Page 132 - Six Sigma for electronics design and manufacturing

Chapter
4
The Use of Six Sigma in
Determining the Manufacturing
Yield and Test Strategy

Manufacturing is a multistep process, with each step generating its
own variability, and therefore contributing to the overall defect rate.
In a large multistep operation, individual process quality has to be
very high in order for the overall yield to be reasonably acceptable.
Otherwise, the probability of producing one good part is very low. In
the case of PCB or IC fabrication, with 30–50 steps each, there are
usually several in-process inspections or tests to cull out the interme-
diate defects, so that good parts can be produced when all production
steps are completed. This chapter will examine methods to allocate
for and plan these tests based on the expected quality of production.
It is important to measure quality in terms of the total number of
defects found anywhere in the manufacturing process, and prior to
any test or inspection. This will reduce confusion when setting quality
targets or benchmarking similar operations in different plants. In ad-
dition, it will result in a true measure of quality that is not masked by
the test or inspection costs.
Units of these quality measures are expressed in terms of first time
yield (FTY) and defects per unit (DPU), expressed in parts per million
(PPM). Recently the term defects per million opportunities (DPMO)
has been used to reduce confusion on how to calculate defect rates in a
complex multistep process such as PCB fabrication and assembly. Re-
pairs are not considered as part of the definition of first time yield
(FTY).

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