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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:
Mon, 2 Feb 1998 16:58:52 -0800
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text/plain (55 lines)
which has a=0Aconstant failure rate for some reason. That is essentially =
the assumption=0Aunderlying the MIL-HBK-217, which has been shown to be i=
nadequate for=0Aelectronic assemblies, because electronic products contai=
n wearout failure=0Amechanisms which cause a continuously increasing fail=
ure rate with time, and=0Athus render the MTBF concept meaningless. For c=
onstant failure rates you can=0Aexpress the reliability in FITs (1 FIT =
=3D 1 failure/10**9 device-hours), a=0Aconcept developed way-back by Bell=
 Labs. But both FITs and MBTF are purely=0Astatistcal concepts and do not=
 take into account the physics-of-failure of the=0Amachanisms and loading=
 conditions the actually cause failures. At Bell Labs,=0Awe tried to get =
around this by using iFITs (time-dependent instantaneous FITs)=0Ato accou=
nt for changing failure rates=97while it works within the FIT frame=0Awor=
k, it is cumbersome and really does not answer the basic question of the=
=0Aexpected system reliability.=0AIn IPC-D-279, 'Design Guidelines for Re=
liable Surface Mount Technology Printed=0ABoard Assemblies', equations ar=
e given allowing you to get a cumulative=0Afailure probability for all th=
e SM solder joints in a system.=0A=0AWerner Engelmaier=0AEngelmaier Assoc=
iates, L.C.=0AElectronic Packaging, Interconnection and Reliability Consu=
lting=0A7 Jasmine Run=0AOrmond Beach, FL  32174  USA=0APhone: 904-437-874=
7, Fax: 904-437-8737=0AE-mail: [log in to unmask]

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