Why need Statistical Process Control (SPC)?
• As mentioned SPC is the tool to detect defect and estimator on the process abnormality. Use SPC to set the control level for the process, the preventive action can be carried out before the process out of the specification. This is to reduce the chances of defect happen. • • How to set the control level? • The graph below showing the upper specification limit (USL) and lower specification limit (LSL)and the process sigma. Sigma six quality process shall set the control limit as per yellow line. • • To set the upper control limit and lower control limit is actually depend on the process consistency. • Look at the table above, if the control limit (UCL and LCL) is one sigma, and the specification limit (USL and LSL) is three sigma, the process consider as 3 sigma process. However, if the control level is 2 sigma but the specification limit is 3 sigma, then the process sigma is actually 1.5 sigma. The table shown that to achieve the six sigma quality process, either improve the process
