One of the best methods used to increase system reliability numbers is to select component parts that, while operating under actual system operating conditions, do so with significant operating margins specified to exceed the circuit requirements.
To illustrate: If a circuit has a maximum current rating of, say, 150mA, then using a part with a higher current rating of 250mA+, allows for an operating margin that will not unduly stress the part and potentially cause a catastrophic failure or performance degradation.
Take the example of a balloon blown up to its designed maximum elastic size of a one-foot diameter, after which one more puff of air is added, forcing the balloon to exceed its maximum diameter rating. The balloon will experience a catastrophic failure and will cease to be useful as the designer and purchaser intended. But if the maximum design size is increased to allow for a maximum two-foot diameter, and the same amount of air volume is applied, inflating the two-foot balloon to a one-foot diameter, we can logically expect the newly specified balloon to last considerably longer than our first balloon.
This technique of specifying component electrical and environmental parameters above and beyond the circuit operating requirements is known as “derating.” An excellent axiom for this is in the case of aluminum electrolytic capacitors. For every increase in temperature of 10° Celsius, the expected life of the capacitor is divided by half. Conversely, reducing the temperature by 10° Celsius doubles the life of the capacitor.
So if I have an operating temperature of 40°C, I will want to select a capacitor that is rated in excess of 40° and consider that every 10 degrees operating margin I add will double the life of the previous life span, then I will select a capacitor that is readily available with a commercial rating of 70°C. But if my operating temperature is 50° to 60°C, then I should select an 85° capacitor to guarantee it's not the part that is most likely to fail first in the field. This is one method or technique for designing-in reliability at the earliest stages of development.
Please click here for derating data for most of the major component types used today. Feel free to download and distribute the PDF to all design engineers working with you. I created this guideline because at the time of its generation, there were no comprehensive compilations covering every component type. The data was built based upon Bell Labs, Mil-217, and various research study results derived from HALT and HASS reports published by reliability organizations such as RIAC (Reliability Information Analysis Center).
The nature and magnitude of the forces that generate the various stresses in each type of component are usually listed on a component's datasheet so the designer or the component engineer can select the appropriate margin required on the derating guideline. Examine the guideline component categories to determine which forces or stresses will need to be considered for each derating specification.
Use the derating guidelines to quickly expedite the component selection process based upon providing enough operating margins to assure the longest, reliable, usable component and system lifetimes. Remember, every component in a circuit experiences constant sources of stress. In-rush or high current, power surges, over-voltage conditions, etc., will lead to a component's premature failure if the component was not selected with these potential accelerated stresses in mind.
A component or design engineer will select a part considering the worst-case operating conditions and then build in the additional margin of safety for long component life, thereby increasing the reliability of the entire circuit or system. In my guideline, the force or stress is identified under the “Derating Parameter” column. All stress parameters are not covered in this guideline. Data for connectors like “Insertion and Withdrawal Force” and “Duty Cycles” are also not covered.
For this reason, carefully examine the manufacturer's datasheets to determine the parametric you will need to reference for specifying the operating margins. Remember to consider the complete system operation conditions, including the worst-case environmental stresses when defining the derating characteristics. This is one discipline that overlaps with the reliability engineering department, but the component engineer is responsible for the components selection and must work hand-in-hand with the reliability people if reliability has been established as a separate department in the company. Also on the Website referenced above is a sample spreadsheet with cell formulas for automatically calculating a bill-of-materials derating suitability. Use this spreadsheet as a template and plug in your own BOM components.
Designing without derating in mind will make failure prediction data unreliable. You can easily double a product's usable lifetime by using good derating practices.