Flexible Printed Circuit Board (FPC) Process Flow
The process of mounting SMDs on flexible printed circuit boards (FPCs) is required on the surface mounting of a large number of consumer products at the time of the development of miniaturization of electronic products. Because of the small assembly space, SMDs are mounted on FPCs. The surface mounting of SMD on the FPC assembled by the complete machine has become one of the development trends of SMT technology.
Conventional SMD placement
Features: Mounting accuracy is not high, the number of components is small, component types are mainly resistance and capacitance, or there are individual shaped components.
The key process:
1. Solder Paste Printing: FPC is positioned on the special pallet for printing by external appearance. Usually it is printed on a small semi-automatic printing press. Manual printing can also be used, but the manual printing quality is worse than semi-automatic printing.
2. Mounting: Generally, it can be mounted by hand. Individual components with high position accuracy can also be mounted by manual placement machine.
3. Welding: Reflow welding process is generally used, and spot welding can be used in special cases.
Features: FPC must have MARK mark for substrate positioning, and FPC itself must be flat. FPC is difficult to fix, so the consistency is difficult to guarantee when mass production, and there is high requirements for equipment. In addition, the control of solder paste and mounting process is difficult.
The key process:
1. FPC fixing: It is fixed on the pallet from the printing patch to the reflow soldering, so the pallet used requires the thermal expansion coefficient to be small. There are two fixing methods, and the mounting accuracy more than 0.65MM for the QFP lead spacing use method A; the mounting accuracy less than 0.65mm for the QFP lead use method B. Method A: Place the plate on the positioning plate. FPC is fixed on the plate with thin high-temperature tape. Then separate the plate from the positioning plate for printing. High-temperature adhesive tape should have a moderate viscosity, must be easily peeled after reflow, and there is no residual adhesive on the FPC. Method B: The pallet is customized, and its process requirements must be minimally deformed after multiple thermal shocks. There are T-shaped positioning pins, and the height of the pin should be slightly higher than the FPC.
2. Paste printing: Because the pallet is loaded with FPC, the FPC has positioning high-temperature tape, so that the height and the pallet plane is inconsistent, so the elastic scraper must be used when printing. Solder paste composition has a greater impact on the printing effect,
So it must use appropriate solder paste. In addition, the printing templates of using B method need special treatment.
3. Placement equipment: First, the solder paste printer, printing machine is best equipped with an optical positioning system, otherwise the welding quality will have a greater impact. Second, the FPC is fixed on the pallet, but there will be some tiny gaps between the FPC and the pallet, which is the biggest difference with the PCB substrate. Therefore, the setting of the device parameters will have a greater impact on the printing effect, placement accuracy, and welding effect. Therefore, the FPC placement requires strict process control.
To ensure assembly quality, it is best to dry the FPC before mounting.