Page 38 - Six Sigma for electronics design and manufacturing

The Nature of Six Sigma and Its Connectivity to Other Quality Tools
chy and some of the quality tools that are in common use today. Spe-
cific mathematical background and formulations are discussed in de-
tail in later chapters.
Six sigma is a condition of the generalized formula for process capa-
bility, which is defined as the ability of a process to turn out a good
product. It is a relationship of product specifications to manufacturing
variability, measured in terms of Cp or Cpk, or expressed as a numer-
ical index. Six sigma is equivalent to Cp = 2 or Cpk = 1.5 (more on
that in the next chapter). The classical definition of the capability of
the process or Cp is: 9
specification width (or design tolerance)
Cp = (1.1)
process capability (or total process variation)
Specifically,
USL – LSL
Cp = (1.2)
6 (total process range from –3 to +3 )
This formula can be expressed conceptually as
product specifications
Cp = (1.3)
manufacturing variability
Six sigma is achieved when the product specifications are at ±6 (
is the symbol for standard deviation) of the manufacturing process
corresponding to Cp = 2 (or Cpk = 1.5, discussed in Chapter 2)
Six sigma or Cp is an excellent indicator of the capability of a
process, which can be expressed numerically. This numerical expres-
sion can be translated into a defect level using normal distribution
statistical assumptions. It is a useful tool for manufacturing process
comparisons, as well as a common language of design and manufac-
turing personnel during the development phase of a product. The de-
sign project team and their managers can use it to set new product
quality goals. It can be used to assess the quality of internal manufac-
turing plants anywhere in the world or to measure the capability of a
supplier. Companies can use it to communicate a particular contrac-
tual level of quality for their supply chain.

1.5 Increasing the Cp Level to Reach Six Sigma
The quality tools in wide use today can easily be integrated within the
six sigma definitions. The object of six sigma is to steadily increase
the process capability index until it reaches the desired level: the
specification limits of the design are equal to six sigma of the manu-
facturing variability.