To observe whether the manufacturing process is stable and reproducible, process monitoring is instituted. In it, the adherence to the tolerance windows (as established previously in a process qualification step) during the individual manufacturing processes is monitored. If deviations in the process are recognized at an early stage, it is possible to take corrective action before sub-standard product is leaving the line. To achieve its goal, process monitoring has to be performed conscientiously and regularly. Preferably it should be performed by automatic means (PC).