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Derating: Extending Component & System Lifetimes

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Eldredge
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Re: "Derating" or "safety factor" ?
Eldredge   11/28/2011 6:51:00 AM
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@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.

t.alex
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Re: Derating
t.alex   11/26/2011 6:09:01 PM
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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.

Douglas Alexander
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Re: "Derating" or "safety factor" ?
Douglas Alexander   11/7/2011 3:44:54 PM
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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.

Douglas Alexander
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Re: Re : Derating: Extending Component & System Lifetimes
Douglas Alexander   11/7/2011 12:07:41 PM
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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

 

Michal Mondek
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Re: "Derating" or "safety factor" ?
Michal Mondek   11/7/2011 12:05:31 PM
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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.

Douglas Alexander
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Re: Re : Derating: Extending Component & System Lifetimes
Douglas Alexander   11/7/2011 11:37:14 AM
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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.

antonio.gabello
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Re: Re : Derating: Extending Component & System Lifetimes
antonio.gabello   11/7/2011 6:06:58 AM
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Hi,

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?

Best regards

 

anandvy
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Re: Re : Derating: Extending Component & System Lifetimes
anandvy   11/7/2011 4:07:07 AM
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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.


Douglas Alexander
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Re: Re : Derating: Extending Component & System Lifetimes
Douglas Alexander   11/7/2011 3:00:21 AM
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Anandvy,

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.

anandvy
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Re : Derating: Extending Component & System Lifetimes
anandvy   11/7/2011 2:31:11 AM
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Feel free to download and distribute the PDF to all design engineers working with you.

@Douglas, thank you for the post and the pdf. They are very useful. I am definitely going to share this pdf with all my colleagues.

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