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Assembly of electronic products because of the large number of components , as well as a wide variety of electronic product life cycle is short and so on , to the production operation scheduling an enormous challenge and variety, low-volume , variable -volume electronics manufacturing has become a mainstream mode of production . In this mode , the switching time between the many varieties occupy an increasingly large proportion of the entire assembly time .

By the high-speed placement machine and multifunction placement machine components. In fact , the entire PCB assembly line also includes sending plate machine, screen printing , dispensing , reflow and curing oven, closing panels and other equipment , these devices are connected in series consisting of PCB assembly lines , but these ancillary equipment and does not constitute the entire pipeline bottleneck process , and thus omitted in modeling . Each placement machine can hold 20 bits for the chute , chute for each element occupies a position on the placement machine . Generally the number of component types , single species PCB need not exceed the capacity of the feeder , but the total number of components required for many kinds of varieties of PCB often exceeds the capacity feeder .

Varieties element during mounting PCB need to be placed on the feed trough , and applying device taken from the feed tank after removing the components mounted on the PCB to the specified position . According to the survey data for the empirical study , the average speed placement machine mount a time element is 0.06s, the average switching time of a feed trough for 180s. Multifunction placement machine has two tables , but only one work table , another table is only used to place large or shaped elements, each table accommodates 10 for trough, the average placement of a component requires 0.18s the average switching time a feeder for 220s. When PCB switch between different varieties , SMT switch requires some preparation time , usually switching element for a time of about six times the trough , ie 1200s. Thus, the switching time plays a decisive role in the electronics assembly process.

In many varieties of small batch environment , usually the first group of the PCB , the PCB with similar characteristics are grouped together , so that in the same group , different varieties of PCB assembly between the need to replace the feeder does not need switching time ; but between the two feeders PCB needs to be replaced , you need to consider switching time between them. Whereby the PCB production scheduling problem is divided into two levels: the first level scheduling problem called " board-level " scheduling problem , namely to determine the order of the PCB production within a group between ; said second level of scheduling problems the " group level " scheduling problem , namely to determine the production sequence PCB organizations.