Page 69 - Six Sigma for electronics design and manufacturing

Six Sigma for Electronics Design and Manufacturing
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Figure 2.4 Conceptual view of control and capability concepts.

good enough to withstand that deviation and continue to produce
parts with low defects.
There are two methods used to increase the quality level and hence
approach six sigma for new product designs: either increase the prod-
uct specification limits to allow manufacturing variability to remain
the same, or keep product specifications limits constant and reduce
manufacturing variability by improving the quality level of materials
and processes. The latter can be achieved through inspection, in-
creased maintenance, and performing design of experiments (DoE) to
determine variability sources and counteract them. The ratio of the
interaction of two sources of defect is the measure of design for quali-
ty, called the process capability index or Cp. Six sigma is a special
condition in which Cp is equal to 2:
specification width (or design tolerance)
Cp = (2.1)
process capability (or total process variation)
USL – LSL
Cp = (2.2)
6 (total range from –3 to +3 )
where
USL = upper specification limit
LSL = lower specification limit
= manufacturing process standard deviation
The Cp value can predict the reject rate of new products by using
normal probability distribution curves. A high Cp index indicates that