SMD Component Mounting Solutions on FPC Boards

Commonly Used Methods:

• Multi-chip Placement:
• Component Types: Suitable for chip components larger than 0603 and QFP components with pin spacings of 0.65mm or greater.
• Number of Components: Ranges from a few to more than a dozen per FPC.
• Mounting Accuracy: Moderate accuracy required.
• FPC Characteristics: FPCs are larger in area, have designated component-free zones, and feature MARK marks for optical positioning and positioning holes.
• High-precision Mounting:
• Component Types: Suitable for all conventional components, including QFPs with pin spacing less than 0.65mm.
• Number of Components: Suitable for dozens or more components.
• Mounting Accuracy: Ensures high precision placement accuracy of QFPs at 0.5mm.
• FPC Characteristics: Large areas, multiple positioning holes, and optical positioning marks for critical components.

Additional Information:

FPCs and plastic-encapsulated SMD components are considered “moisture-sensitive devices.” To prevent warping and delamination, store FPCs in moisture-proof conditions and dry them before use. Large-scale production often involves high-temperature drying methods.

For the preservation of solder paste, store it in a low-temperature environment and allow it to return to room temperature before use. Avoid rapid reheating and ensure thorough stirring for optimal printing results.

Maintain an ambient temperature around 20°C with relative humidity below 60% for solder paste printing. Use a relatively closed space to minimize air convection.

When using metal bushings, ensure the thickness is between 0.1mm-0.5mm for effective solder paste stripping. Precision methods like chemical corrosion, laser, or electroforming are recommended for optimal results.

Solder Paste Selection for SMD Placement on Flexible Printed Circuits (FPCs)

• Particle Shape and Diameter: Opt for spherical solder paste with a diameter less than one-third of the metal bushing thickness and one-fifth of the aperture width. Aim for particles around 40um to prevent blockages and ensure high printing quality.
• Solder Ratio: Choose a solder paste with 90%-92% solder content for moderate viscosity, preventing sagging during printing and ensuring strong welds post-reflow.
• Viscosity: Ideal solder paste viscosity ranges from 700-900Kcps. Low viscosity can lead to short circuits, while high viscosity may block metal leak holes.
• Thixotropic Coefficient: Aim for a coefficient between 0.45-0.60.

Printing Parameters for FPC SMD Placement

• Squeegee Type and Hardness: Utilize a polyurethane flat scraper with 80-90 degrees hardness for the uneven surface of FPCs.
• Scraper Angle: Maintain a scraper angle between 60-75 degrees.
• Printing Direction: Opt for left-right or front-rear printing, angled for effective paste application on all QFP pad sides.
• Printing Speed: Set printing speed between 10-25mm/s to prevent squeegee issues.
• Printing Pressure: Adjust pressure from 0.1-0.3kg/cm length for proper solder paste application.
• Stripping Speed: Choose a stripping speed of 0.1-0.2mm/s for effective paste release.

Mounting and Reflow Soldering Tips

• Mounting: Use medium to high-speed placement machines and pay attention to suction force in the nozzle to avoid placement problems.
• Reflow Soldering: Opt for forced hot air convection infrared reflow soldering for even temperature changes and reduced soldering defects.
• Temperature Curve Test Method: Conduct temperature curve tests with FPC pallets to ensure proper component attachment.
• Temperature Curve Setting: Control reflow soldering time to about 3 minutes and adjust heating speeds based on solder paste characteristics.

Summary: Proper fixation and precise process parameter adjustments are key to maintaining high-quality SMD placement on FPCs. Focus on solder paste selection, printing, and reflow parameters to minimize defects in SMT production.