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MIL spec 217

Thank you Christopher Marki for the wonderful engineering and personal insights that you shared in the Microwave Journal. We read them, passed them around, and felt at one with you as an engineer in a small business who cares about the technology  (the stuff) and your family, co-workers and customers (the people).

At Wenzel Associates we are a close group of engineers, technologists and manufacturers, people who also love the stuff and the people. We make very low noise oscillators, frequency based systems and synthesizer to about 16 GHz. There are lots of custom modules shipped from our Austin, Texas facility, some in complicated nickel-plated aluminum boxes with Spira shields to reduce cross-coupled spurs, some that slide into rugged racks mounted in a fighter jet or mount on the cold plate of a satellite ranging system.

We hope to contribute bits of useful information to RFblogger, life’s lessons perhaps, things relevant to the RF field. I am Liz Ronchetti, President and co-founder of Wenzel Associates with my husband, Charles Wenzel, our CEO and the brilliant one in the family. John Richardson is our Head of Engineering at Wenzel and I bet we will see posts from him in the future. My BSEE degree is from Worcester Polytechnic Institute in frosty Worcester Massachusetts and I am grateful to be writing from warm and sunny Austin, Texas.

So what’s all this about 217Plus?

We recently ran into a source control drawing from a prime contractor that called out 217Plus, rather than MIL-HDBK-217F for its MTBF calculation. After a bit of research and a call to a very nice QA engineer at Lockheed who forwarded me to another, here is what I learned.

“Everyone knows that MIL-HDBK-217F is outdated,” having last been updated in 1995. Lockheed (among others) reviewed suppliers, looked at processes and histories and decided if an improved number was appropriate, sometimes for a single part, sometimes for a complete line.  They applied a methodology on the numbers, making judgments and surveys and supplementing them with more data, always applying the same criteria. They sent their methodology to the Navy to verify that they were good with it. The Navy saw that there was a mismatch with what happens in the field and the 217F predictions, and that the new predictions gave results that were actually closer to what happens. In many of cases this approach was preferred by the Navy because it was more accurate and also kept needless costs down.

MIL-HDBK-217F Methodology

MIL-HDBK-217F defines the “failure rate” by calculating a rate for each subcomponent. The rate is assigned by part type. For example, the formulas needed to calculate failure rate for a ceramic capacitor (Fixed, ceramic, general purpose CK, CKR) differ from the formulas for an FET (Transistor, High Frequency, GaAs FET). Formulas for each part type’s specific reliability effects are detailed and multiplied together for a final “Failures/10e6 Hours”. In the case of the ceramic capacitors the factors are:

λb base failure rate => ambient and max rated temperature, stress (ratio of operating to rated voltage)
πCV capacitance  factor  => uses capacitance in pF
πQ quality factor => part quality type , S, R, …, MIL-C-11015 non-established reliability, and lower
πE environment factor => ground based GB, airborne inhabited cargo AIC, etc.

This modeling system, by its own admission, is limited. The information in a released standard can only be as up to date as the data at time of publication. The writers recognize that “Electronic technology is noted for its dynamic nature”, and write that reliability will certainly vary by the differences in system application, operational scenarios and even in the definition of failure.

Something better

It seems that, from lots of collected data and analysis, new standards have evolved, probably not as suddenly or as easily as it seems. Some companies, in conjunction with the DOD and the Reliability Information Analysis Center (RIAC), have built software around these standards. PRISM® software, originally released in 1999, is available through System Reliability Center (SRC). In 2006 the upgrade 217Plus was released by the RIAC and is available through SRC. Relex, from Parametric Technology Corporation, is the software that we use for our MTBF calculations, and it also supports PRISM® and the 217Plus upgrade. There may be other programs available.

Eventually, MIL-HDBK-217 will be updated to Rev G; it has had an initial release for review and comments, but is still a work in progress.

Why is this important

This is significant to RF designers and manufacturers, any of us who have scoured data sheets and poured through websites looking for a good MIL-style varactor. MIL parts are expensive. They are less available than they were and we are finding more MIL parts have become obsolete. What were our favorite parts may be no more because better more reliable parts are available and have replaced them. Many of these new parts are characterized in 217Plus.

So new designs can be cheaper and have a shorter development time; we are all grateful for this. Now, new reliability information is readily available, accessible and easy to use. The creation of 217Plus also suggests that we are getting better at producing high quality high reliability parts as a group. This means safer air travel, better radar systems and lower cost mobile radios.

Of course, we will use caution in applying 217Plus. More research is needed to learn the applicability of the new data and find out who is currently using it. There may be cases were the new numbers are not approved, such as in space work, or for some mission critical designs that were qualified a long time ago and any changes means the addition of risk. But it is very welcome.

Liz Ronchetti
Wenzel Associates, Inc.

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