PCB Tech - Flexible printed circuit board (FPC) process flow

Flexible printed circuit board(FPC) process flow
The process of mounting SMD on flexible printed circuit boards(FPC) requires that when the miniaturization of electronic products develops, a considerable part of the surface mount of consumer products, due to the assembly space, the SMD is mounted on the FPC to complete The assembly of the whole machine. The surface mount of SMD on FPC has become one of the development trends of smt technology. Shanghai Hanhe Electronic Technology has rich experience in FPC placement and has provided placement services for a large number of customers, especially those of medical equipment.
Next, iPCB will introduce some problems about FPC placement:
Conventional SMD placement
Features: The placement accuracy is not high, the number of components is small, and the component varieties are mainly resistors and capacitors, or there are individual special-shaped components.
The key process:
1.Solder paste printing: FPC is positioned on the special pallet for printing by its appearance. Generally, it is printed by a small semi-automatic printing machine, or manual printing can also be used, but the quality of manual printing is worse than that of semi-automatic printing.

2. Mounting: Generally, manual mounting can be used, and individual components with higher position accuracy can also be mounted by manual placement machine.
3.Welding: Reflow welding process is generally used, and spot welding can also be used in special circumstances.
High precision placement
Features: There must be a MARK mark for substrate positioning on the FPC, and the FPC itself must be flat. It is difficult to fix the FPC, and it is difficult to ensure consistency in mass production, and the equipment requirements are high. In addition, it is difficult to control the printing solder paste and placement process.
2. The key process:
1. FPC fixing: fixed on the pallet from the printing patch to the reflow soldering. The pallet used requires a small thermal expansion coefficient. There are two fixing methods. When the mounting accuracy is QFP lead spacing above 0.65MM, use method A; paste When the mounting accuracy is less than 0.65MM for QFP lead spacing, use method B. Method A: set the pallet on the positioning template. The FPC is fixed on the pallet with a thin high-temperature resistant tape, and then the pallet is separated from the positioning template for printing. The high temperature tape should have moderate viscosity, it must be easy to peel off after reflow soldering, 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. The pallet is set There are T-shaped positioning pins, and the height of the pins is slightly higher than that of FPC.
2 Solder paste printing: Because the pallet is loaded with FPC, there is a high temperature resistant tape for positioning on the FPC, so that the height is inconsistent with the pallet plane, so an elastic scraper must be used when printing. The composition of the solder paste has a greater impact on the printing effect and must be selected Suitable solder paste. In addition, special treatment is required for the printing template of the B method.
3. Mounting equipment: First, the solder paste printing machine, the printing machine should have an optical positioning system, otherwise the welding quality will have a greater impact. Secondly, the FPC is fixed on the pallet, but the total distance between the FPC and the pallet There will be some tiny gaps, which is the biggest difference from the PCB substrate. Therefore, the setting of equipment parameters will have a greater impact on the printing effect, placement accuracy, and welding effect. Therefore, the placement of FPC has strict requirements for process control.
3. Others: In order to ensure the quality of assembly, it is best to dry the FPC before mounting.