The assembly of electronic products has brought great challenges to production operation and scheduling because of its large number of components, variety and short life cycle of electronic products. Many varieties, small batch and lot size have become the mainstream production mode of electronic manufacturing. In this mode, the switching time between multiple varieties occupies an increasing proportion in the whole assembly time. 
It is composed of a high-speed patch machine and a multi-functional patch machine. In fact, the whole assembly line of PCB also includes the feeder, screen printing machine, glue dispenser, reflow welding and curing furnace, and the receiver and so on. These equipment are connected in series to form the assembly line of PCB, but these auxiliary equipment do not constitute the bottleneck process of the whole assembly line, so they are omitted in the modeling. Each placement machine can accommodate 20 feeding slots, each of which occupies a feeding slot on the placement machine. In general, the number of components required for a single type PCB circuit board does not exceed the capacity of the feeder, but the number of components required for multiple varieties of PCB usually exceeds the capacity of the feeder.     
In the process of mounting, the components required for different kinds of PCB circuit boards are placed on the feeding grooves, and the attachers are removed from the feed slot to be attached to the designated position on the PCB. According to the survey data of the empirical research object, the time of the average mounting of one component on the high-speed patch machine is 0.06s, and the time of the average switching of one feed slot is 180s. The multi-function patch machine has two workstations, but only one worktable works, the other is only for large or heterogeneous components, each table holds 10 feed tanks, an average mount of one component needs 0.18s, and the average time for switching a feeder is 220s. When switching between different kinds of PCB, the patch machine needs a certain switching time, usually about 6 times the time of switching a component feeding slot, that is, 1200s. It can be seen that switching time plays an important role in the assembly process of electronic products. 
In the multi variety and small batch environment, the PCB is usually divided into groups, and the PCB with similar features is divided into one group. In the same group, the assembly between different varieties of PCB does not need to replace the feeder or the switching time; however, when the two groups of PCB need to replace the feeder, they need to consider the switch between them. Time. According to this, the production scheduling problem of PCB is divided into two levels: the first level scheduling problem is called the "board level" scheduling problem, that is to determine the production order of each PCB in a PCB group; the second level scheduling problem is called the "group level" scheduling problem, that is, to determine the production order between the PCB organizations.