Page 89 - Six Sigma for electronics design and manufacturing

Six Sigma for Electronics Design and Manufacturing
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fications, since it is assumed that in specifying the bond, only a mini-
mum value is given. For solder defects, a composite specification can
be assumed to be two-sided, since solder defects can be one- or two-
sided, as in excessive or insufficient solder. The difference between
implied one- or two-sided specifications is that the number of defects
representing the f(z) value under the normal curve should be halved
for two-sided specifications, or used directly for one-sided specifica-
tions, resulting in different implied Cpk interpretations. The decision
for one- or two-sided specifications for implied Cpk should be left to
the appropriate design and manufacturing engineers.
An example of an attribute process calculation to generate an im-
plied Cpk is for solder defects. They are usually measured in PPM or
parts per million of defects obtained in production divided by the total
number of solder joints in the product (total number of opportunities
for solder defects). Solder defects may result from the combination of
several specifications of design parameters such as component pad
size, drill hole size, fabrication quality of plated metal surface, and
the material and process parameters of the soldering equipment. A
100 PPM solder process (1 solder defect in 10,000 terminations or
joints) is calculated to have a Cpk = 1.3 as follows:
1. 100 PPM defects (assuming a two-sided specification), 50 PPM per
each tail of the normal curve
2. 50 PPM is f(z) = 0.00005 or z = 3.89, from standard normal curve
tables.
3. Implied Cpk = z/3 = 1.3
The assumptions are that the defects can occur on either side of the
implied specifications, the process is normally distributed, and the
process average is equal to the specification nominal. If this example
of Cpk was for a wire bond machine, then it could be assumed that the
defects occur due to one side of the specification limits of minimum
pull strength. In this case, the Cpk can be calculated as follows:
1. 100 PPM defects (assuming a one-sided specification) is 100 PPM
per one tail of the normal curve
2. 100 PPM is f(z) = 0.0001 or z = 3.72, from standard normal curve
tables
3. Implied Cpk = z/3 = 1.24, which is lower quality than two-sided
defects
It can be seen that the method of implied Cpk could lead to various
interpretations of one- versus two-sided specifications when the Cpk
methodology is used. If the six sigma interpretation of quality is used,