Page 198 - Six Sigma for electronics design and manufacturing

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The Use of Six Sigma with High- and Low-Volume Products and Processes
2. For state of the art products, the part specifications are set aggres-
sively, with the implication that the early production runs will
have a poor yield. The parts in this case will attain the desired lev-
el of quality through rigorous testing against specifications. Even-
tually, their process capability will improve over time, thus achiev-
ing the specified first-time yield sometime after product release.
3. Using six sigma procedures for process capability implies that
every purchased or manufactured part or assembly meets the six
sigma requirements. Process capabilities might not be available for
many of the new purchased parts and may have to be calculated
from prototype purchases. For major companies, this issue is less
of a problem, as they can specify the process capability or six sigma
directly in the purchasing contracts for parts.
5.4.1 Process capability for prototype and early
production parts
When prototype parts are acquired, whether through purchase or
made in the company’s internal factories, the following methodology
is recommended for process capability calculations:
1. New parts that are very similar to current parts, or made in the
same production line or process, can assume the current part
process capability. Examples would be fabricated and assembled
PCBs. Process capability can be derived from existing manufactur-
ing statistical control data.
2. For parts new to the company, either purchased from the supply
chain or locally manufactured, the sampling plan of Table 5.4 can
be used for high-volume manufacturing.
3. For low-volume manufacturing, use smaller sample sizes, includ-
ing the moving range method. Use the statistical techniques of t
and distributions as well as sample size determination, dis-
2
cussed in this chapter, to determine the ranges of population aver-
age and standard deviation . Use the confidence limits to deter-
mine the worst-case process capability.
4. To determine the specification limits, especially for six sigma de-
sign, ensure that the specifications are related to the customer
wishes, and that the average and population standard deviations
are within the six sigma limits of design.
5. The six sigma or the Cpk quality level target can be altered for the
short versus the long term. In some cases, including prototype and
early production, close attention is given to the parts and manufac-
turing process by the design team and manufacturing engineers in