Page 147 - Six Sigma for electronics design and manufacturing

Six Sigma for Electronics Design and Manufacturing
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Confusion over the utility of both functions. DPMO is easier to cal-
culate than OMI and therefore will become the more commonly
used function.
DPMO/OMI deployment will require extensive training of assem-
bly labor as well as management and support staff such as
process and quality engineers to interpret the rules for calculating
defects.
Guidelines will have to be defined for certain defect conditions to
assure the independence of component, placement, and termina-
tion defects
Some components might have different defect rates than others.
For example, mechanical, through-hole (TH), and surface mount
technology (SMT) components can all be part of the assembly line
process. Each will have a different defect rate, and they should not
be lumped together in one defect number.
DPMO concepts require knowledge of the actual number of termi-
nation opportunities, which are readily available in manufacturing
but do not get finalized until late in the design and development
process for electronic products (after PCB layout). Intermediate
metrics such as the ratio of components versus termination oppor-
tunities might be more useful in the design stage, especially for de-
sign for manufacturing (DFM) input, before the design in “hard-
ened” after PCB layout. This intermediate metric was shown in
Example 4.3.2.
DPMO is an example of the attribute quality problem in six sigma.
The notion of striving for “six sigma in everything that we do” is
not directly shown with the use of one or two indices such as DPMO
and OMI. Individual process quality as well as total assembly line
quality should be examined. In DPMO, the emphasis is on a modi-
fied defect rate. In the next section, an alternate method for calcu-
lating and comparing quality of assembly lines using back-calculat-
ed or “implied” Cpk is discussed with examples.
4.3.6 The use of implied Cpk in product and assembly
line manufacturing and planning activities
As discussed earlier, some industries have adopted a form of six sig-
ma that is based on target values of Cpk. Examples are the auto in-
dustries with the QS 9000 (Cpk 1.67 for new and 1.33 for old prod-
ucts), and the defense industry with various Cpk values for weapon
systems (Cpk = 1.33 for the F22 jet fighter). In these cases, an “im-
plied Cpk” value is used to characterize the quality of the process or
the product being evaluated.