PCB Tech - PCB technology to mount SMD solution on FPC

According to the requirements of PCB placement accuracy and the types and quantities of PCB 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 and 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. Placement accuracy: medium placement accuracy is required.

D. FPC characteristics: the area is slightly larger, there are no components in the appropriate area, each piece of 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 plate, read the internal positioning data of the FPC to manufacture the 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.

It is particularly important to note that the storage time between the beginning of the FPC fixing on the pallet and the soldering printing and placement is as short as possible.

Scheme 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 can also be used

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 characteristics: large area, several positioning holes, MARK marks for FPC optical positioning and optical positioning marks for important components such as QFP.

2. FPC fixing: 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.

High-precision placement and process requirements and precautions on FPC 1. FPC fixing direction: Before making the metal leakage plate and pallet, the fixing direction of the FPC should be considered first, so that it may cause poor soldering during reflow soldering. 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 of the 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 activators, 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 is required to be carried out in a relatively closed space with little air convection.

1) Chemical corrosion and local chemical polishing method: The chemical corrosion method is more common in China at present, but the hole wall is not smooth enough. Local chemical polishing method can be used to increase the smoothness of the hole wall. The manufacturing cost of this method is low.

2) Laser method: high cost. However, it has high processing accuracy, smooth hole walls and small tolerances, which can be suitable for printing QFP solder paste with 0.3mm pitch.

7. Printing parameters:

1) Type and hardness of squeegee: Due to the particularity of the FPC fixing method, the printed surface cannot be as flat as the PCB and the thickness and hardness are consistent. Therefore, metal squeegees should not be used, and polyurethane flat squeegees with a hardness of 80-90 degrees should be used.

2) The angle between the scraper and the FPC: generally choose between 60-75 degrees.

3) Printing direction: generally left-right or front-to-back printing, the most advanced printing machine squeegee prints at a certain angle with the conveying direction, which can effectively guarantee the printing volume of solder paste on the four-side pads of QFP, and the printing effect is the best.

4) Printing speed: within the range of 10-25mm/s. Too fast printing speed will cause the squeegee to slip, resulting in missing printing. If the speed is too slow, the edges of the solder paste will be uneven or the surface of the FPC will be contaminated. The squeegee speed should be directly proportional to the pad spacing and inversely proportional to the viscosity of the thickness of the squeegee. When the printing speed is 20mm/s, the filling time of the solder paste is only 10mm/s. So the moderate printing speed can ensure the printing volume of the solder paste during fine printing.

5) Printing pressure: Generally set at 0.1-0.3kg/cm length. Since changing the printing speed will change the printing pressure, under normal circumstances, first fix the printing speed and then adjust the printing pressure, from small to large, until the solder paste is just scraped off the surface of the metal leakage plate. Too little pressure will make the amount of solder paste on the FPC insufficient, and too much printing pressure will make the solder paste print too thin, and at the same time increase the possibility of the solder paste contaminating the back surface of the metal drain and the surface of the FPC.

9. Reflow soldering: compulsory hot air convection infrared reflow soldering should be used, so that the temperature on the FPC can be changed more uniformly, reducing the occurrence of poor soldering.

1) Temperature curve test method: due to the different heat absorption properties of the pallets and the different types of components on the FPC, after they are heated in the reflow soldering, the temperature rises at different speeds and the heat absorbed is also different, so carefully set the reflow soldering The temperature curve has a great influence on the welding quality. A more appropriate method is to place two pallets with FPC in front of the test board according to the pallet spacing during actual production, and at the same time attach components to the FPC of the test pallet, and use high-temperature solder wire to test the temperature probe Solder on the test point, and fix the probe lead on the pallet with high temperature resistant tape (PA protective film).

2) Temperature curve setting and transmission speed: Since the weight ratio of the solder paste we use reaches 90%-92%, and the flux composition is less, the entire reflow soldering time is controlled at about 3 minutes. We should according to the reflow soldering temperature zone How much and the time required for each functional section to set the heating and conveying speed of each temperature zone of the reflow soldering. It should be noted that the transmission speed should not be too fast, so as not to cause jitter and cause poor welding.