Basic knowledge of mounting on FPC

Basic knowledge of mounting on FPC

The process requirements for mounting SMD on flexible printed circuit boards (FPCs) are as electronic products are miniaturized. Due to the limited assembly space, a considerable number of consumer products are mounted on FPCs to complete the assembly of the entire machine. The surface mounting of SMDs on FPCs has become one of the development trends of SMT technology. There are several points to note about the process requirements and points for surface mounting. 1. Conventional SMD mounting

Features: The mounting accuracy requirement is not high, the number of components is small, the component varieties are mainly resistors and capacitors, or there are some special-shaped components.

Key process: 1. Solder paste printing: FPC is positioned on a special printing tray by its appearance. It is generally printed by 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. Individual components with higher position accuracy can also be placed by manual placement machines.

3. Welding: Reflow soldering is generally used, and spot welding can also be used in special cases.

2. High-precision placement

Features: There must be a MARK mark on the FPC for substrate positioning, and the FPC itself must be flat. FPC is difficult to fix, and it is difficult to ensure consistency during mass production, and the equipment requirements are high. In addition, it is difficult to control the printing solder paste and mounting process.

Key process: 1. FPC fixing: fixed on the pallet from printing patch to reflow soldering. The pallet used requires a small thermal expansion coefficient. There are two fixing methods. When the mounting accuracy is more than 0.65MM of QFP lead spacing, use method A; when the mounting accuracy is less than 0.65MM of QFP lead spacing, use method B.

Method A: The pallet is placed 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 moderate viscosity and must be easy to peel off after reflow soldering, and there should be no residual adhesive on the FPC.

Method B: The pallet is customized, and its process requirements must be minimal deformation after multiple thermal shocks. 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 FPC is loaded on the pallet, there is a high temperature resistant tape on the FPC for positioning, which makes the height inconsistent with the plane of the pallet, so an elastic scraper must be used for printing. The composition of the solder paste has a great influence on the printing effect, so a suitable solder paste must be selected. In addition, the printing template using method B needs to be specially treated.

3. Mounting equipment: First, solder paste printer, the printer is best 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 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 great impact on the printing effect, mounting accuracy, and welding effect. Therefore, the mounting of FPC requires strict process control.