Page 177 - Six Sigma for electronics design and manufacturing

Six Sigma for Electronics Design and Manufacturing
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5.2.1 Process capability for large-volume production
The following procedures are recommended when time and resources
are not gating items. It is ideally suited for large-volume manufactur-
ing, where the parts cost is low and the ease of collecting data is high.
These procedures will increase the accuracy of the process capability
and reduce its apparent variation with time.
1. Initial determination of process capability. Historical guidelines
for variable and attribute data are given in Table 5.4. Each sub-
group of data should be taken at a different point in time, prefer-
ably on different days. In this manner, day-to-day variations of the
process could be integrated into the process capability calculations.
There should be no allowance for process average shift in the Cpk
calculations. For low volume applications, the moving range
method should be used because of the low volume required. A dis-
cussion of the moving range method is given in the next section.
2. Regular updates of the process capability. The process capability
should be regularly checked to determine if the process has
changed. If the change is deemed significant using statistical tests,
then a process quality correction project should be initiated to de-
termine the cause of the process deviation. The amount of data re-
quired for checking the process could be less than the original data
needed for initial determination. Determination of can be
achieved either directly from the data or through the R estimator
for variable data. For large-volume production, a sample size of 30
is sufficient to perform this check of process capability for variable
Table 5.4 Amount of data required for process capability studies
Period of time Sample size Total
High Volume
X and R charts 1st period 50 measurements
2nd period 25 measurements
3rd period 25 measurements 100 measurements
P, nP charts 1st period 20–25 samples
U and C charts (50–100 units tested)
2nd period 20–25 samples
(50–100 units tested)
3rd period 20–25 samples
(50–100 units tested) 3000 min. units tested
Low Volume
Moving range Long period 10 consecutive numbers 10
Long period 10 consecutive numbers 10