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32 The PCB Magazine • November 2015 number means. It is generally accepted that a Cpk of less than 1.33 would indicate a process that is not capable of consistently meeting cus- tomer requirements, and a Cpk of 2.0 would represent a six sigma level. Calculating process Cpk levels is not a one-time occurrence, it must be performed on a regular basis (quarterly is usually sufficient). Capability Studies The capability of the process needs to be es- tablished to use as a baseline for future improve- ment comparison, and is accomplished through Cpk analysis. As discussed earlier, a Cpk value is an index representing the ability, or capability, of the process to meet customer requirements. To determine process capability, collect process data as you would if you were setting up a con- trol chart, calculate the process mean and varia- tion, and then compare the relationship be- tween these values and the specification limits. Any SPC software, or even Excel, can calculate the Cpk of a process data set. Figure 2 shows the various levels of process capability. Once the baseline capability study has been completed, a final study should be performed again to verify the effectiveness of the process improvements that may have been implement- ed to improve the process capability. As men- tioned earlier, using a minimum Cpk of 1.33 is DATA AnALyTICS THroUGH STATISTICAL TeCHnIQUeS FeATure Figure 2: cpk levels.