How to Use SPC for Process Control and Improvement

      SPC is a powerful tool for process control and improvement, but if it’s not properly used, it may cause extra burden to the owners of processes, but bring them with little benefit. To do effective process control and improvement with SPC, one needs to follow the three steps as described below:

1. Process Release

      This is done during R&D stage for transferring the process to mass production. A process must be evaluated and can only be released for mass production when it satisfies two conditions:

• The process is under statistical control. In other words, there is no special cause existed and the process is stable and predictable. The distribution of products produced by this process will be the same in the future. 
• The indicators of the process capability and performance meet the customer or internal requirements (e.g. many customers require Cpk>1.67).

      Now let’s see how the evaluation on the process should be done:

      1) First, collect >= 25 subgroups with a total of >=100 samples of the controlled parameter. The variation within a subgroup should be minimized, so the samples of the same subgroup are suggested to be taken from the same batch or lot, with the least variation in the impacting factors of the process (e.g. materials used, operators, equipment, processing parameters, etc). On the other hand, the variation between subgroups should be maximized. So a subgroup can be taken when the shift is changed, the material lot is changed, or the production line is restarted, etc.

      2) Calculate the trial control limits with the data points collected. Plot them in the control charts. Identify any subgroup which signals the existence of a special cause. Exclude these subgroups and repeat this step (more subgroups may be collected to replace the excluded ones if the total number of subgroups falls within 25). Actions should be taken to remove the special cause if it continuously exists in the process.

      3) When all the subgroups with special causes are excluded from the control charts, use the reaming subgroups to calculate the control limits and the indicators of the process capability and performance, including Cp, Cpk, Pp and Ppk, and see whether these indicators meet the requirements (e.g. whether Cpk>1.67).

      4) If not, take actions on the common cause to improve the process until the indicators meet the requirements.

      5) If yes, check the difference between Cp and Cpk. If there exists a big difference, it means the process average is significantly shifted from the process target, and a significant part of the process capability is wasted due to the shift . Actions should be taken to bring the process average close to the target.

      6) Also, it’s suggested to check the difference between Cpk and Ppk. If the difference is significant, it indicates the between-subgroup variation is significant, special causes exist and the process is not actually in statistical control. Actions should be taken to remove the special causes.

      After all the above steps, special causes originally existed in the process are removed (i.e. the process is now statistically controlled) and all the process capability and performance meet the requirements. The process can now be released for mass production.

      After the process is released, the control limits established after the above steps should be continually employed during the mass production. They should not be changed unless there’s intentional change in the process or the reason for the change is clear.

2. Process Monitoring

      With the control limits as established during process releasing, the process owner continues the data collection of subgroups, with the defined sampling frequency, sampling scheme and size of subgroups, and monitors:

• Whether the new subgroups indicate the occurrence of any special cause (the rules to identify the special cause can be found in P.75 of AIAG's SPC manual);
• Whether the indicators of process capability and performance fail to meet the requirements; 
• Whether the difference between Cp and Cpk or Cpk and Ppk is significant. 

      If the answer is YES to any of the above question, actions should be taken as discussed in Step 3)-6) in Step 1 Process Release.

3. Process Improvement

      Even the answers for the three questions in Process Monitoring are all NO, one should never stop there. One needs to study the process and take actions to improve the process, to increase the process capability and performance. The improvement of process requires actions on the common causes, and usually requires the resource support from the management team.

      Once the improvement actions have been taken, one needs to go back to Step 1 Process Release, do the evaluation on the process and start over again. As a most significant result from the process improvement, new and tighter control limits are usually established.