I've just read Vickie Chapman's comment to Mark's reply regarding the
variation in ppm between products.  It reminded of some of the other errors
I've seen in thinking in the past.

I work in a low volume, hi-rel shop where each product is unique, unless you
build several versions of the same product with different firmware in the
proms.  Each product should have its own set-up sheet and unique profile
parameters.  This enables you to make adjustments for problem components or
design features.  Each CCA will thus have its own unique defect rate for the
equipment, flux, state of profile development,etc...

   We monitor the defect rate by individual CCA, stopping when it falls
outside of the expected ppm.  My problem comes when management looks at the
collective data for a week or month and demands to know why we had an
increase.  I've also been congratulated by the same management for bringing
down those darn ppm's.  Only, they don't mean anything unless you are
building the same quantities and mix of CCAs from period to period.  The
biggest swings come from starting and stopping programs with the older
Hi-Rel soldering specifications, or in mix of older low volume cards with
newer cards.  Switching from digital to power supply cards is anothr
guarentee of flakey data.  None of this has anything to do with the quality
of the shipping product or necessarily the profitability of the process.

Steve Mikell
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