Page 144 - Six Sigma for electronics design and manufacturing

Determining the Manufacturing Yield and Test Strategy
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cause many termination defects, as discussed earlier. In this method-
ology, it would be counted as one placement defect and zero termina-
tion defects for the PCB. The number of opportunities for components
include all of the components plus the fabricated (raw) PCB. The
number of termination is the actual number of solder joints on the
PCB. Some the definitions are as follows:
number of defects
DPMO operation =
number of opportunities
operation defects · 10 6 (4.9)
DPMO index = · 10 6 (4.10)
opportunities defects
defects 1 defects 2
6
OMI = 1 – 1 – · 1 – ·... ·10
opportunities 1 opportunities 2
(4.11)
The DPMO for each operation is equivalent to DPU (PPM) defined
earlier in this chapter. The DPMO index is a useful tool for calculat-
ing the actual yield of the PCB, since it is based on the total number
of defects divided by the total number of opportunities. It is usually
dominated by the termination count. The DPMO index is the basis for
DPMO charts, discussed in the next section.
The overall manufacturing index (OMI) is an attempt to equalize
the weight of all three basic operations in PCB assembly. The yield of
each operation is calculated using the power expansion formula 4.8,
then the yields are multiplied together to form a multiplier yield for
the assembly line. A multiplier defect rate for the assembly line is de-
rived from the one-multiplier yield, and then multiplied by 1 million
to obtain the OMI index.
The OMI index represents an overall theoretical defect rate in
which each operation is given equal weight, based on the its own cal-
culated yield. The OMI index is independent of the number of oppor-
tunities of each operation, and therefore can be used to compare the
quality of alternate PCB assembly lines.
4.3.4 DPMO charts
DPMO charts are attribute charts used to monitor the quality of PCB
assembly lines. They are best used instead of attribute defect charts
such as U or C charts. Each type of PCB can be charted every time it
is run through the assembly line. A multiplication factor (MF) is pro-
vided in the calculations to make the conversion to million opportuni-
ties. DPMO charts can be used with defects codes for quality tracking
and continuous improvements.