Page 76 - Six Sigma for electronics design and manufacturing

The Elements of Six Sigma and Their Determination
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this is a poor quality design, where more than 50% of the parts made
are defective, it is an example of some of the quick indicators that Cpk
can provide for prioritizing corrective action for improving products
and processes.
2.2.3 Choosing six sigma or Cpk
Although both six sigma and Cpk are excellent measurement systems
for quality improvements in design and manufacturing, a consensus
has not been reached as to which system should be selected based on
some of the issues discussed in this section. Currently, major indus-
tries and companies have either opted for one or the other, or for their
own company brand of six sigma. In the latter case, a combination of
rules from both systems is developed to clarify some of the issues, es-
pecially when dealing with internal manufacturing and the supply
chain. This is important, since the requirements for six sigma or Cpk
levels are becoming part of the contractual agreements between com-
panies and their supply chain, as well as performance measures for
design and manufacturing centers in modern enterprises.
Some of the issues to be considered when a company plans to
launch a quality program based on six sigma or Cpk approaches, and
how they can converge, are:
The classical definition of six sigma corresponds to the last line in
Table 2.2. Six sigma is equivalent to Cp = 2 or Cpk = 1.5, while al-
lowing a process average shift to the specification nominal of ±1.5
. However, Cpk = 1.5 does not always equate only to six sigma.
Many different conditions of specifications tolerance and process
average shift can result in Cpk = 1.5, as shown in Table 2.2
The implication of the six sigma average shift of ±1.5 is that the
production process variability will not improve beyond the ±1.5
shift of the process average. This may be considered as a negative,
since it does not encourage those in the supply chain to improve
their process variability. By specifying a particular Cpk, a company
can encourage its suppliers to minimize their variability, since it is
apparent from Table 2.2 that the smaller the average shift, the
wider the specification tolerance can be.
It is widely recognized that older manufacturing processes are more
stable than newer processes, and hence are prone to less average
shift. This has led to specifying a particular Cpk for new processes,
and then a different Cpk when the process matures, in 3 to 6 months
after production start-up. In the auto industry, the starting Cpk is
set at 1.67 and the mature Cpk at 1.33. This was done to force the
supply chain to pay attention to the process in the initial stage of