PCB Technical - Basic knowledge of FPC circuit board mounting

The process of mounting SMD on FPC (flexible printed circuit board) requires that at the time of the development of 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 The assembly of the whole machine is completed. The surface mount of SMD on FPC has become one of the development trends of SMT technology. The process requirements and points of attention for surface mount are as follows.

A. 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 printing pallet 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 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, and spot welding can also be used in special circumstances.

2. 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 during mass production, and it requires high equipment. In addition, it is difficult to control the printing solder paste and placement process.

The key process: 1.FPC fixation: from the printing patch to the reflow soldering, the entire process is fixed on the pallet. The pallet used requires a small thermal expansion coefficient. There are two fixing methods, and the mounting accuracy is used when the QFP lead spacing is more than 0.65MM A; Method B is used when the placement accuracy is less than 0.65MM for QFP lead spacing.

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 a moderate viscosity and must be easy to peel off after reflow soldering, and There is no residual glue on the FPC.

Method B: The pallet is customized, and its process requirements must undergo multiple thermal shocks and the deformation is minimal. The pallet is equipped with a T-shaped positioning pin, and the height of the pin 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 you must choose an elastic scraper when printing. The composition of the solder paste has a greater impact on the printing effect. Choose a suitable solder paste. In addition, the printing template of the B method needs to be specially processed.

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.