Hi Ingemar,
I did not suggest that Design for Reliability (DfR) is overkill, quite the
opposite, I said that DfR is the only way to assure reliability. What I did
say, however, was that in many instances FEA and complex CAD-based tools are
overkill. And I also do not believe in 'Rules-of-Thump' in this situation. In
most cases, all the questions 'his boss' asked can be answered with DfR (see
IPC-D-279) without FEA. I know, because that is what I have been doing for my
clients for 10 years, and prior to that for my employer for 25.
You do not get substantially more reliability-design information from this
increased complexity, and unless done right you may if fact get wrong answers
because of the many more assumptions that have to be made with a more complex
analysis.
For complex situations, proper FEA can run upwards of $ 200,000 to 300,000.
While I certainly have not paid this kind of money, some of my clients have.
Further, an FEA analysis can only give you more details about the loading
conditions; it can not translate that to cycles-to-failure or estimates of
product reliability. For this you need a strain-energy-based fatigue
relationship, of which the Coffin-Manson equation is one. The simplified
Coffin-Manson equation is possible for non-creeping metals, since a unique
relationship between stress and strain exists--the stress-strain diagram.
For metals used above their homologous temperature, where they creep, no
unique relationship between stress and strain exists and their are an
infinite stress-strain curves dependent on temperature and strain rate. This
also makes FEA much more difficult.

Werner Engelmaier
Engelmaier Associates, L.C.
Electronic Packaging, Interconnection and Reliability Consulting
7 Jasmine Run
Ormond Beach, FL  32174  USA
Phone: 904-437-8747, Fax: 904-437-8737
E-mail: [log in to unmask], Website: www.engelmaier.com

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