How to Setting the Process Capability Index of Six Sigma
Many companies are beginning to think about the process capability index, be it six sigma or Cpk, as a good method for both design manufacturing engineers to achieve quality goals jointly, by having both parts work together. Design engineers should open up the specifications to the maximum possible, while permitting the product to operate within customer expectations. Manufacturing engineers should reduce the process variations by maintenance and calibration of process and materials, training of operators, and by performing design of experiments (DoE) to optimize materials and processing methods.
Another advantage of using the six sigma or CPK as a quality measure and target is the involvement of the suppliers in the design and development cycle. To achieve the required quality target, the design engineer must know the quality level and specification being delivered by the suppliers and their materials and components. In some cases, e suppliers do not specify certain parameters, such as rise time on integrated circuits, but provide a range. The design engineers must review several samples from different lots from the approved supplier and measure the process variability based on those specifications. A minimum number of 30 samples is recommended.
Many companies use six sigma or a specific Cpk level to set expected design specifications and process variability targets for each part or assembly. Usually, this number has been used to set a particular defect rate such as 64 PPM, which is a Cpk = 1.33 with a centered distribution and specification limit of ±4σ. The six sigma goal of Cp = 2 results in a defect rate of 3.4 PPM based on a specification limit of ±6 cr and an average shift of ±1.5σ.
Six sigma or a high Cpk increases the robustness of design and manufacturing. A temporary process average shift does not significantly affect the defect rate. Six sigma (Cp = 2) implies that a shift of the average by as much as ±1.5σimparts a defect level of 3.4 PPM to the end product. A comparable shift of the average for a Cp of 1.33 increases the defect rate from 64 PPM to 6210 PPM.