Flexible PCB assembly
Flexible PCB assembly or FPC assembly is a process that is mounting electronic components on flexible plastic substrates such as polyimide, PEEK, or a transparent conductive polyester film.
With the development of electronic products to be short, thin, and light, the integration and miniaturization of electronic components become the trends. And the flexible PCBs came into being and made the circuit boards with unique shapes and sizes. Good flexibility, space-saving, excellent performance are the main advantages of flexible PCBs.
Due to the characteristics, the flexible boards cannot perform surface mounting technology(SMT) like rigid boards. The process of flexible assembly is quite technical. There are two assemble ways of FPC. One is reel to reel(RTR); this way needs a highly specialized production line equipment and high input cost.So, RTR is suitable for substantial batch FPC production. Another assemble way is fixture attachment method, and there are some process points need to be noted as below:
  • Pre-baked
  • FPC attachment and fixing
  • The solder paste printing of FPC
  • FPC assembly
The flexible board material is easy to get damp. When the wet board after high-temperature welding, there will be a foaming layer and resulting in scrap. Therefore, PCB manufacturers usually require the flexible board supplier to use vacuum packaging for the incoming materials. But vacuum packaging is not foolproof, and it is best to pre-baked the flexible boards before assembling. After baking, the FPC must be cooled to room temperature before it can be put into production. Otherwise, the hot FPC will cause the solder paste to collapse.
For the single FPC assembly, usually manual assembled. For the multiple varieties and small batch production, a fixture is needed. The method of attaching and fixing varies according to different fixture designs. Eliminating the need for special products and considering operability and ease of maintenance, the fixture usually is designed into two parts: the base and the pallet. The base is used to fix the pallet and position the FPC. The pallet plays the role of fixing FPC. The design and manufacture of the fixture directly affect the production straight-through rate of the FPC.
Although the FPC becomes a hardboard(PCB) after being fixed, the surface of the FPC may still be uneven because of the deformation of the FPC itself, the thickness of the attached material, the reinforcement plate, or the thickness of the back adhesive. The problems of continuous tin, less tin, and more tin will occur when the FPC is not flat.
When the unevenness of the FPC is solved, the assembly is very simple, and there is not much different from the rigid PCB. However, because there are few components on FPC, FPC must be assembled by the panel, so how to use the SMT machine efficiently is the main problem of FPC assembly.