MTBF is called Mean Time Between Failures which is the average value of the time when the new product starts to work under the specified working environment until the first failure. Specifically, it refers to the average working time in between two failures. Longer the MTBF means higher the reliability and better ability. The unit of MTBF is by hour, only used for repairable products.

Due to MTBF is an important reference value to be considered when designing products, engineers or designers will use various methods and standards to evaluate the MTBF value of products. There is an easy way to calculate the MTBF. We could estimate the MTBF according to a formula based on failure rate. The formula is MTBF=1/λ. λ: Failure rate (times/year). So, we need to collect data from the actual performance of the product first, and then, we could use this formula to get the MTBF.

As mentioned above, MTBF is a reliability estimation standard. However, the reliability refers to the ability of the product to complete the specified function under the specified conditions and within the specified time; and the failure of the product to complete the specified function means that it is faulty. For example, 100 pcs hard drivers in operation have 2 failures within a year, and its failure rate (λ) is 0.02 times/year.

And MTBF=1/λ. Assume a hard driver with an MTBF of 1.2 million hours. 1.2 million hours is about 137 years, but it does not mean that each hard driver can operate for 137 years without failure. Substituting into the MTBF=1/λ formula, it can be known that λ=1/MTBF=1/137 year, and the average annual failure rate can be calculated to be about 0.7%. It means that within a year, an average of 7 out of 1000 hard drivers will fail.

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Reference : https://www.dekra-ist.com/article_d.php?lang=tw&tb=1&cid=84&id=976

Reference : https://baike.baidu.hk/item/MTBF/2119144