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According to the requirements of placement accuracy and the types and quantities of components, the currently commonly used solutions are as follows:
Solution 1. Multi-chip placement: Multi-chip FPC is positioned on the pallet by the positioning template, and is fixed on the pallet during the whole process for SMT placement.
1. Scope of application:
A. Component types: Chip components generally have a volume greater than 0603, and QFQ and other components with a lead pitch greater than or equal to 0.65 are acceptable.
B. Number of components: a few to a dozen components on each FPC.
C. Mounting accuracy: medium mounting accuracy is required.
D. FPC characteristics: The area is slightly larger, there are no components in the appropriate area, and each FPC has two MARK marks for optical positioning and more than two positioning holes.
2. Fixing of FPC:
According to the CAD data of the metal slip plate, read the internal positioning data of the FPC to manufacture a high-precision FPC positioning template. Match the diameter of the positioning pin on the template with the hole diameter of the positioning hole on the FPC, and the height is about 2.5mm. There are two lower pallet pins on the FPC positioning template. Manufacture a batch of pallets based on the same CAD data. The thickness of the pallet is about 2mm, and the warpage of the material after multiple thermal shocks should be small. Good FR-4 materials and other high-quality materials are better. Before SMT, put the pallet on the pallet positioning pin on the template, so that the positioning pin is exposed through the hole on the pallet.
Put the FPC on the exposed dowels one by one, fix it on the pallet with a thin high-temperature resistant tape to prevent the FPC from shifting, and then separate the pallet from the FPC positioning template for soldering printing and mounting, high temperature resistance The adhesive pressure of the tape (PA protective film) should be moderate, and it must be easy to peel off after high temperature impact. And there is no residual glue on the FPC.
Special attention should be paid to the storage time between FPC fixing on the pallet and soldering printing and placement as short as possible.
Solution 2. High-precision placement: Fix one or several pieces of FPC on a high-precision positioning pallet for SMT placement
1. Scope of application:
A. Component types: Almost all conventional components, QFP with pin spacing less than 0.65mm is also available.
B. Number of components: more than dozens of components.
C. Placement accuracy: In comparison, the placement accuracy of QFP with high placement accuracy up to 0.5mm pitch can also be guaranteed.
D. FPC features: large area, several positioning holes, MARK marks for FPC optical positioning and optical positioning marks for important components such as QFP.
2. Fixing of FPC:
The FPC is fixed on the component pallet. This kind of positioning pallet is customized in batches, with extremely high precision and high precision, and the positioning difference between each pallet can be ignored. This kind of pallet has very little dimensional change and warpage deformation after dozens of high-temperature impacts. There are two positioning pins on this positioning pallet. One has the same height as the thickness of the FPC, and the diameter matches the aperture of the positioning hole of the FPC. The other T-shaped positioning pin is slightly higher than the previous one. One point, because the FPC is very flexible, has a large area, and has an irregular shape, the function of the T-shaped positioning pin is to limit the deviation of some parts of the FPC to ensure the accuracy of printing and placement. For this fixing method, the metal plate corresponding to the T-shaped positioning pin can be properly treated.
The FPC is fixed on the positioning pallet. Although there is no limit to the storage time, the time should not be too long due to environmental conditions. Otherwise, the FPC is prone to moisture, which will cause warping and deformation, which will affect the quality of placement.
High-precision placement and process requirements and precautions on FPC:
1. FPC fixing direction: Before making stencils and pallets, the fixing direction of FPC should be considered first, so that the possibility of poor welding during reflow soldering is small. The preferred solution is to place the chip components in the vertical direction, SOT and SOP horizontal direction.
2. FPC and plastic package SMD components are also "moisture sensitive devices". After FPC absorbs moisture, it is easier to cause warpage and deformation, and it is easy to delaminate at high temperatures. Therefore, FPC is the same as all plastic SMD. It must be dried before drying. Generally, the high drying method is adopted in large-scale production factories. The drying time at 125°C is about 12 hours. Plastic package SMD is at 80 degree Celsius-120 degree Celsius for 16-24 hours.
3. Preservation of solder paste and preparation before use:
The composition of solder paste is more complicated. When the temperature is high, some components are very unstable and volatile. Therefore, the solder paste should be sealed and stored in a low temperature environment. The temperature should be greater than 0 degree Celsius, 4 degree Celsius-8 degree Celsius is the most suitable. Before use, return to room temperature for about 8 hours (under sealed conditions), when its temperature is consistent with room temperature. It can be opened and used after stirring. If it is used before it reaches room temperature, the solder paste will absorb moisture in the air, which will cause spattering and tin bead formation during reflow soldering. At the same time, the absorbed water easily reacts with certain activators at high temperatures, consuming the activator, and prone to poor welding. The solder paste is also strictly forbidden to quickly return to temperature at high temperatures (above 32°C). Manually stir evenly and vigorously. When the solder paste is stirred like a thick paste, use a spatula to pick it up. If it can be divided into sections naturally, it means it can be used. It is best to use a centrifugal automatic mixer, which has better effect, and can avoid the phenomenon of air bubbles remaining in the solder paste by manual stirring, so that the printing effect is better.
4. Ambient temperature and humidity:
Generally, the ambient temperature requires a constant temperature of about 20°C and a relative humidity below 60%. The solder paste printing requires a relatively closed space with little air convection.
5. Steel mesh
The thickness of the metal drain plate is generally selected between 0.1mm-0.5mm. According to the actual effect, when the thickness of the drain plate is less than one-half of the minimum pad width, the effect of solder paste stripping off the plate is good, and there is little solder residue in the leak space. The area of the leakage hole is generally about 10% smaller than that of the pad.
Due to the precision requirements of SMT components, the commonly used chemical corrosion does not meet the requirements. It is recommended to use chemical corrosion plus local chemical polishing, laser and electroforming to make metal leakage plates. From the comparison of price and performance, the laser method is preferred.
