MTBF MIL217 Quality

[sh-eda]

Hi

I'm evaluating the MTBF for a relatively new smd electronic design. It's not something I've really done before myself.
So I've been going through the MIL-217 reliability spec, and evaluating some software and online calculators.

I thought I'd start with the component 'count' type then 'stress' analysis.

What I'm not sure about is how you evaluate what quality factor of components to use in the analysis, when the failure rates are not given in datasheets.
The components we have used in the design are not necessarily to military spec ie "Established reliability". So does that mean I choose "LOWER". The PI-Q values becomes very large.
The calculations I've done so far are giving much higher MTBF figures than I thought it should.

Maybe I am missing something simple, hope someone can help.

Thanks.

 

[dick_freebird]

You can bet that whatever FIT numbers you -can- get,
are sandbagged at their source. Sometimes you can
drill into a manufacturer's applications or quality
support sub-pages (like TI / National was really good
about publishing qual reports, though this seems
less and less true with newer product introductions).

Even so, the qual basis may not match your application
"frame", like if you take a 0-70C commercial part that
maybe was HTOL tested at 100C to cover commercial
and industrial temp ranges, your 125C (junction) or 75C
(box, with temp rise from there that includes other
"heater" contributors) may just not be pertinent.

This is a specialty, you might want to look around for
more reliability-analysis-oriented forums (I just know
what I know, from watching my chips go through the
HiRel qualification cycle). This is one level up from my
lily pad.

 

[sh-eda]

Thanks for replying.
Yes I agree it's becoming really hard to find information.
I'm just trying to understand how Mil spec quality factor D, C, S, B, R, P, M etc relate to commercial components.

I know there are lots of different method and spec, much newer, is there a better one I should be looking at?

As you suggested, I have asked on a reliability forum.

Thanks