The assembly of electronic products has brought huge challenges to production and operation scheduling due to the large number and variety of components and the short life cycle of electronic products. Multiple varieties, small batches, and variable batches have become the mainstream production mode of electronic manufacturing. In this mode, the switching time between multiple varieties occupies an increasing proportion of the overall assembly time.
It consists of a high-speed placement machine and a multi-functional placement machine. In fact, the entire PCB assembly line also includes equipment such as board feeders, screen printers, glue dispensers, reflow soldering and curing ovens, and board unwinders. These equipment are connected in series to form a PCB assembly line, but these auxiliary equipment does not constitute the bottleneck process of the entire pipeline, so it is omitted during modeling. Each placement machine can accommodate 20 feeding slots, and each component occupies one feeding slot on the placement machine. Generally speaking, the number of types of components required for a single-variety PCB does not exceed the capacity of the feeder, but the total number of types of components required for a multi-variety PCB usually exceeds the capacity of the feeder.
During the placement process, the components required by different types of PCBs are placed on these feeding troughs, and the pick-and-place device removes the components from the feeding trough and mounts them to the designated positions on the PCB. According to the survey data of the empirical research objects, the average time for a high-speed placement machine to mount a component is 0.06s, and the average time to switch a feeding slot is 180s. The multi-function placement machine has two workbenches, but only one workbench works, and the other workbench is only used to place large or special-shaped components. Each workbench accommodates 10 feeding troughs, and it takes 0.18s to place a component on average, the average time to switch a feeder is 220s. When switching between different types of PCBs, the placement machine needs a certain preparation switching time, usually about 6 times the time to switch a component feeding slot, that is 1200s. It can be seen that the switching time plays a pivotal role in the assembly process of electronic products.
In a multi-variety and small-batch environment, PCBs are usually grouped first, and PCBs with similar characteristics are grouped into a group, so that within the same group, the assembly between different varieties of PCBs does not require changing feeders, nor does it require Switching time. But when the feeder needs to be replaced between two sets of PCBs, the switching time between them needs to be considered. According to this, the PCB production scheduling problem is divided into two levels: the first-level scheduling problem is called the “board-level” scheduling problem, that is, to determine the production order among the PCBs in a PCB group; the second-level scheduling problem is called for the “group-level” scheduling problem, that is, to determine the production sequence between PCB organizations.
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