Page 68 - Six Sigma for electronics design and manufacturing

The Elements of Six Sigma and Their Determination
should be well within the product tolerance limits. Process capability
is commonly depicted by a standard normal distribution. The inter-
section of the process capability and the specification limits deter-
mines the defect level, as shown in Figure 2.3
Process capability could be monitored using control charts. The
manufacturing process variability can be reduced by increased opera-
tor training, using optimized equipment calibration and maintenance
schedules, increased material inspection and testing, and by using de-
sign of experiments (DoE) techniques to determine the best set of
process parameters to reduce variability. 37
The classical design for manufacturing (DFM) conflict of interests
between design and manufacturing engineers is usually about con-
trolling product quality and cost. The design engineers would prefer
the narrowest possible process capability, so they can specify the min-
imum tolerance specifications to ensure the proper functioning of
their designs. The manufacturing and process engineers would prefer
the widest possible tolerance specification, so that production can con-
tinue to operate at the largest possible manufacturing variability with
a reduced amount of defects. The process capability index and six sig-
ma are good arbiters of the two groups’ interests.
A good conceptual view of this argument is the use of the term “ca-
pability.” A process could be either “in control,” or “capable,” or both.
Obviously, the desired condition is both in control and capable, as
shown in Figure 2.4. Six sigma assures that the desired outcomes are
processes that are highly capable and always in control. If there is a
short-term out-of-control condition in manufacturing, then the robust-
ness of the process, which is its capability versus its specifications, is

Figure 2.3 Intersection of process capability and specification limits to determine the
defect level.