Process Monitors and Controls

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Process Variability and Capability

Every process varies. If you try to bake ten cookies using the same mold, mix, and procedure, none of them will come out of the oven identical. To a certain extent they will all look similar, but no two of them will be exactly the same. This inherent tendency of your cookies not to look identical is known as your process variability, while the ability of your baking prowess to make your cookies stay within predictable limits of similarity is your process capability.

Using the same process to bake cookies over and over again will result in many batches of cookies that look similar, but once in a while you'll get a batch that's quite different. One time you may get a batch of darker cookies, and another time you may get a batch of sweeter cookies. When this happens, the cookies are outside the normal variability of the rest of your cookies, so they are called 'outliers.'

Something special outside of your defined process happened that resulted in such outliers, e.g., you probably set your oven too high or you inadvertently added more sugar. Your process is said to have gone out-of-control because of special or assignable causes. Special causes are unpredictable and should be avoided. In processes that are more complex than baking, the occurrence of out-of-control incidents can be very costly.

In semiconductor manufacturing for instance, a single misprocessing incident due to an assignable cause can mean losses in the hundreds of thousands of dollars. This is why in this industry, it is everybody's responsibility to keep the process in control.

This can only be done if there's a means of observing the process for any abnormalities and to correct the situation before the process goes out of control. Observing how the process takes place and behaves is known as process monitoring, while responding to changes to keep the process within its normal variability is known as process control.

Process Monitoring

Thanks to the invention of statistics, nobody has to stare at a process all the time to know how it is behaving. It is just necessary to look at how a process looks like at specific intervals and a fair assessment of how the process generally behaves may be achieved. In fact, it is not even necessary to scrutinize all aspects of the process at these intervals. One simply needs to check the process aspects that matter most.

Process monitoring consists of observing and measuring the critical parameters affecting a process at pre-defined, regular intervals and recording the observations and results. The critical parameters monitored are chosen in such a way that they collectively represent the state of the entire process. Since monitoring costs money, it is necessary to limit the number of parameters monitored to the minimum required to ensure that the process is meeting the company's quality standards.

One way of reducing the parameters that need to be monitored is to look for correlations between parameters. If the behavior of one parameter can be reliably predicted from the behavior of another parameter, then only one parameter has to be monitored. The process owner should eliminate all redundancies in process monitoring.

Metrology and Calibration

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