Page 199 - Six Sigma for electronics design and manufacturing

Six Sigma for Electronics Design and Manufacturing
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the short term. As production ramps up, more parts are made with
newer and less-skilled operators, resulting in poor quality, even if
a good control system is in place. In the long term, with good use of
corrective action processes and TQM, as well as increased opera-
tors’ skills through the learning curve, the parts’ quality levels will
increase. Considering the previous arguments, it is advisable to set
a higher quality level in the early production phase in order to
counteract the problems when production ramps up. An example
would be to set quality for early production runs to Cpk = 1.67 (five
sigma), then back off to Cpk = 1.33 (four sigma) in the long term
when the product matures. In Figure 5.9, the standard deviation
used is the combined based on the prototype and production
runs.
6. The formulas for combining s (small samples)or (large samples)
from two distinct samples with varying sample sizes (n 1 and n 2 )
follow. For large samples (>30) of standard deviation 1 , 2 and
sample sizes n 1 , n 2 :

2
2
1
2
combined = + (5.19)
n 2
n 1
For small samples (<30) of standard deviation s 1 , s 2 and sample
sizes n 1 , n 2 :

s 1 (n 1 – 1) + s 2 (n 2 – 1)
2
2
s combined = (5.20)
n 1 + n 2 – 2
Early production
Prototype distribution distribution

Suggested
Specifications
at 6 combined
Figure 5.9 Distributions of prototype and early production of parts.