APPLICATION OF X CONTROL CHART WITH MODIFIED LIMITS IN PROCESS CONTROL

摘要

The conventional Shewhart X chart is developed based on the assumption that the within-sample variation is due to the inherent process variation, and any significant variation between samples is attributed to the existence of assignable causes. In the manufacturing industry there are processes where there is variation between samples due to the inherent process variation straightforward application of the conventional Shewhat X chart would thus result in more frequent false alarms, The problems associated with various Shewhart X charts in monitoring such a process are discussed using a real data set form an integrated circuit (IC)assembly process. A Shewhart X chart with modified limits is adapted for such a process. In addition to the usual ability to signal for assignable causes, the X chart with modified limits is also developed as a tool to signal the need for adjustment of controllable process variables for improving the process capability. Practical application of this chart in monitoring an IC assembly process is discussed.