The Traffic Cop - Early Warning Indicators

As an example, suppose that the Traffic Cop starts with the following chart, showing the top ten bottleneck tool groups over the past two weeks for a fab. Each column shows actual throughput divided by expected maximum throughput for the two week period. This chart clearly identifies the 5xStep tool group as the bottleneck during the time period of interest - the only tool group with utilization greater than 85%.

Next, the Traffic Cop looks at fab starts by shift for the current week, to see if the start pattern has been regular or irregular. In this example (shown below), the starts are highly irregular by shift. 500 wafers were started during the day shift on May 22nd, with no wafers at all started during either of the following two shifts. 

Next, the Traffic Cop checks WIP turns for the same period. WIP turns (operation moves divided by starting WIP) are an excellent early indicator of cycle time problems. Both lower than expected moves and higher than expected WIP will cause turns to be lower than expected. In this example, WIP turns looked great for the first three shifts, but then they decreased by nearly 15% during the next shift, and stayed low for the next several shifts, just barely meeting the goal. 

The Traffic Cop’s next task is to dig down into the factory performance data, to understand why the WIP turns decreased so suddenly. (Continue to the next page.)