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. Another way to say this is, if a circuit has a maximum current rating of 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. Think of a balloon that is blown up to its designed maximum elastic size of a one foot diameter. Add one more puff of air forcing the balloon to exceed its maximum diameter rating and the balloon will experience a catastrophic failure. The balloon will cease to be useful as the designer and purchaser of said balloon intended. But if the maximum design size is increased to allow for a maximum 2 foot diameter, and the same amount of air volume is applied, inflating the 2 foot balloon to a 1 foot diameter, we can logically expect the newly specified balloon to last considerably longer than our first balloon. This technique of overrating parts compared to their circuit 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 degrees C, the expected life of the capacitor is divided by half” Conversely, “Reducing the temperature by 10 degrees C, doubles the life of the capacitor.” So if I have an operating temperature of 40 degrees C, I will want to select a capacitor that is rated in excess of 40 degrees C 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 70C. But if my operating temperature is 50C-60C, then I should select an 85C capacitor to guarantee it is not the part that is most likely to fail first in the design. This is one method or technique for designing-in reliability at the earliest stages of development.
Under the Procedures and Guidelines tab, and below you will see the Derating Guidelines PDF listing the suggested operating margin data for most of the major component types used today. I created this guideline because at the time of its generation, there were no comprehensive compilations covering every component type. I built the data 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 is usually listed on a component’s datasheet so the Designer or the Components 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.
On the 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 not covered on the guideline. 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 Components Engineer is responsible for the components selection and must work hand-in-hand with the Reliability people if Reliability has been establishes as a separate department in the company.
Also, be sure to view the PDF showing Derating analysis results. These data were recorded subsequent to actually testing each component’s Derating parameter characteristic while the circuit was operational. Read the short synopsis next to the PDF for a further explanation of the testing process.
Derating Guidelines are designed to quickly expedite the component selection process based upon providing enough operating margins to assure the longest, reliable, usable component and system lifetimes. Every component in a circuit experiences constant sources of stress. A popular rule of thumb is that every increase in temperature of 10 degrees, will shorten the component’s life by 50%. 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. I created this table so a designer or a Components Engineer may select a part by accounting for the worst case operating conditions and then building in an additional margin of safety for long component life and thereby increasing the reliability of the entire circuit or system
This is a complete study verifying the operating margins of every component used in an entire product assembly. The colors on the spreadsheet are identifiers pointing to schematic pages where each component is found in the design. Using the de-rating guideline under Procedures and Guidelines on the website, the operating limits and margins are established. The actual testing of the components under actual circuit conditions was performed by an electronics technician. The data was recorded on the spreadsheet and the formulas for comparing allowances and limits were inserted. As you examine the spreadsheet, you will see just a few red blocks indicating that the corresponding component selection made by the Design Engineer was not optimal and exceeded the safe operating requirements. This study allows the CE to confidently support the MTBF prediction and convince the customer that the life-time of the product is a derived number and not just Marketing or Sales hype.