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December 1999

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Subject:
From:
Lou Hart <[log in to unmask]>
Reply To:
TechNet E-Mail Forum.
Date:
Fri, 3 Dec 1999 08:45:50 -0500
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Edward, et al., you are largely correct about capability indices and spec limits.    I hope that any spec-opinions are based on some knowledge and experience, although I once asked a non-SMT engineer "Where did those spec limits come from?" and his response was to raise his hand over his head and figuratively grab the air.  Let me add my opinion that the ability of a capability index to indicate process improvement drops off rapidly over the range of 1.2 to 1.6.

And let me add another opinion, about SPC.  Its MAIN value is in telling people when NOT to do something:  leave the process alone, don't waste your time and materials tinkering when one or two points seem to be different from what you're used to.  The same thing might be said about Designed Experiments.

Lou Hart

----------
From:   Brunker Ed[SMTP:[log in to unmask]]
Sent:   Friday, December 03, 1999 4:01 AM
To:     [log in to unmask]
Subject:        [TN] SPC INFORMATION  /   WHY SPC IS NOT USED MORE

I have found SPC useful as a tool, variable control charts (X bar / sigma),
using calculated limits to determine the natural osscilation of a process.
Attributes charts, DPMO and Sigma calculations are also useful as a metric.
Where I run into difficulty is Capability Cp calculations. These are based
on Specified limits. As someone said earlier in this discussion this works
fine for machined metal parts, but on an SMT line.... The specified limits
are one mans opinion, they are subjective. A person chooses the limits and
thus chooses the capability of the process. Stupid idea!!  For the likes of
solder paste measurement and peak temperature monitor in a reflow oven, who
is going to state exactly what those limits should be?
It would be useful to use this forum (as we have done in the past for other
subjects) to come to a general summary of what people do practically on an
SMT line.
Regards
Edward Brunker

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