In the development of miniaturized electronic products, a significant portion of surface-mount devices (SMD) in consumer products is mounted on flexible printed circuits (FPC) due to limited assembly space, enabling the completion of the final product. Mounting SMDs on FPCs has become one of the key trends in the evolution of surface-mount technology (SMT). The process requirements and precautions for surface mounting are outlined below.
1. Conventional SMD placement
Features: Placement accuracy is not critical, the component count is low, and components primarily include resistors and capacitors, with occasional special-shaped components.
Key process: 1. Solder paste printing: The FPC is positioned on a dedicated pallet for printing based on its layout. Typically, small semi-automatic printers are used, though manual printing is also an option. However, manual printing tends to be less precise than semi-automatic printing.
2. **Placement**: Typically, manual placement can be used, and for components requiring higher positioning accuracy, a manual placement machine may also be employed.
3. **Soldering**: Reflow soldering is generally the preferred method, though spot welding can be used in specific cases.
**Two. High Precision Placement**
**Features**: A MARK mark for substrate positioning is essential on the FPC, and the FPC itself must be flat. Fixing the FPC is challenging, and ensuring consistency during mass production is difficult, requiring advanced equipment. Additionally, controlling the solder paste printing and placement process is also complex.
**Key Process**:
1. **FPC Fixation**: The FPC should be secured on a pallet from the printing stage through to the reflow soldering process. The pallet must have a low coefficient of thermal expansion. There are two fixation methods based on placement accuracy. Method A is used for QFP lead spacing above 0.65mm, while Method B is suitable for QFP lead spacing below 0.65mm.
**Method A**: Position the pallet on a positioning template. Secure the FPC onto the pallet with a thin, high-temperature resistant tape. Afterward, remove the pallet from the template for printing. The tape should have moderate viscosity, be easy to peel off after reflow soldering, and leave no residue on the FPC.
**Method B**: Use a custom-designed pallet that can withstand multiple thermal shocks with minimal deformation. The pallet should have T-shaped positioning pins, with their height slightly higher than that of the FPC.
2. **Solder Paste Printing**: Since the pallet holds the FPC, and there is a high-temperature resistant tape for positioning, the height of the FPC may be inconsistent with the pallet plane. This requires the use of a flexible scraper during printing. The composition of the solder paste significantly impacts the printing quality, so selecting the right paste is crucial. Additionally, the printing template used in Method B requires special treatment.
3. **Mounting Equipment**: First, the solder paste printing machine should be equipped with an optical positioning system to ensure high-quality soldering. Without this, the soldering quality may be significantly compromised. The FPC is secured to the pallet, but there are often tiny gaps between the FPC and the pallet, which is a key difference from the PCB substrate. This gap makes equipment parameter settings critical, as they directly influence the printing effect, placement accuracy, and soldering quality. As a result, the placement of FPCs demands rigorous process control.
**Three. Additional Considerations**: To ensure high assembly quality, it is advisable to dry the FPC before mounting.
1. Conventional SMD placement
Features: Placement accuracy is not critical, the component count is low, and components primarily include resistors and capacitors, with occasional special-shaped components.
Key process: 1. Solder paste printing: The FPC is positioned on a dedicated pallet for printing based on its layout. Typically, small semi-automatic printers are used, though manual printing is also an option. However, manual printing tends to be less precise than semi-automatic printing.
2. **Placement**: Typically, manual placement can be used, and for components requiring higher positioning accuracy, a manual placement machine may also be employed.
3. **Soldering**: Reflow soldering is generally the preferred method, though spot welding can be used in specific cases.
**Two. High Precision Placement**
**Features**: A MARK mark for substrate positioning is essential on the FPC, and the FPC itself must be flat. Fixing the FPC is challenging, and ensuring consistency during mass production is difficult, requiring advanced equipment. Additionally, controlling the solder paste printing and placement process is also complex.
**Key Process**:
1. **FPC Fixation**: The FPC should be secured on a pallet from the printing stage through to the reflow soldering process. The pallet must have a low coefficient of thermal expansion. There are two fixation methods based on placement accuracy. Method A is used for QFP lead spacing above 0.65mm, while Method B is suitable for QFP lead spacing below 0.65mm.
**Method A**: Position the pallet on a positioning template. Secure the FPC onto the pallet with a thin, high-temperature resistant tape. Afterward, remove the pallet from the template for printing. The tape should have moderate viscosity, be easy to peel off after reflow soldering, and leave no residue on the FPC.
**Method B**: Use a custom-designed pallet that can withstand multiple thermal shocks with minimal deformation. The pallet should have T-shaped positioning pins, with their height slightly higher than that of the FPC.
2. **Solder Paste Printing**: Since the pallet holds the FPC, and there is a high-temperature resistant tape for positioning, the height of the FPC may be inconsistent with the pallet plane. This requires the use of a flexible scraper during printing. The composition of the solder paste significantly impacts the printing quality, so selecting the right paste is crucial. Additionally, the printing template used in Method B requires special treatment.
3. **Mounting Equipment**: First, the solder paste printing machine should be equipped with an optical positioning system to ensure high-quality soldering. Without this, the soldering quality may be significantly compromised. The FPC is secured to the pallet, but there are often tiny gaps between the FPC and the pallet, which is a key difference from the PCB substrate. This gap makes equipment parameter settings critical, as they directly influence the printing effect, placement accuracy, and soldering quality. As a result, the placement of FPCs demands rigorous process control.
**Three. Additional Considerations**: To ensure high assembly quality, it is advisable to dry the FPC before mounting.
