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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