A copy of the Process Failure Mode and Effect Analysis (PFMEA, or Process FMEA), reviewed and signed off by supplier and customer. In full and partial turnkey PCB assembly, the PFMEA follows the process flow steps, and indicates “what could go wrong” during PCB design, PCB fabrication and PCB assembly (PCBA).
PFMEA is a living document that should be initiated prior to process of production and maintained through the lifecycle of the product. PFMEA evaluates each process step and assigns a score on a scale of 1 to 10 for the following variables:
- Severity — It assesses the impact of the failure mode (the error in the process), with one representing the least safety concern and 10 representing the most dangerous safety concern. In most cases, processes with severity scores exceeding eight may require a fault tree analysis, which estimates the probability of the failure mode by breaking it down into further sub-elements.
- Occurrence — It assesses the chance of a failure happening, with one representing the lowest occurrence and 10 representing the highest occurrence. For example, a score of one may be assigned to a failure that happens once in every five years, while a score of 10 may be assigned to a failure that occurs more frequently, such as once per hour or once per minute.
- Detection — It assesses the chance of a failure being detected, with one representing the highest chance of detection and 10 representing the lowest chance of detection.
- Risk priority number (RPN) = severity X occurrence X detection. By rule of thumb, any RPN value exceeding 80 requires a corrective action. The corrective action ideally leads to a lower RPN number.