I agree that understanding the relationships, in your process, between the
key variables is what matters most. SPC is a system designed to quantify
your natural variation and provide visual tools to detect when your process
is affected by some other cause of variation. The correlations between
variable are usually determined under specific conditions. SPC is best used
to detect changes in conditions that render the models you've developed
invalid and/or inaccurate. For example, regardless of what the maintenance
records say, I can tell when the schedule for pump cleaning has not been
properly followed using my dwell time charts. Since the model which tells me
which parameters are optimal (including pump speed) assumes this maintenance
is done in a certain manner, when maintenance is late the process drifts
away from optimal. The SPC charts warn me this is happening.
yves
> -----Original Message-----
> From: Earl Moon [SMTP:[log in to unmask]]
> Sent: Friday, October 05, 2001 12:08 PM
> To: [log in to unmask]
> Subject: Re: [TN] SPC @ WAVE SOLDER
>
> I still live in the glass plate and matrix world. Simple correlations
> between variables as conveyor speed, temps, topside measurements, managing
> fluxes and their applications, wave height, contact area parallelism and
> dwell time all make solder joint attributes clearly acceptable or not. SPC
> is used in a number of the variables and provides, again, correlation to
> the
> matrix (relative to PCB thermal mass, as an example) and the effects of
> good
> process management. Though wave soldering has a few more factors, compared
> with hand or reflow soldering, the basics are all the same, to me, for
> each.
> I'm a simple fool and can't change at this point when it works.
>
> MoonMan
>
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