Page 77 - Six Sigma for electronics design and manufacturing

Six Sigma for Electronics Design and Manufacturing
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production, a form of learning-curve-based improvements. This is-
sue of time improvements has long been recognized in the supply
chain, with commonly used incentives for cost reduction based on
time. The six sigma program maintains a constant ±1.5 allowable
average shift, which is an easier goal to manage irrespective of time.
It is the author’s opinion that it is better to manage quality with a
single number and concept, as opposed to a time-dependant stan-
dard. In addition, the reduced life cycle of electronic products, and
the emphasis on “doing it right the first time” should encourage the
supply chain to set a goal for first production quality and then main-
tain it. This might prove less costly in the long run.
The choice of focusing on the process average shift correction to
equal the specification nominal or reducing variability or both will
be discussed in greater detail together with the quality loss func-
tion (QLF), discussed in Chapter 6.
Cpk and six sigma can have different interpretations when consid-
ering attribute processes. These are processes in production, where
only the defect rates are determined and there are no applicable
specification limits. Examples of attribute processes are assemblies
such as printed circuit boards (PCBs) where rejects could be consid-
ered to be the result of implied specifications interacting with pro-
duction variability of materials and processes. In these cases, the
quality methodologies are centered around production defect rates
and not specifications, thereby clouding the relationships and nego-
tiations between design and manufacturing. Different levels of
defect rates based on Cpk levels could be allowed for different
processes, resulting in an overall product defect goal setting and
test strategy based on these defects. Six sigma quality provides the
power of the single 3.4 PPM defect rate as a target for all processes.
A similar issue arises when using six sigma or Cpk for determining
total system or product quality. This is the case when several six
sigma designs and parts are assembled together into a system or
product. Six sigma practitioners handle this issue by using the con-
cept of rolled yield, that is, the total yield of the product based on
the individual yields of the parts. Those using the Cpk terminology
can continue to use Cpk throughout the product life cycle, assign-
ing different Cpk targets as the product is going through the design
and manufacturing phases. More discussions on this subject are
found in Chapter 10.
2.2.4 Setting the process capability index
Many companies are beginning to think about the process capability
index, be it six sigma or Cpk, as a good method for both design and