M3SetupBatch Tool Availability

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The capacities of all machines, except the constraint machine, are 420 items per week, which is 30 items per shift (12 hours).

The batch size is assumed to be five items. This batch processing is a parallel process; therefore, the five items are processed at the same time, then un-batched when the process is completed.

For the availability machine, downtime (Table 6.5) is random and follows an exponential distribution. The average time between failures is 8064 min and the average time to repair is 2016 min. It corresponds to 80% availability and to one failure each week. These values also provide us 9% of the shift availabilities under the critical availability with shift availabilities as low as 1.45%.

By default 20 items are released at the beginning of every shift. Therefore the critical availability for the operation affected by downtime is 66.66%. This level of release should avoid the building of high queue in the constraint buffer and simultaneously keeps the constraint busy most of the time, avoiding lost capacity. This release will be modulated according to the machine availability. Different release strategies and constrain capacities will be applied from Table 6.2.

6.3.4 Scenario 4: Push and CONFLOW Policies Matched

Throughput

This scenario is based on the simulation model 5 BTC (order: batch, tool availability and constraint). All the operations‘ characteristics (machine number, capacity, downtime, batching…) are similar. The aim of this experiment is to facilitate the comparison of

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obtain the same throughput than CONFLOW. The appropriate rate was determined by gradually reducing the simulation release until the throughputs matched. 17 items per shift are released for the push system.

6.3.5 Scenario 5: 5-Stage Serial Line with Batch, Downtime,

Constraint and Re-entrant Line

CONFLOW was developed for serial lines without product mix or re-entrant lines. Nevertheless, in practice in front-end semiconductor manufacturing, re-entrant lines are an important factor of variability (Section 2.5, p33). Therefore, this scenario tests CONFLOW in a re-entrant system. Will CONFLOW be able to handle the problem of re-entrant lines?

In a real system, re-entrant lines follow a very complex pattern (Figure 4.1, p101). Complex modeling cannot be attempted directly. CONFLOW need to be evaluated with a simpler re-entrant system. The model 5 BTC is used as a reference. The aim is to demonstrate whether reentrancy is an issue that warrants further exploration, e.g. in future work.

In the model studied (Figure 6.10), all items will go through the whole line twice. In other words, when an item is completed by Operation 5 for the first time, it will be sent to Operation 1 buffer. When an item is completed by Operation 5 for the second time, it will exit the line.

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Figure 6.10: Re-entrant line

All the operations‘ characteristics (machine number, capacity, downtime, batching…)

are similar with scenario 3 (simulation model 5 BTC). The amount of items released is halved to 10 items/shift. As all items have to be processed twice, each operation still has to process 20 items/shift. So the overall amount of work remains identical to scenario 3.

6.3.6 Scenario 6: 5-Stage Model with Failures on Multiple

Stages

This scenario is based on the simulation model 5 BTC but all machines preceding the constraint (machines 1, 2 and 3) are affected by downtimes (Figure 6.11). Again, BTC is used to demonstrate normal conditions. The aim is to determine whether failures-on- multiple-machines is an issue that warrants further exploration, e.g. in future work. Downtime characteristics are the same than in model 5 BTC. Failures are random and follow an exponential distribution. The average time between failures is 8064 min and the average time to repair is 2016 min. All the other machines‘ characteristics (machine number, capacity, batching…) are similar to model 5 BTC.

Batch process

B2 M2

B1 M1 B3 M3 B4 M4 B5 M5

Tool availability Constraint

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Figure 6.11: Five operations model with failures on multiple operations

In order to calculate the number of items released in the shift, CONFLOW compares the availability of machine 1, 2 and 3 in the previous shift. The lowest availability is used to calculate the number of items released according to the formula given in section 6.2.3.

6.3.7 Scenario 7: TOC vs. CONFLOW

CONFLOW performance needs to be evaluated against well known TOC policies such as Starvation Avoidance (SA) policy and Drum-Buffer-Rope policy (DBR). For a single type of product (no mixed product) and in the absence of re-entrant lines, CONFLOW will be compared to the SA policy. Indeed, it was shown that SA is easy to use and well adapted to those conditions. Then a re-entrant line is introduced and CONFLOW is compared to the Drum Buffer Rope policy as it is better adapted to re-entrant lines.

Starvation Avoidance Policy Setup

The model was built from the model developed in SM5 BTC (Figure 6.9). All the operations‘ characteristics (machine number, capacity, downtime, batching…) are

similar.

The release policy differs. In compliance with the SA policy, the number of WIP from the start of the line down to the constraint machine is maintained constant at a target

B1 M1 B2 M2 B3 M3

B4 M4 B5 M5

Tool availability

Constraint Batch process

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WIP level (20 items). New items are released into the line at the beginning of every shift to meet the WIP target. The number of WIP after the constraint machine is not monitored.

Drum Buffer Rope Policy Setup

The model was built from the model developed in scenario 5 (Figure 6.10). All the operations‘ characteristics (machine number, capacity, downtime, batching…) are

similar.

The release policy differs. Initially (first shift), the line was loaded with 10 items. Thus each machine still has to process 20 items each shift (10 items x 2 due to re-entrant line) like in all previous models. Then for each shift, the number of items leaving the constraint machine for the second time is counted. These are the items which already went through the re-entrant line. They are completing their process and will not come back to the constraint machine. In compliance with the DBR policy, at the beginning of the following shift, the same number of items is released in the line.

6.4 Experiment Results

The results for the seven scenarios described are presented. Firstly, the 2 machines/operations model, then the 5 machines serial line with constraint and downtime, then batching is added. A re-entrant line and failures on multiple machines are also tested. Finally, CONFLOW is compared to SA and DBR release policies.

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6.4.1 Scenario 1: Two Machines (Operations) Simulation

Models

Two types of simulations are studied. Firstly recovery performance is analysed and then response to downtime.