The solution for mounting SMD components on an FPC board, depending on placement accuracy requirements and the types and quantities of components, generally involves the following commonly used methods:
1. **Multi-chip Placement**: Multiple FPCs are positioned on a support using a positioning template and are fixed onto the support plate through the entire SMT process.
1. **Scope of Application**:
1.1 **Component Types**: Suitable for chip components typically larger than 0603, and QFP or similar components with pin spacings of 0.65 mm or greater.
1.2 **Number of Components**: Ranges from a few components to more than a dozen per FPC.
1.3 **Mounting Accuracy**: Moderate accuracy is sufficient.
1.4 **FPC Characteristics**: Each FPC is slightly larger in area and free of components in designated zones. Each FPC is equipped with two MARK marks for optical positioning and more than two positioning holes.
1. Fixing of FPC: Based on the CAD data of the metal bushing, read the internal positioning data of the FPC to produce 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, ensuring a height of approximately 2.5mm. Additionally, two lower position pins should be included on the pallet of the FPC positioning template. Manufacture a batch of pallets using the same CAD data, with a thickness of about 2mm, and select materials with minimal warping deformation after thermal shocks, preferably high-quality FR-4 material. Before SMT, place the pallet on the template’s positioning pins so the pins protrude through the pallet holes. Position the FPCs on these exposed pins and secure them with thin high-temperature resistant tape to prevent offsetting. Then, detach the pallet from the FPC positioning template for soldering, printing, and mounting. The adhesive tape (PA protective film) should be moderately sticky and easy to peel off post high temperature exposure without leaving residue on the FPC. Minimize the storage time between fixing the FPC on the pallet and the soldering, printing, and placement for best results.
2. High-precision mounting: Secure one or multiple FPCs on a high-precision positioning pallet for SMT mounting.
1. Scope of application:
1.1 Component types: Suitable for almost all conventional components, including QFPs with pin spacing less than 0.65mm.
1.2 Number of components: Suitable for dozens or more components.
1.3 Mounting accuracy: Ensures placement accuracy of QFPs with high precision of 0.5mm.
1.4 FPC characteristics: Features large areas, multiple positioning holes, MARK marks for optical positioning, and optical positioning marks for critical components like QFPs.
3. FPCs and plastic-encapsulated SMD components are “moisture-sensitive devices”. Moisture absorption in FPCs can cause warping and delamination at high temperatures. Like all plastic-encapsulated SMD components, FPCs should be stored in moisture-proof conditions and dried before use. Large-scale production often uses high-temperature drying methods, with drying at 125°C for about 12 hours or at 80°C-120°C for 16-24 hours.
4. Preservation of solder paste and preparation before use: Due to the complex composition of solder paste, it should be sealed and stored in a low-temperature environment above 0°C, ideally between 4°C-8°C. Before use, allow it to return to room temperature for about 8 hours (under sealed conditions), then stir it before use. Premature exposure to room temperature can cause moisture absorption, leading to splashing during reflow soldering and poor soldering quality. Rapid reheating of solder paste at high temperatures (above 32°C) should be avoided. Manual stirring should be even, and when the paste becomes a thick consistency, use a spatula to stir until it breaks into sections naturally. An automatic centrifugal mixer is preferable to avoid bubbles and ensure better printing results.
5. Ambient temperature and humidity: Maintain an ambient temperature around 20°C with relative humidity below 60%. Solder paste printing requires a relatively closed space with minimal air convection.
6. Metal bushing: Typically selected with a thickness between 0.1mm-0.5mm. For effective solder paste stripping, the leakage plate thickness should be less than half the pad width, with leakage hole areas about 10% smaller than the pad area. For precision, use chemical corrosion plus local chemical polishing, laser, or electroforming methods. The laser method is preferred for its precision and smooth hole walls, suitable for printing solder paste with QFP pitch of 0.3mm.
7. Solder paste: Select based on product requirements, with general or no-clean options.
1) Particle shape and diameter: Spherical solder paste with a diameter less than one-third of the metal bushing thickness and one-fifth of the aperture width is ideal. Spherical particles with a diameter of about 40um are preferred to avoid blocking the leakage window and ensure good printing quality.
2) Solder ratio: A solder paste with 90%-92% solder content has moderate viscosity, avoids sagging during printing, and ensures reliable welding strength after reflow soldering.
3) Viscosity: Optimal viscosity for solder paste is 700-900Kcps. Low viscosity (500Kcps) tends to collapse and form short circuits, while high viscosity (1400Kcps) can block metal leak holes.
4) Thixotropic coefficient: Generally 0.45-0.60.
8. Printing parameters:
1) Squeegee type and hardness: Use a polyurethane flat scraper with 80-90 degrees hardness due to the FPC’s uneven surface.
2) Scraper angle: Typically between 60-75 degrees.
3) Printing direction: Generally left-right or front-rear printing, angled to ensure effective paste application on all QFP pad sides.
4) Printing speed: 10-25mm/s. Adjust speed to avoid squeegee slippage or uneven paste edges.
5) Printing pressure: Set to 0.1-0.3kg/cm length. Adjust pressure from low to high until solder paste is correctly scraped off the metal leakage plate.
6) Stripping speed: 0.1-0.2mm/s. A slower stripping speed helps release solder paste effectively, though modern machines can gradually accelerate stripping speed for better results.
9. Mounting: Use medium to high-speed placement machines based on component characteristics and efficiency. Ensure that the suction force in the nozzle is zero before removing it from the component to avoid placement issues. Attention is needed to the height of the lower sticker and the speed of suction nozzle removal.
10. Reflow soldering: Use forced hot air convection infrared reflow soldering to achieve even temperature changes and reduce poor soldering occurrences.
1) Temperature curve test method: Place two pallets with FPC on a test board and attach components to test points. Fix probes with high-temperature tape without covering test points to reflect real conditions.
2) Temperature curve setting and transmission speed: Control reflow soldering time to about 3 minutes. Adjust heating and transmission speeds based on the solder paste’s characteristics. For no-clean solder paste, use a curve different from conventional paste to avoid oxidation and ensure good soldering.
Summary: For SMD placement on FPCs, proper fixation is crucial, impacting placement quality. After ensuring correct fixation, focus on solder paste selection, printing, and reflow parameters. By adjusting process parameters based on FPC characteristics, SMD components, and other factors, you can maintain a low defect rate in SMT production.
1. **Multi-chip Placement**: Multiple FPCs are positioned on a support using a positioning template and are fixed onto the support plate through the entire SMT process.
1. **Scope of Application**:
1.1 **Component Types**: Suitable for chip components typically larger than 0603, and QFP or similar components with pin spacings of 0.65 mm or greater.
1.2 **Number of Components**: Ranges from a few components to more than a dozen per FPC.
1.3 **Mounting Accuracy**: Moderate accuracy is sufficient.
1.4 **FPC Characteristics**: Each FPC is slightly larger in area and free of components in designated zones. Each FPC is equipped with two MARK marks for optical positioning and more than two positioning holes.
1. Fixing of FPC: Based on the CAD data of the metal bushing, read the internal positioning data of the FPC to produce 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, ensuring a height of approximately 2.5mm. Additionally, two lower position pins should be included on the pallet of the FPC positioning template. Manufacture a batch of pallets using the same CAD data, with a thickness of about 2mm, and select materials with minimal warping deformation after thermal shocks, preferably high-quality FR-4 material. Before SMT, place the pallet on the template’s positioning pins so the pins protrude through the pallet holes. Position the FPCs on these exposed pins and secure them with thin high-temperature resistant tape to prevent offsetting. Then, detach the pallet from the FPC positioning template for soldering, printing, and mounting. The adhesive tape (PA protective film) should be moderately sticky and easy to peel off post high temperature exposure without leaving residue on the FPC. Minimize the storage time between fixing the FPC on the pallet and the soldering, printing, and placement for best results.
2. High-precision mounting: Secure one or multiple FPCs on a high-precision positioning pallet for SMT mounting.
1. Scope of application:
1.1 Component types: Suitable for almost all conventional components, including QFPs with pin spacing less than 0.65mm.
1.2 Number of components: Suitable for dozens or more components.
1.3 Mounting accuracy: Ensures placement accuracy of QFPs with high precision of 0.5mm.
1.4 FPC characteristics: Features large areas, multiple positioning holes, MARK marks for optical positioning, and optical positioning marks for critical components like QFPs.
3. FPCs and plastic-encapsulated SMD components are “moisture-sensitive devices”. Moisture absorption in FPCs can cause warping and delamination at high temperatures. Like all plastic-encapsulated SMD components, FPCs should be stored in moisture-proof conditions and dried before use. Large-scale production often uses high-temperature drying methods, with drying at 125°C for about 12 hours or at 80°C-120°C for 16-24 hours.
4. Preservation of solder paste and preparation before use: Due to the complex composition of solder paste, it should be sealed and stored in a low-temperature environment above 0°C, ideally between 4°C-8°C. Before use, allow it to return to room temperature for about 8 hours (under sealed conditions), then stir it before use. Premature exposure to room temperature can cause moisture absorption, leading to splashing during reflow soldering and poor soldering quality. Rapid reheating of solder paste at high temperatures (above 32°C) should be avoided. Manual stirring should be even, and when the paste becomes a thick consistency, use a spatula to stir until it breaks into sections naturally. An automatic centrifugal mixer is preferable to avoid bubbles and ensure better printing results.
5. Ambient temperature and humidity: Maintain an ambient temperature around 20°C with relative humidity below 60%. Solder paste printing requires a relatively closed space with minimal air convection.
6. Metal bushing: Typically selected with a thickness between 0.1mm-0.5mm. For effective solder paste stripping, the leakage plate thickness should be less than half the pad width, with leakage hole areas about 10% smaller than the pad area. For precision, use chemical corrosion plus local chemical polishing, laser, or electroforming methods. The laser method is preferred for its precision and smooth hole walls, suitable for printing solder paste with QFP pitch of 0.3mm.
7. Solder paste: Select based on product requirements, with general or no-clean options.
1) Particle shape and diameter: Spherical solder paste with a diameter less than one-third of the metal bushing thickness and one-fifth of the aperture width is ideal. Spherical particles with a diameter of about 40um are preferred to avoid blocking the leakage window and ensure good printing quality.
2) Solder ratio: A solder paste with 90%-92% solder content has moderate viscosity, avoids sagging during printing, and ensures reliable welding strength after reflow soldering.
3) Viscosity: Optimal viscosity for solder paste is 700-900Kcps. Low viscosity (500Kcps) tends to collapse and form short circuits, while high viscosity (1400Kcps) can block metal leak holes.
4) Thixotropic coefficient: Generally 0.45-0.60.
8. Printing parameters:
1) Squeegee type and hardness: Use a polyurethane flat scraper with 80-90 degrees hardness due to the FPC’s uneven surface.
2) Scraper angle: Typically between 60-75 degrees.
3) Printing direction: Generally left-right or front-rear printing, angled to ensure effective paste application on all QFP pad sides.
4) Printing speed: 10-25mm/s. Adjust speed to avoid squeegee slippage or uneven paste edges.
5) Printing pressure: Set to 0.1-0.3kg/cm length. Adjust pressure from low to high until solder paste is correctly scraped off the metal leakage plate.
6) Stripping speed: 0.1-0.2mm/s. A slower stripping speed helps release solder paste effectively, though modern machines can gradually accelerate stripping speed for better results.
9. Mounting: Use medium to high-speed placement machines based on component characteristics and efficiency. Ensure that the suction force in the nozzle is zero before removing it from the component to avoid placement issues. Attention is needed to the height of the lower sticker and the speed of suction nozzle removal.
10. Reflow soldering: Use forced hot air convection infrared reflow soldering to achieve even temperature changes and reduce poor soldering occurrences.
1) Temperature curve test method: Place two pallets with FPC on a test board and attach components to test points. Fix probes with high-temperature tape without covering test points to reflect real conditions.
2) Temperature curve setting and transmission speed: Control reflow soldering time to about 3 minutes. Adjust heating and transmission speeds based on the solder paste’s characteristics. For no-clean solder paste, use a curve different from conventional paste to avoid oxidation and ensure good soldering.
Summary: For SMD placement on FPCs, proper fixation is crucial, impacting placement quality. After ensuring correct fixation, focus on solder paste selection, printing, and reflow parameters. By adjusting process parameters based on FPC characteristics, SMD components, and other factors, you can maintain a low defect rate in SMT production.
