Understanding Mean Time Between Failure (MTBF)

Each MTBF calculation standard has its own set of assumptions, which can significantly impact the outcome.
For example:

Comparing MTBF values calculated using these different standards is like comparing apples to oranges, the underlying assumptions are completely different.

Suppliers sometimes calculate mean time between failures under idealized conditions to present higher figures.
For instance:

• Assuming the product will operate in low-stress environments with stable temperatures.
• Ignoring real-world factors like power surges, mechanical vibration, or temperature fluctuations.
• Relying on derated components (operating well below their maximum ratings) that might not reflect typical usage.

If Supplier A uses more optimistic assumptions than Supplier B, their MTBF value will look better on paper, even if the product isn’t as reliable in real-world conditions.

The reliability of a product isn’t just about numbers; it’s about the quality of its components and how it’s designed:

• One supplier might prioritize cost efficiency, which could increase stress on components and reduce reliability.
• Another option might use better components. This could lead to improved long-term performance, but it may cost more.

MTBF calculations often fail to capture these nuances, making it hard to judge the true robustness of a product.

Suppliers rely on component failure rate databases to calculate MTBF. The accuracy of these databases can vary:

• Some suppliers might use generic failure rates that don’t reflect actual component performance.
• Others might rely on outdated or overly optimistic data.

If one supplier uses better (or worse) failure rate data, their MTBF figures could look drastically different, even if the products are otherwise similar.

Always check which standard was used for the calculation and what assumptions were made (e.g., operating conditions, component quality). In RECOM data sheets the MTBF calculation conditions are clearly stated. For example, “according to MIL-HDBK-217F, GB, +25°C”, which means that the standard used was the Military Handbook number 217, Edition F, with the controlled environmental conditions of Ground Benign, and an ambient temperature of 25°C.

Only compare MTBF values calculated under identical conditions using the same standard. If a competitor uses the same calculation assumptions, the MTBF calculations can be compared. However, if they choose different temperatures or environmental conditions, the results may not be the same.

This is because the mean time between failures calculation uses the base reliability of the components multiplied by several operational influencing factors, such as the environment (e.g. temperature, shock, vibration), component interaction (e.g. cross interference or mutual warming) and quality (component tolerances, quality control, etc.). For example, one of the most important variables is the quality factor. RECOM carries out supplier audits and our manufacturing facilities are ISO certified, meaning that we have consistent component performance.

Evaluate real-world performance metrics like field failure rates, warranty returns, and operational uptime. MTBF is a predictive tool. If the failure rate of a power supply should be less than 1% after 2 years of service, then the required MTBF will be (2 x 365 x 24) / 1% = 1.75 million hours.

So, for example, if the application needs a compact 5W AC/DC power supply and RECOM’s RAC05-K/277 series is used (MTBF at 25°C = 2.25 million hours), the designer can be confident that the field failures due to the power supply will be significantly less than one in a hundred (actually closer to one in two hundred and fifty installed units) after two years of active service.

A product with a lower MTBF might still outperform in your specific use case if it’s better designed for your environment (e.g., extreme temperatures, vibrations). Returning to the RAC05-K/277 example above, the mean time between failures at +40°C is 1.8 million hours, so even at elevated ambient temperatures, the field failure rate can be expected to be still below one in a hundred installed units after two years of active service.

This part also meets the military grade shock and vibration standard MIL-STD-202G, has an operational temperature range of -40°C to +90°C and is certified to over voltage category OVCIII. Each of these additional specifications reinforces the confidence in the MTBF figure.