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March 1997

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From:
[log in to unmask] (Dave Willhard)
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Date:
Tue, 18 Mar 1997 13:06:49 PST
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The biggest problem we have is trying to get customers to understand
the limitations of SPC. Many of them think SPC can and should be applied to al
processes and they expect to see charts everywhere when they visit. Circuit
board manufacturing presents some difficulties when you are trying
to implement SPC. For example, standard SPC charts require that
measurements be independent (not related to each other in any way other
than the simple fact that they came from the same process). In most
PCB processes this is not the case since consecutive measurements of 
product or process ARE related (i.e., 2 consecutive circuit width
measurements will be similar since they were etched when the chemicals
were at the same concentration, etc.).

I don't wish to discourage anyone from using SPC. It can be used
effectively in most processes. My main point is - don't let
customers who have a limited understanding of PCB processes and/or
SPC dictate how you will use it! We've made that mistake and currently
have 10 to 15 control charts that are basically worthless. Not only
are they worthless, they have a negative impact on the company by
convincing many operators and engineers that SPC is a waste of time.


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