As designed, the PRO COOL® process cost of ownership software does not consider rework loops. PRO COOL® is designed to consider a single process sequence. For modeling more complicated process flows, including rework loops and process branches, we recommend our Factory Commander® cost and resource modeling software. However, rework impact can be included in TWO COOL® cost of ownership calculations. This application note will outline a method for including the impact of a simple rework loop in a process sequence cost of ownership analysis.
Rework in TWO COOL®
SEMI E10 considers rework to be part of the productive time of a process tool or step. However, to correctly calculate the shippable output or good units per week, the rework production needs to be subtracted from the overall output of that process step. Thus TWO COOL® includes an input for rework. This input is called "Redo Rate" and is part of the "Fab Parameter Inputs". "Redo Rate" is entered as a percentage and is most properly defined as the percentage of units processed due to these being reworked whether caused by this process step or by some other process step.
For example, a wet clean station may be used to strip photoresist from wafers being reworked at lithography. The lithography process causes the rework but the wet clean station is also impacted. A detailed TWO COOL® cost of ownership analysis of the wet clean would include an entry for "Redo Rate" to reflect this.
Rework in PRO COOL®
PRO COOL® does not have a separate entry for reworked units. Process sequence capacity analysis is based on the good units per week calculated by the TWO COOL® models that make up the PRO COOL® sequence. Since good units per week has been adjusted for rework by TWO COOL®, PRO COOL® will automatically take that into consideration in calculating process capacity analysis and process cost of ownership. No further adjustment to the process model is required.
The following process sequence will illustrate the modeling of a process sequence containing rework. A five step printed circuit board process has the following steps:
Receive and test bare board
Screen Print solder paste
Load surface mount devices
Some of the boards are identified at Final Test as having incomplete reflow. This is corrected by reworking the affected boards through the Reflow step and retesting the completed board. Thus both Reflow and Final Test will have part of their capacity used for rework.
At a start rate of 7000 boards per week and if 5% of the boards are reworked, we observe the following differences in estimated COO.
|Cost Item||Without Rework||With Rework|
*Load COO includes the $30.00 cost of devices.
This illustrates that if rework is properly treated in the TWO COOL® cost of ownership model, it will be reflected in the PRO COOL® process cost of ownership analysis.
Note: A disk with all of the TWO COOL® and PRO COOL® files used in this example is available from Wright Williams & Kelly, Inc.