Jan, a couple of things come to mind re MTBF.  Failure criteria and
environment. It seems that Classes 2 & 3 may have different failure
criteria, therefore an anomaly which is a failure for Class 3 may not
necessarily be a failure for a Class 2 equipment.  An assessment of the
differential MTBF would presume a common failure criteria (since the
criteria for Classes 2 & 3 are sometimes different it is unlikely that such
data exists).  In addition, the end use environment for a Class 3 equipment
is generally acknowldges to be more severe, in some cases severe enough to
precipitate failure(s), consequently there are probably few folks subjecting
Class 2 equipemnts to a Reliability Index Determination test using Class 3
simulated environments.  Intuitively I have to believe that an equipment
manufactured to Class 2 requirements would exhibit a lower MTBF if tested or
operated in a Class 3 environment and also that an equipment manufactured to
Class 3 requirements would exhibit a higher MTBF if tested or operated in a
Class 2 environment.  The differential is probably more pronounced for
equipment with a predominance of leadless surface mount components.  Thermal
induced mechanical stress due to subtle Tce mismatch is the main killer of
solder connections and the lead compliance available in most leaded devices
tends to extend solder connection life.  Since the minimum acceptable solder
criteria for Class 2 is less demanding than the minimum criteria for Class
3, it is reasonable to conclude that given identical environmental stress
the Class 2 connection would fail first.  Just my .02 worth.  Regards, Jim
Moffitt/EMPF   

At 02:56 PM 8/30/96, you wrote:
>
>Does anyone know of any studies done on the  MTBF between IPC-A-610 class 2 
>and class 3 Surface mount assemblies?
>
>Thanks in advance,
>
>Jan Satterfield
>Mfg Engineer
>Edo Corp
>
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