PCB Blog - The solution of PCB board mounting SMD on FPC

The solution of PCB board mounting SMD on FPC,According to the placement accuracy requirements and the different types and quantities of components, the currently commonly used solutions are as follows: Multi-chip placement: Multiple FPCs are positioned on the half of the support by the positioning template, and are fixed on the support plate with SMT in the whole process. mount.
1. Scope of application:
1.1 Component types: chip components generally have a volume greater than 0603, and QFQ and other components with a pin spacing greater than or equal to 0.65.
1.2 Number of components: from several components to more than a dozen components on each FPC.
1.3 Mounting accuracy: The mounting accuracy is moderate.
1.4 FPC characteristics: The area is slightly larger, and there are no components in appropriate areas. 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 bushing, 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 diameter of the positioning hole on the FPC, and the height is about 2.5mm. There are also two lower position pins on the pallet on the FPC positioning template. Make a batch of pallets from the same CAD data. The thickness of the pallet should be about 2mm, and the warping deformation of the material after multiple thermal shocks should be small, and good FR-4 material and other high-quality materials are preferred. Before performing SMT, put the pallet on the pallet positioning pins on the template, so that the positioning pins are exposed through the holes on the pallet. Put the FPCs on the exposed pins one by one, and fix them on the pallet with thin high-temperature resistant tape, so that the FPC is not offset, and then separate the pallet from the FPC positioning template for soldering, printing and mounting, which is resistant to high temperature. The adhesive tape (PA protective film) should be moderately sticky and easy to peel off after being impacted by high temperature. And there is no residual glue on the FPC. It is especially important to note that the shorter the storage time between the FPC being fixed on the pallet and the soldering, printing and placement, the better. Option 2. High-precision mounting: Fix one or several pieces of FPC on a high-precision positioning pallet for SMT mounting 1. Scope of application:
2.1 Component types: almost all conventional components, QFPs with pin spacing less than 0.65mm are also available.
2.2 Number of components: dozens of components or more.
2.3 Mounting accuracy: In comparison, the placement accuracy of QFP with a high placement accuracy of 0.5mm can also be guaranteed.
2.4 FPC characteristics: large area, several positioning holes, MARK marks for FPC optical positioning and optical positioning marks for important components such as QFP.

3. FPC and plastic-encapsulated SMD components belong to "moisture-sensitive devices". After FPC absorbs moisture, it is more likely to cause warpage and deformation, and it is easy to delaminate at high temperatures. Therefore, FPC, like all plastic-encapsulated SMD components, should be kept in moisture-proof storage at ordinary times. 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 SMD at 80℃-120℃ for 16-24 hours.

4. Preservation of solder paste and preparation before use: The composition of solder paste is more complex. When the temperature is high, some components are very unstable and volatile, so the solder paste should be sealed and stored in a low temperature environment. The temperature should be greater than 0°C, and 4°C-8°C is suitable. Before use, return to normal temperature for about 8 hours (under sealed conditions), when the temperature is consistent with normal temperature. can be opened and used after stirring. If it is turned on before reaching room temperature, the solder paste will absorb moisture in the air, causing splashing during reflow soldering, resulting in undesirable phenomena such as tin beads. At the same time, the absorbed moisture easily reacts with some activators at high temperature, consumes the activator, and is prone to poor welding. Solder paste is also strictly prohibited to quickly reheat at high temperature (above 32°C). When stirring manually, use even force. When the solder paste is stirred like a thick paste, use a spatula to stir it up, and it can be divided into paragraphs naturally, which means it can be used. The centrifugal automatic mixer can be used, and the effect is better, and the phenomenon of residual bubbles in the solder paste can be avoided by manual stirring, so that the printing effect is better.

5. Ambient temperature and humidity: Generally, the ambient temperature requires a constant temperature of about 20 °C, and the relative humidity is kept below 60%. Solder paste printing requires a relatively closed space with little air convection.

6. Metal bushing The thickness of the metal bushing is generally selected between 0.1mm-0.5mm. According to the actual effect, when the thickness of the leakage plate is less than half of the width of the pad, the effect of solder paste stripping is good, and there is less solder residue in the leakage. The area of the leak hole is generally about 10% smaller than that of the pad. Due to the precision requirements of the mounted components, the commonly used chemical corrosion does not meet the requirements. It is recommended to use chemical corrosion plus local chemical polishing, laser method and electroforming method to make metal bushings. From the price performance comparison, the laser method is preferred.
1) Chemical corrosion and local chemical polishing method: Chemical corrosion method is used to manufacture the leakage plate, which is relatively 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. This method is less expensive to manufacture.
2) Laser method: high cost. However, the processing precision is high, the hole wall is smooth, and the tolerance is small, which can be suitable for printing the solder paste of QFP with a pitch of 0.3mm.

7. Solder paste: According to the requirements of the product, general solder paste and no-clean solder paste can be selected respectively. Solder paste characteristics are as follows:
1) The particle shape and diameter of the solder paste: the shape of the particle paste is spherical, and the proportion of non-spherical type cannot exceed 5%. The diameter of the solder ball should be based on general rules. The diameter of the solder ball should be less than one-third of the thickness of the metal bushing and one-fifth of the aperture width. Otherwise, the solder balls with excessive diameter and irregular particles will easily block the leakage window and cause solder Poor paste printing. Therefore, the thickness of the metal plate of 0.1-0.5mm and the width of the leakage plate window of about 0.22mm determine the diameter of the solder ball to be about 40um. The proportion of the diameter of the solder balls cannot exceed 5%. If the diameter of the solder ball is too small, the surface oxide will increase rapidly and non-linearly as the diameter becomes smaller, and a large amount of flux components will be consumed during reflow soldering, which will seriously affect the soldering quality. If it is a no-clean solder paste, its deoxidizing substances are less, and the soldering effect will be worse. Therefore, spherical solder paste particles with uniform size and diameter of 40um are the best choice.
2) Solder ratio: Solder paste with a solder content of about 90%-92% has a moderate viscosity and is not easy to sag during printing, and after reflow soldering, the thickness is roughly 75% of that during printing, enough solder to ensure reliable welding strength.
3) Viscosity: The flow dynamics of solder pastes are complex. It is obvious that the solder paste should be easy to print and adhere firmly to the surface of the FPC. The low viscosity solder paste (500Kcps) tends to collapse and form a short circuit, while the high viscosity solder paste (1400Kcps) tends to remain in the metal leak holes. , slowly blocking the leak hole, affecting the printing quality. So 700-900Kcps solder paste is ideal.
4) Thixotropic coefficient: the general choice is 0.45-0.60.

8. Printing parameters: 1) Squeegee type and hardness: Due to the particularity of the FPC fixing method, the printing surface cannot be as flat as the PCB board and has the same thickness and hardness, so it is not suitable to use a metal squeegee, and the application hardness is 80-90 degrees. Polyurethane flat scraper. 2) The angle between the scraper and the FPC: generally choose between 60-75 degrees. 3) Printing direction: generally left and right or front and rear printing, the printing machine scraper and the conveying direction are printed at a certain angle, which can effectively ensure the printing volume and printing effect of the solder paste on the four-side pads of the QFP. 4) Printing speed: in the range of 10-25mm/s. Printing too fast will cause the squeegee to slip, resulting in missing prints. Too slow will cause uneven solder paste edges or contaminate the FPC surface. The squeegee speed should be proportional to the pad spacing and inversely proportional to the viscosity of the bushing thickness. When the printing speed is 20mm/s, the solder paste filling time is only 10mm/s. Therefore, the moderate printing speed can ensure the printing volume of the solder paste during fine printing. 5) Printing pressure: generally set to 0.1-0.3kg/cm length. Since changing the printing speed will change the printing pressure, usually, 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 cause insufficient amount of solder paste on the FPC, while too much printing pressure will make the solder paste too thin and increase the possibility of the solder paste contaminating the reverse side of the metal bushing and the surface of the FPC. 6) Stripping speed: 0.1-0.2mm/s. Due to the particularity of FPC, a slower stripping speed is conducive to the release of solder paste from the leak. If the speed is fast, the air pressure between the FPC and the support plate will change rapidly between the metal leak plate and the FPC, which will cause the gap size between the FPC and the support plate to change instantaneously, affecting the flow of solder paste from the leak hole. Detach and print graphic integrity, resulting in poor. Now, more advanced printing machines can set the stripping speed to be accelerated, and the speed can be gradually accelerated from 0, and the stripping effect is also very good.

9. Mounting: According to the characteristics of the product, the number of components and the patching efficiency, a medium and high-speed placement machine is generally used for mounting. Since each piece of FPC has an optical MARK mark for positioning, there is little difference between SMD mounting on the FPC and mounting on the PCB. It should be noted that after the component placement action is completed, the suction force in the suction nozzle should become 0 in time before the suction nozzle can be removed from the component. Although improper setting of this process can also cause poor placement when it is mounted on a PCB board, the probability of such a poor occurrence on a soft FPC is much greater. At the same time, attention should also be paid to the height of the lower sticker, and the speed of removing the suction nozzle should not be too fast.

10. Reflow soldering: Forced hot air convection infrared reflow soldering should be used, so that the temperature on the FPC can change more evenly and reduce the occurrence of poor soldering. 1) Temperature curve test method: Due to the different heat absorption properties of the pallets, the different types of components on the FPC, and after they are heated in reflow soldering, the temperature rises at different speeds, and the absorbed heat is also different, so carefully set the reflow soldering parameters. 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 interval during actual production, and attach components to the FPC of the test pallet. Solder on the test point, and fix the probe lead on the pallet with high temperature resistant tape (PA protective film). Note that the high temperature tape does not cover the test points. The test points should be selected on the solder joints and QFP pins that are close to each side of the pallet, so that the test results can better reflect the real situation. 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. How much and how much time each functional section requires to set the heating and transmission speed of each temperature zone of 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. We all know that there are less activators in no-clean solder paste, and the degree of activation is low. If a conventional temperature curve is used, the preheating time will be too long, and the degree of oxidation of the solder particles will also be high, and there will be too much activator at the peak temperature. is depleted before the peak region, and there is not enough activator in the peak region to reduce the oxidized solder and metal surfaces. Solder cannot quickly melt and wet the metal surface, resulting in poor soldering. Therefore, for no-clean solder paste, a positioning curve different from that of conventional solder paste should be used to obtain good soldering results. This point has been ignored by some SMT craftsmen.

Summary For SMD placement on FPC, one of the key points is the fixation of FPC. The quality of fixation directly affects the placement quality. Followed by the selection of solder paste, printing and reflow. In the case of a well-fixed FPC, it can be said that more than 70% of the defects are caused by improper process parameter settings. Therefore, it is necessary to determine the process parameters according to the difference of FPC, the difference of SMD components, the difference of heat absorption of the pallet, the difference of the characteristics of the selected solder paste, and the difference of the equipment characteristic parameters, and to control the production process in a timely manner to detect abnormal conditions in time. Only by analyzing and making correct judgments, and taking necessary measures, can the defective rate of SMT production be controlled within dozens of PPM on PCB board.