Page 70 - Six Sigma for electronics design and manufacturing

The Elements of Six Sigma and Their Determination
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the process is capable of replicating faithfully the product characteris-
tics, and therefore will produce products of high quality.
The utility of the Cp index is that it shows the balance of the quali-
ty responsibility between the design and manufacturing engineers.
The quality level is set by the ratio of the efforts of both. The design
engineers should increase the allowable tolerance to the maximum
value that still permits the successful functioning of the product. The
manufacturing engineers should minimize the variability of the man-
ufacturing process by proper material and process selection, equip-
ment calibration and control, operator training, and by performing
design of experiments (DoE).
An example of design and manufacturing process interaction in the
electronics industry is the physical implementation of electronic de-
signs in printed circuit board (PCB) layout. The design engineer
might select a higher number of layers in a multilayer PCB, which
will speed up the layout process because each additional layer in-
creases the PCB surface available for making electrical connections.
Speedier layout time could result in a faster new product introduc-
tion, bringing in new revenues into the company faster. Minimizing
the number of layers requires more layout time, but would produce
lower-cost PCB’s and fewer defects, because there are fewer process
steps. This is a classical case of the balance between new product de-
sign and development expediency and manufacturing cost and quali-
ty. Six sigma helps focus all engineers toward making the proper deci-
sion in these cases by quantifying the quality and cost benefits of the
alternatives. A case study of resolving this problem is given in Chap-
ter 6, Section 6.3.4.
2.1.4 Six sigma approach
The six sigma concept requires that each process element and each
part necessary for the product have a defect rate of no more than 3.4
PPM (parts per million). The underlying assumption is that the varia-
tions occurring in all the parameters associated with these process el-
ements and parts follow a normal statistical distribution function and
that the specification limits are situated six sigma away from the
process average. A further assumption is made that the average value
of a parameter can shift from the specification nominal by as much as
±1.5 . With this shift, one of the specification limits is at 4.5 away
from the process average, instead of 6 , while the other specification
limit is at 7.5 , where defects can be ignored. This will result in a de-
fect rate, based on one side of the normal distribution, of 3.4 PPM.
This defect rate results from the interaction of the normal distribu-
tion of parts versus the 4.5 limit.