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.
@Michal - You bring up a good point regarding the footprint of substituted components. If the footprint for an upgraded part changes, the cost can be more than just the repalcement component - it can require a circuit board redesign, or at least the addition of an interposer. The same problem sometimes occurs when a part goes obsolete, and another device that performs the same funcxtion has to be found.
I think in typical hardware design, even for consumer electronic products, people still apply this technique at 10 to 20 percent margin, for safety purpose. Of course, cost will come into play and has an important say as well.
Michal, you make a very good point about considering the cost ramifications that go with derating. There are direct cost, such as the cost of a higher rated component and the burdened inventory cost associated thereto, but there are also consequential costs associated with poor reliability. These costs include, field returns for repair or replacement, customer satisfaction and reorder potential, company reputation, administrative costs for ECO/ECN processing, and other penalties that go with any late decisions affecting WIP quantity, RTV, stock purge, etc. A Component Engineer has to be able to select the lowest cost components without sacrificing reliability. In cost reduction efforts, most of the cost savings can be realized by renegotiating the cost of the components with the supplier, or moving to higher levels of integration and consequently reducing component BOM count. But as you rightly stated, higher rated components, usually have a cost increase impact. For this reason, a Component Engineer plays a very vital role in the price setting of products and the inventory cost accross all product lines. Knowing when, where, and how to make the correct trade-off decisions and being able to justify them to management is one of the most important roles the CE plays in company and product credibility and longevity. Sometimes, the real struugle is getting management to take the long-range perspective vs. instantaneous part cost impact.
Antonio, I apologise for the massive typos on that last response. Short answer: If you derate, you can pretty much guarantee the Manufacturer's F.I.T. number and use it in the Part Count Roll-up method. Here is the most excellent Vicor paper:www.vicr.com/documents/quality/Rel_MTBF.pdfSimilar
Not helpful? You can block www.vicr.com results when you're signed in to search.www.vicr.com
Hi, I agree with this article in principal of the functionality, but when you want to setup let call it "safety area" for components we have to have in focus also same dimension of the footprint and body of the components, also next class is price of the components. When we are talking about commercial and industrial range (temperature) of the ICs we have to have in mind the end price of the ICs.When I was work in Power-one I was part of supply chain as technical support. I had a lot of replacement of the componnets where we had to secure the sources of components.Some time we chose better electrical properties as temperature range, telerance of the value and the duty for this new sources was UNIT price of the components.Special cases what I was solving in my profesional life was replacement of the ceramic capacitor what was under temperature stress aprox. 150 degree.We had to had look to the military speciefied high temperature range provided by NOVACAP. At the end of my comment I want to bring to front also the supply chain view, in the EMS bussines wher I work now everithing is going around price focus, that means going to high runner componnets not to define in design very special componnets which going to be single source in future, but some components have to have be as single source there is no other way in design.
Antonio, Excellent question! There are various ways to compute MTBF. The quickest way is called the "Part Count Roll- up" method. This is where the manufacturer issues the F.IT. Number the components, and the CE or Reliability Engineer just adds up all the F.I.T. Numbers and plugs them in to a formula, usually in a simple spreadsheet cell format. But this is an over simplification for anything but ground benign conditions. The other method, not derived by actual HALT or HASS lab testing methods, is to consider all the stresses that the component will experience in actual operation. Every additional stress factor, like going from ground benign to aerospace conditions, adds a percentage or decimal point fixed or known calculation factor to the reliability quotient. So where one would consider ground benign as a 100% MTBF roll-up result, adding an environmental condition to the formula may change the 100% or factor of 1, to 90% or a multiplying factor of .9. In Reliability calculations, the various conditions have fixed quotients so identifying the operating conditions and plugging in those fixed factors, gives a. Ore realistic MTBF. So, hypothetically, if all components had only one stress factor, temperature and the highest environmental temperature was 100C, then specifying all parts for 125C, would bring the temperature factor for stress to 1 and temperature would not induce a degradation in MTBF or actual performance. I say hypothetically, because temperature always accelerates degradation, however for the formula purpose, be derating all components by specifying 125C, you are on much safer grounds that your MTBF calculation, using the Manufacturer's FIT data can be considered as a reliable number. Ask the manufacturer to give you the various FIT numbers under various stress conditions, and 9 out of 10 times, they won't furnish it. At this point, if there are any Reliability Engineers that would like to comment, we would all like to hear a less verbose response to Antonio's question.
A paper by Scott Speaks, Vicor Reliability Engineer entitled "Reliability and MTBF Overview" is an excellent and thorough response to this question. It should be in every Component Engineer's reference library.
It's very interesting guidelines to derating electronic components, but I have a question: How does much increase component's MTBF or lifetime use the mentioned derating in the guideline? For example if we take the film capacitor derating about DC voltage of 60%, how the MTBF will increase?
I hope that they are dissemeniated thoroughout the world because these tools, guidelines, and procedures just make all our our jobs easier and the products more reliable.
@Douglas, I totally agree with you. Tools like this really make life of designer's easy. Looking forward for more such tools from you in your future blogs. Thank you again.
It is really gratifying to see the value and effort of these articles and associated PDFs appreciated. It is very considerate of you to take the time to respond. By not charging a fee for tools like these, I hope that they are dissemeniated thoroughout the world because these tools, guidelines, and procedures just make all our our jobs easier and the products more reliable. Once again, thank you for the very encouraging words.
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