Product design is a complex process that often involves bureaucracy and document preparation to show compliance with regulations and customer requirements.
Reliability, Availability, Maintainability and Safety (RAMS) analyses are standard requirements in industries such as aerospace, defense, rail, automotive, medical, and maritime. A key parameter in these analyses is Reliability.
When test or field data exists, the data can be used for reliability calculation. However, when designing a new product this information is not available, and accelerated life tests are expensive and time consuming. Therefore, reliability prediction is done following industry specific standards (MIL-HDBK-217, FIDES, Telcordia, IEC, Physics of Failures, reliability physics etc.). Each prediction method requires a set of data which represents the design such as: environment, temperature, components type, PCB structure, vibration, duty cycle and most importantly: the electrical stress on each component.
During reliability analysis you can choose between two strategies:
1. Improve the report
This attitude is geared toward the short-term goal of providing fast satisfactory reports.
You can quickly get a reliability prediction by using a default assumption of 50% electrical stress on the components.
However, this will not prevent your product from malfunctioning due to over-stressed components and other design errors that you failed to identify.
Another issue is that in most designs the actual stress is less than 50%, therefore this conservative assumption only provides a lower limit of the reliability.
The 50% stress assumption also provides high calculated power and temperature which may result in over-design (for example: adding unnecessary heat sinks).
To improve the report you may calculate actual stresses on a few key components, but most design errors and over-stressed components remain hidden.
2. Improve the product
This attitude is geared toward the product improvement, or as one of our customers said: “We have to do these reports, so we might as well learn something from them”.
By calculating the actual electrical stresses you gain:
- Early identification of overstressed components and design errors
- Identify over-designed components that can be replaced with more economic ones
- Increase your product reliability and robustness
- Increase accuracy of the reliability prediction
Calculating the component stresses is good practice that should be implemented as part of the design process.