Wright Williams & Kelly

Factory Explorer® v2.4
Technical Alert #18

Capacity Analysis for Batch Tools
in Split-Lot Regions


Summary: When performing capacity analysis for batch tools contained in split-lot regions, if a batch tool specifies lot-based minimum/maximum batch sizes and per-unit process times, Factory Explorer® underestimates the true capacity of the tool. To avoid this problem, use unit-based minimum/maximum batch sizes for batch tools that must specify per-unit process times inside split-lot regions. This problem is fixed in Factory Explorer® v2.5 beta 2.

Details: When calculating the capacity of a batch tool, Factory Explorer®'s capacity analysis engine uses a full-batch assumption. For each step in the process flow using a batch tool, it first calculates the full batch size (in units), and then the resulting process time for a batch of this size. It then uses the process time for later capacity calculations. For batch tools, minimum/maximum batch sizes can be specified in units or in lots. If the maximum batch size is specified in lots, Factory Explorer® calculates the full batch size (units) by multiplying the maximum batch size (# lots) by the product's maximum lot size. In Factory Explorer® v2.4, the calculation used the product's nominal maximum lot size (i.e. when it was released into the factory). But inside a split-lot region, the maximum lot size is generally smaller than the nominal maximum lot size. And so, this led the full batch size, measured in units, to be over-estimated. For example, if the nominal maximum lot size is 15 units, inside a split-lot region of lot size 5, the full batch size for a batch tool holding 2 lots should be 10 units (2 lots of 5 units each) -- not 30 units. If a process step specifies a per-batch time only, then there is no problem -- the process time is independent of the number of units in a full batch. If, however, the process step specifies a per-unit time, this error causes the process time to be over-estimated, and hence the capacity of the tool to be under-estimated.

Workaround: For batch tools that must specify per-unit process times inside split-lot regions, use unit-based (instead of lot-based) minimum/maximum batch sizes. In the capacity calculation, there will then be no need to multiply by the lot size to find the full batch size in units, and capacity will be calculated correctly.

Techalert tafx18-9808


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