Page 78 - Six Sigma for electronics design and manufacturing

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The Elements of Six Sigma and Their Determination
manufacturing engineers to achieve quality goals jointly, by having
both parts work together. Design engineers should open up the speci-
fications to the maximum possible, while permitting the product to
operate within customer expectations. Manufacturing engineers
should reduce the process variations by maintenance and calibration
of processes and materials, training of operators, and by performing
design of experiments (DoE) to optimize materials and processing
methods.
Another advantage of using the six sigma or Cpk as a quality
measure and target is the involvement of the suppliers in the design
and development cycle. To achieve the required quality target, the
design engineers must know the quality level and specification being
delivered by the suppliers and their materials and components. In
some cases, the suppliers do not specify certain parameters, such as
rise time on integrated circuits, but provide a range. The design en-
gineers must review several samples from different lots from the
approved supplier and measure the process variability based on
those specifications. A minimum number of 30 samples is recom-
mended.
Many companies use six sigma or a specific Cpk level to set expect-
ed design specifications and process variability targets for each part
or assembly. Usually, this number has been used to set a particular
defect rate such as 64 PPM, which is a Cpk = 1.33 with a centered dis-
tribution and specification limit of ±4 . The six sigma goal of Cp = 2
results in a defect rate of 3.4 PPM based on a specification limit of ±6
and an average shift of ±1.5 .
Six sigma or a high Cpk increases the robustness of design and
manufacturing. A temporary process average shift does not signifi-
cantly affect the defect rate. Six sigma (Cp = 2) implies that a shift of
the average by as much as ±1.5 imparts a defect level of 3.4 PPM to
the end product. A comparable shift of the average for a Cp of 1.33 in-
creases the defect rate from 64 PPM to 6210 PPM.
2.3 Calculating Defects Using Normal Distribution
Quality defects can be calculated from the defect rate generated by
six sigma or Cpk, from the interaction of the production process and
the specification limits. The production process characteristics are
assumed to be normally distributed. This distribution is also known
as the bell curve, and is symmetrical. The area under the curve is
equal to 1, and it is much smaller on both ends, as shown in Figure
2.8. Once a process is determined to be normally distributed, it can
be characterized by two numbers: a process average and a popula-
tion standard deviation . A standard normal curve is one that has