The process requirements for SMD mounting on FPC (Flexible Printed Circuit Board). With the development of miniaturization in electronic products, a considerable portion of consumer products are surface mounted. Due to the limited assembly space, the SMD is mounted on FPC to complete the assembly of the entire machine. Surface mounting of SMD on FPC has become one of the trends in SMT technology. The following are the process requirements and precautions for surface mounting

1、 Conventional SMD mounting

Features: Low requirements for mounting accuracy, limited number of components, mainly consisting of resistors and capacitors, or with individual irregular components

Key process: 1. Solder paste printing: FPC is positioned on a dedicated printing pallet based on its appearance, and is generally printed using a small semi-automatic printing machine. 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 for individual components with higher positional accuracy, manual placement machines can also be used for placement

3. Welding: Generally, reflow welding is used, and spot welding can also be used in special cases

2、 High precision mounting

Features: FPC should have MARK marks for substrate positioning, and the FPC itself should be flat. FPC fixation is difficult, and consistency is difficult to ensure during mass production, which requires high equipment requirements. Additionally, printing solder paste and mounting process control are difficult

Key process: 1. FPC fixation: Fix the entire process from printing the patch to reflow soldering on the pallet. The pallet used requires a small coefficient of thermal expansion. There are two fixation methods, and the installation accuracy is QFP. When the lead spacing is above 0.65MM, use method A; Method B is used when the installation accuracy is below 0.65MM for QFP lead spacing

Method A: Place the support plate on the positioning template. Fix the FPC onto the support plate with thin high-temperature resistant tape, and then separate the support plate from the positioning template for printing. The high-temperature resistant tape should have a moderate viscosity, be easy to peel after reflow soldering, and have no residual adhesive on the FPC

Method B: The pallet is customized and requires minimal deformation after multiple thermal shocks. The pallet is equipped with a T-shaped positioning pin, which is slightly higher in height than FPC

2. Solder paste printing: Due to the loading of FPC on the pallet, there is a high-temperature resistant tape for positioning on the FPC, which causes the height to be inconsistent with the pallet plane. Therefore, elastic scrapers must be used for printing. The composition of solder paste has a significant impact on the printing effect, and suitable solder paste must be selected. In addition, the printing template using method B needs to undergo special treatment

3. Installation equipment: Firstly, the solder paste printing machine should be equipped with an optical positioning system, otherwise the welding quality will be greatly affected. Secondly, the FPC is fixed on the pallet, but there will always be some small gaps between the FPC and the pallet, which is the biggest difference from the PCB substrate. Due to the setting of equipment parameters, it will have a significant impact on the printing effect, installation accuracy, and welding effect. Therefore, the installation of FPC requires strict process control requirements