Limitations of Reliability Prediction

To use quantitative reliability prediction methods such as MIL-HDBK-217 and Bellcore wisely, you should be  aware of their limitations. Like all engineering models, the failure rate models are approximations to reality. The  failure rate models are based on the best field data that could be obtained for a wide variety of parts and systems;  this data is then analyzed and massaged, with many simplifying assumptions thrown in, to create usable models.  Then, when you use the model, you make more assumptions for the design parameters you enter, such as stress and temperature.

Thus you should not treat a reliability prediction number for your product as an absolute prediction of it's field failure rate. It is generally agreed that these predictions can be very good when used for relative comparisons, such as comparing design alternatives, or comparing products. Note also that reliability predictions do not account for substandard quality control for purchased parts, bad workmanship, poor product level quality control, overstressed field operation, etc.

Many people get caught up in the numbers game, manipulating the reliability prediction numbers for one purpose or another; you will be best served if you use reliability prediction as one of the tools that can guide you to more reliable products.

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Even though a product has a reliable design, when the product is manufactured and used in the field, its reliability may be unsatisfactory. The reason for this low reliability may be that the product was poorly manufactured. So, even though the product has a reliable design, it is effectively unreliable when fielded which is actually the result of a substandard manufacturing process. As an example, cold solder joints could pass initial testing at the manufacturer, but fail in the field as the result of thermal cycling or vibration. This type of failure did not occur because of an improper design, but rather it is the result of an inferior manufacturing process. So while this product may have a reliable design, its quality is unacceptable because of the manufacturing process.

Just like a chain is only as strong as its weakest link, a highly reliable product is only as good as the inherent reliability of the product and the quality of the manufacturing process.

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