Soft board FPC installation of basic knowledge, come to the onlookers

In the development of the miniaturization of electronic products, a considerable part of the surface mount of consumer products, due to the relationship of assembly space, its SMD are mounted on FPC to complete the assembly of the whole machine.  The surface mount of SMD on FPC has become one of the developing trends of SMT technology.  For surface mounting process requirements and attention points have the following points.  

1. Conventional SMD mounting  

Features: low mounting accuracy requirements, small number of components, components mainly resistance and capacitance, or some unusual components.  

Key process: 1. Solder paste printing :FPC is positioned on a special printing plate by its appearance. It is generally printed by a small semi-automatic printing machine, and manual printing can also be used, but the quality of manual printing is worse than semi-automatic printing.  

2. Mounting: generally, manual mounting can be used. Individual components with higher position accuracy can also be mounted by manual mounting machine.  

3. Welding: reflow welding process is generally used, spot welding can also be used in special circumstances.  

2. High-precision mounting  

Features: FPC should be marked with MARK for substrate positioning,FPC itself should be flat.  FPC is difficult to fix, consistency is difficult to ensure in mass production, high requirements for equipment.  In addition, it is difficult to control the printing solder paste and mounting process.  

Key process: 1.FPC fixation: it is fixed on the pallet from printing patch to reflow welding.  The supporting plate is required to have a small thermal expansion coefficient.  There are two fixing methods. Method A is used when the mounting accuracy is more than 0.65mm between QFP leads;  Method B is used when the QFP lead spacing is less than 0.65mm.  

Method A: The bracket is set on the positioning template.  The FPC is attached to the pallet with thin heat-resistant tape, and the pallet is then separated from the positioning template for printing.  High temperature resistant tape shall have moderate viscosity, be easy to peel off after reflow welding, and have no residual adhesive agent on FPC.  

Method B: The pallet is custom-made and requires minimal deformation after multiple thermal shocks.  T positioning pin is provided on the pallet, the pin height is slightly higher than FPC.  

2. Solder paste printing: Because FPC is loaded on the pallet, there is high temperature resistant adhesive tape for positioning on the FPC, which makes the height inconsistent with the plane of the pallet, so the elastic scraper must be selected during printing.  Solder paste composition has a great influence on the printing effect, so the appropriate solder paste must be selected.  In addition, special treatment is required for the printing template using method B.  

3. Mounting equipment: First, solder paste printing machine, printing machine preferably with optical positioning system, otherwise the welding quality will have a great impact.  Second, the FPC is fixed on the pallet, but there is always some small gap between the FPC and the pallet, which is the biggest difference from PCB substrate.  Therefore, the setting of equipment parameters will have a great impact on the printing effect, mounting accuracy and welding effect.  Therefore, FPC mounting has strict requirements for process control.