Flexible pcb mounting problems and solutions introduction
The process of mounting SMD on flexible pcb (FPC) requires that when the electronic product is miniaturized, a considerable part of the surface mounting of consumer products, due to the assembly space, its SMD is mounted on the FPC to complete the surface mount of SMD on the .FPC of the whole machine has become one of the development trends of SMT technology.
Conventional SMD mounting
Features: Mounting accuracy is not high, the number of components is less, the components are mainly resistors and capacitors, or there are individual shaped components.
1.Solder paste printing: flexible pcb is positioned on a special printing plate by external shape. It is usually printed by small semi-automatic printing machine or manual printing, but the quality of manual printing is worse than that of semi-automatic printing.
2.Mounting: Generally, manual mountingcan be used, and individual components with higher positional accuracy can also be mounted by manual placement machine.
3. Welding: Reflow soldering technicsis generally used, and spot welding can also be used in special cases.
High-precision mounting features: FPC must have MARK for substrate positioning, flexible pcb itself should be flat. FPC is difficult to fix, consistency is difficult to ensure during mass production, and equipment requirements are high. In addition, printing paste and mounting technics control is more difficult Big. 2. Key processes:
1.FPC fixing: from printing patch to reflow soldering, it is fixed on the pallet the whole process. The pallet used requires small thermal expansion coefficient. There are two fixing methods. The mounting accuracy of QFP lead spacing is 0.65MM or more. When the precision of the QFP lead is 0.65mm or less, the method is used. Method A: The support plate is placed on the positioning template. The flexible pcb is fixed on the pallet with a thin high-temperature resistant tape, and then the pallet is separated from the positioning template for printing. High temperature tape should have moderate viscosity, easy to peel off after reflow soldering, and no residual glue on FPC. Method B: The pallet is customized, and its technics requirements must be deformed after multiple thermal shocks. There is a T-shaped locating pin, the height of the pin is slightly higher than the FPC.
2.Solder paste printing: Because the flexible circuit board is loaded on the pallet, the FPC has a high temperature resistant tape for positioning, so the height is not consistent with the plane of the pallet. Therefore, an elastic scraper must be used for printing. The solder paste composition has a great influence on the printing effect, and must be selected appropriately. The solder paste. In addition, the printing template using the B method needs special treatment.
3.Mounting equipment: First, the solder paste printing machine, the printing machine preferably has an optical positioning system, otherwise the welding quality will have a greater impact. Secondly, the FPC is fixed on the pallet, but the total there will be some tiny gapsbetween the flexible pcb and the pallet, which is the biggest difference from the PCB substrate. Therefore, the setting of the device parameters will have a great impact on the printing effect, placement accuracy, and soldering effect. Therefore, the FPC placement requires strict process control.
In order to ensure the quality of the assembly, the flexible pcb is preferably dried before being mounted.
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