A new approach to statistical process control in a test environment: the empirical delta control chart

Past attempts at Harris Semiconductor to control the test measurements process and their inherent weaknesses are discussed. The empirical delta control chart (ED-chart), along with its corresponding control unit/die philosophy, is introduced as an alternative toward controlling the semiconductor wafer and package test environments. Only one control chart (per test parameter tracked) is required for all control units and die, as opposed to one chart for each unit or die. To add to this, no modification of the control limits is required when new control devices are introduced to the system. Their low chart count and their ability to stay with one set of charts solves many of the maintenance problems typically associated with statistical process control (SPC) in a test area. The permanent nature of an ED-chart representing one continuing standard facilitates the idea of continuous improvement. ED-charts emphasize the measurement aspect of a test area-where the ability to test accurately and repeatability over time are the goal