effects are represented as binary events (fail or not fail), which enables the use of the logic gate structure shown above. 

FTA is an intuitive, qualitative method that consists of progressive steps for identifying the faults that can cause or contribute to a system or process failure. Therefore, it is applicable to circuit board development and production. And similarly to FMECA, either a top-down or bottom-up design approach may be taken. If explicit data about the components or steps that comprise the process being analyzed is available, then FTA may also be implemented as a quantitative technique for specific cases. How does FTA stack up against FMECA in improving PCBA reliability through risk management? This question is answered below. 

PCBA Design for FMECA vs Fault Tree Analysis Risk Management

FMECA and FTA are well-defined and utilized methods for identifying the type of risks that are present during the PCBA development and production processes. The similarities do not end there, as the table below shows. 

The question of which is better may be more a matter of preference than functionality or capability. 

Irrespective of which of these risk management methods are utilized, the level of success depends to a significant degree upon the capabilities and cooperation of the contract manufacturer (CM) that builds your boards. However,  with the right PCB Design and Analysis software, you can create and analyze your risk management within design. Cadence Allegro provides all the functionality needed to take control of your risk management and ensure that your development or production process is optimized for board reliability.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.