PCB Technical - What are the requirements for mounting SMD on FPC

In the development of the miniaturization of electronic products, 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 the SMD on the FPC has become One of the development trends of SMT technology. The process requirements and precautions for surface mounting are as follows.

one. 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.

Key process: 1. Solder paste printing: FPC is positioned on a special pallet for printing based on its appearance. Generally, small semi-automatic printers are used for printing, or manual printing can also be used, but the quality of manual printing is worse than that of semi-automatic printing.

2. Placement: Generally, manual placement can be used, and individual components with higher position accuracy can also be placed by manual placement machine.

3. Welding: Reflow welding is generally used, spot welding can also be used under special circumstances.

two. 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 it requires high equipment. In addition, it is difficult to control the printing solder paste and placement process.

Key process: 1. FPC fixation: fix it on the pallet from the printing patch to the reflow soldering process. The pallet used requires a small coefficient of thermal expansion. There are two fixing methods. Use method A when the placement accuracy is above 0.65MM for QFP lead spacing; use method B when the placement accuracy is below 0.65MM for QFP lead spacing.

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 resistant tape should have a moderate viscosity, be easy to peel off after reflow soldering, and there should be no adhesive residue on the FPC.

Method B: The pallet is customized, and its process requirements must undergo multiple thermal shocks and the deformation is minimal. There are T-shaped positioning pins on the pallet, and the height of the pins is slightly higher than that of the 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 a suitable solder paste must be selected. In addition, special treatment is required for the printing template using method B.

3. Mounting equipment: First, the solder paste printing machine, the printing machine is best equipped with an optical positioning system, otherwise the welding quality will have a greater impact. Secondly, the FPC is fixed on the pallet, but there will always be some tiny gaps between the FPC and the pallet, 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, FPC placement requires strict process control.

three. Others: In order to ensure the quality of assembly, it is best to dry the FPC before mounting.