Page 150 - Six Sigma for electronics design and manufacturing

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Determining the Manufacturing Yield and Test Strategy
required to make the decision when back-calculating the Cpk from
the defect data. The specifications are assumed to be either single
(one-sided) or double (two-sided)
The Cpk for each operation. This Cpk is calculated from previous
historical data when the process capability of each operation was
determined. They are recorded as the current quality level of that
operation. Note that in the last operation, solder reflow has
achieved six sigma quality of Cpk = 2.
The next two attributes convert this Cpk number to the more famil-
iar DPU number for defect measurement in PPM. The DPU num-
ber could alternately be used to record the quality instead of the
Cpk number.
z is the variable from the standard normal distribution, derived
from Cpk by Equation 2.13 (z = 3 · Cpk). The next line is the f(–z) to
determine the one-sided probability of defects that can be found di-
rectly.
DPU (PPM) is the defect rate of the operation. It is derived from
the f(–z) and then multiplied by 1,000,000. If the implied specifica-
tions of the operation in this section are two-sided, then the defect
rate is multiplied by two. The DPU can be used as a substitute for
defining the quality of the operation, instead of the Cpk if so de-
sired.
N is the number of operations required for the IC being assessed for
quality. In this case, the IC has to undergo 183 epoxy dispense op-
erations. NDPU, or total defects for producing the IC in this opera-
tion, is calculated by multiplying N by the DPU to produce NPDU
for that IC.
The operation FTY is calculated by subtracting the NDPU from 1
for each operation.
When all of the data for each operation have been determined, then
the total line information can be calculated. Depending on the goals
set for the IC manufacturing line, three indicators can be deter-
mined for each IC type that is produced on that line:
1. Total line NDPU—the total manufacturing defects for the line
resulting from making a particular IC. This is calculated by
adding the defects (NDPU) from each operation.
2. Total line FTY—the total yield for a particular IC made in the
line. It can be calculated either by multiplying the yield of each
operation or from subtracting the total NDPU from 1.
3. Total line Cpk—the quality index for the IC being made in the
line. This is back-calculated from the defect rate, assuming two-
sided specifications and no process average shift.