Printed circuit board assembly problems

Electronic equipment manufacturing companies intended to produce very high volumes in the shortest possible time. Therefore, this is why they invest a lot of resources so that their processes are automated through the integration of electronic systems in its manufacturing plants, which involves having efficient techniques to facilitate the control of production.

Today, printed circuit boards (PCB) manufacture base their work on the use of manufacturing machines. These machines are scheduled according to the characteristics of the PCB being assembled at that time.

The PCB manufacturing task requires that the components to be mounted on them are taken from their pickup locations and placed in their respective positions on the PCB. To achieve production goals, the machines need to be configured and/or programmed in order to efficiently reduce power consumption and minimize assembly time, for example. The latter gives rise to some combinatorial optimization problems that have to be addressed and solved.

The facts that there is a large diversity of manufacturing machines types and that they may assemble one or more types of PCB, result in that difficulty varies for solving the problems associated with this process.

The PCB assembly problems range from deciding the placement sequence of components, to problems of how to operate the whole plant for efficient production. Due to the variety of planning and control problems, hierarchical decomposition is a widely used approach to solve these problems. The problems are related to each other, so that for solving the most difficult problems require solutions of the least complex1.

The increase in the number of components to be placed on a single PCB has made the reduction of the assembly time to be probably the most important objective to reduce production costs and increase productivity2. There is now a wide variety of manufacturing machines for achieving this goal. Even when dealing with different types of machines, the manufacturing process is similar to them.

The functionality of a pick and place machine type is as follows. A PCB is transported into the machine on a conveyor belt and is fixed at a predetermined position for their manufacture. There are one or two feeder shelves which are located at both sides of the conveyor belt and each having a number of available slots. The reels contain the components which are to be mounted on the PCB and each reel contains only one type of component. Each slot can house a reel, while a reel may occupy one or more slots.

The machine has a mechanical arm which may have one or more heads. The heads pick up the components from the feeder shelves and placed them on the PCB. Each head can hold one component at a time. These heads are independent to hold, pick up, turn and place. Heads use some tools that support a component until it is placed on the card. Each type of component can be held by a subset of the tools, that is, a head with a given tool can only hold components of a limited set of component types. The tools are automatically changed in the automatic tool changer when they can not hold the required component.

The assembly process can be divided into two phases. In the first, the arm moves towards the feeder rack and heads sequentially hold the components to be assembled. In the second, the arm moves towards the PCB and heads sequentially placed the components in their respective locations. If required, a tool change is made before the initial phase.

According to the number of PCB types to be manufactured and to the number of manufacturing machines to be used for the assembly process, the PCB manufacturing problems can be classified in the following types1:
  • One type of PCB and one machine (problem 1-1). This class includes optimization problems on a single machine, involving the location of the components in the feeder rack and the assembly sequence of the components.
  • One type of PCB and many machines (problem 1-M). This class focuses on the allocation of components to sequential manufacturing machines, where the objective is to balance the workload on the machines.
  • Many types of PCB and one machine (problem M-1). In this kind of problem, the feeder rack arrangement strategy is determined (reels to slots placement strategy) for a single machine, this in order to minimize the number of changes to be made in the feeder rack.
  • Many types of PCB and many machines (problem M-M). This is a kind of scheduling problems that mainly focus on the allocation of tasks to production lines and the sequence of the production lines.

1 M. Johnsson and J. Smed. Observations on PCB assembly optimization. Electronic Packaging and Production, 41 (5) :38-42, 2001.
2 E. K. Burke, P. I. Cowling and R. Keuthen. New models and heuristics for component placement in printed circuit board assembly. 1999 IEEE International Conference on Information, Intelligence and Systems, pp. 133-139. IEEE Press, 1999.