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Process Qualification - Optimized vs. Nominal



In process qualification protocols, the issue of whether a process has been 'optimized' is sometimes encountered.

What does 'optimized' mean?
  • It might mean finding the process settings that result in the best location of the process mean.
  • It might mean finding the process settings that result in the lowest process variation.
  • It might mean finding the process settings where the change in the process output with respect to changes in the process inputs is minimized (i.e. flat response; smallest slope or first derivative of the response).
    • Sometimes known as Robust Design.
  • It might mean some combination of the above, or perhaps something else depending on the goal of the study and the needs of the process outcomes.



EXAMPLE:

If a process has three inputs, a protocol (e.g. an "OQ" or Operational Qualification protocol) that studies only "Low-Low-Low", "High-High-High", and perhaps "Mid-Mid-Mid" can NOT make the following claims in the absence of earlier DOE studies:
  • It cannot claim to have found an optimum set of process inputs, as no interaction terms were studied.
  • It cannot claim to have studied the process 'worst case' conditions, as no interaction terms were studied. 
    • An interaction term that is significant in the process model might mean that some other combination of Low-High process inputs is the 'worst case' condition.  For example, Low-Low-High.


For those process qualification protocols where "all Low" and "all High" levels of process inputs are studied, the statement of conclusions should use a term other than "optimized", such as "nominal".


"The OQ tested Nominal, Low, and High parameter settings to allow the development of a nominal process and nominal process window."

(Note that this 'nominal' process window may not include the most challenging conditions for the process.)


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