Page 183 - Six Sigma for electronics design and manufacturing

Six Sigma for Electronics Design and Manufacturing
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Example 5.12
Fuses are made in a production line, with specifications of 5 ± 2 ohms.
A sample of six fuses measurement was taken at 3, 6, 6, 4, 5, and 5
ohms. If it is desired to have an X , R control chart, what is the quality
data for the fuse line?
Moving range method data = 3 0 2 1 0
Average X = 4.83; M R = 1.2
= M R /d 2 = 1.2/1.128 = 1.0638
UCL x = 4.83 + 2.66 · M R = 8.02
LCL x = 4.83 – 2.66 · M R = 1.64
UCL R = D 4(n=2) · M R = 3.27 · 1.2 = 3.92
LCL R = 0
Cp = 2/3 · 1.0638 = 0.63; Cpk = (4.83 – 3)/3 · 1.0638 = 0.57
z 1 = (3 – 4.83)/1.0638 = –1.72; f(z 1 ) = 0.0427
z 2 = (7 – 4.83)/1.0638 = 2.04; f(–z 2 ) = 0.0207
Defect rate (RR) = 0.0427 + 0.0207 = 0.0634 or 6.34% or 63,400 PPM
5.2.6 Process capability studies in industry
The discussions in the previous sections outlined a system for investi-
gating and maintaining process capability for the purpose of quality
planning. In the six sigma environment, process capability data will
have to be maintained within one or more of the indicators that were
discussed in previous chapters, including DPU (PPM), DPMO, yield,
and number of sigma’s quality (including six sigma). Knowing that all
of these indicators are related to each other as discussed and shown
by examples in previous chapters, an enterprise can decide on one of
these indicators, or a combination of several, and use the indicator(s)
in process capability studies. This is especially useful when the enter-
prise management or major customers have asked for a certain level
of quality.
An example would be a factory that chose Cpk as the process capa-
bility indicator. This requires that all of the fabrication and assembly
operations, as well as major part suppliers and outside manufactur-
ing contractors, are to report on their process capabilities. For the
suppliers and contractors, a supplier management team and contrac-
tual processes with quality as well as cost and delivery requirements
have to be in place to indicate the need for process capability. The