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February 1998

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Subject:
From:
Werner Engelmaier <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Tue, 3 Feb 1998 08:23:34 EST
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Apparently, my message got garbeled somewhere in the transmission. Therefore,
here it is again:
Hi Andy,
The concept of an MBTF makes sense only if you have a product which has a
constant failure rate for some reason. That is essentially the assumption
underlying the MIL-HBK-217, which has been shown to be inadequate for
electronic assemblies, because electronic products contain wearout failure
mechanisms which cause a continuously increasing failure rate with time, and
thus render the MTBF concept meaningless. For constant failure rates you can
express the reliability in FITs (1 FIT = 1 failure/10**9 device-hours), a
concept developed way-back by Bell Labs. But both FITs and MBTF are purely
statistcal concepts and do not take into account the physics-of-failure of the
machanisms and loading conditions the actually cause failures. At Bell Labs,
we tried to get around this by using iFITs (time-dependent instantaneous FITs)
to account for changing failure rates—while it works within the FIT frame
work, it is cumbersome and really does not answer the basic question of the
expected system reliability.
In IPC-D-279, 'Design Guidelines for Reliable Surface Mount Technology Printed
Board Assemblies', equations are given allowing you to get a cumulative
failure probability for all the SM solder joints in a system.

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]


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