OUR SERVICES
Solder Paste Printing
Solder paste printing is a crucial step in the surface mount technology (SMT) assembly process, where solder paste is applied onto the pads of a printed circuit board (PCB) before components are placed. This process ensures that the components will be securely attached to the board during reflow soldering.
Regarding PCB Solder Paste Printing
PCB solder paste printing is a critical process in SMT (Surface Mount Technology) that determines final soldering quality. Industry statistics show that over 60% of soldering defects (such as bridging and cold joints) originate from improper process control during the printing stage. Its core objective is to consistently deposit precise volumes of solder paste onto PCB pads through a stencil.
Process Flow and Core Principles
Solder paste printing is a process involving the interaction of fluid mechanics and precision mechanics. The basic steps are: the PCB is precisely aligned and brought into contact with the stencil via fiducial marks (optical alignment points), typically at zero gap → the squeegee pushes the solder paste across the stencil at a constant speed and pressure → the solder paste fills the stencil apertures under hydrodynamic pressure → the stencil separates from the PCB, releasing the solder paste onto the pads.
Solder paste is a non-Newtonian fluid with thixotropic properties (it thins when sheared and recovers when at rest). The pressure generated by the squeegee motion causes the paste to fill the apertures, while the speed control during stencil separation determines whether the paste releases cleanly from the aperture walls rather than stringing or slumping.
Key Process Parameters and Control Standards
To achieve consistent printing quality, the following parameters must be strictly controlled:
Temperature and humidity: Shop-floor temperature should be controlled at 23±3°C, with relative humidity <70% (some fine-pitch printing requires 45%±5%). Excessive temperature causes the paste to thin and slump; too low a temperature increases viscosity and leads to poor filling.
Solder paste warming and storage: Refrigerated (2–10°C) solder paste must be warmed at room temperature for at least 2 hours before use to prevent condensation from mixing in. Once opened, it is recommended to use it within 24 hours.
Viscosity: Typical printing viscosity ranges from 900–1400 Pa·s. For fine pitch of 0.5 mm and below, 900 Pa·s generally provides better release consistency.
Squeegee pressure: Typically set at 0.3–0.5 N/mm (of squeegee length). Excessive pressure tends to cause “scooping” (digging paste out of large apertures), while insufficient pressure fails to wipe the stencil surface clean.
Printing speed: Generally 25–50 mm/s. Fine-pitch apertures should use lower speeds (about 25 mm/s) to ensure complete filling; large pads or throughput-oriented production can use higher speeds.
Separation speed: Fine-pitch printing recommends a slower separation speed of 0.1–3 mm/s; too fast produces solder paste “peaks,” leading to slumping or bridging.
Common Printing Defects and Causes
Printing defects propagate directly into the reflow stage. Typical problems include:
Insufficient stencil area ratio: This is the primary cause of solder starvation in fine pitch. IPC-7525 recommends an area ratio (aperture area / aperture wall area) of at least 0.66. Below this value, the adhesion between the paste and the aperture walls exceeds that between the paste and the pad, causing poor release.
Aperture clogging or poor release: Residual paste on the underside of the stencil not cleaned properly, or separation speed too fast.
Abnormally high paste viscosity: May result from insufficient warming, flux evaporation, or excessive pause time during printing.
Excess paste or slumping: Oversized stencil apertures, excessive stencil thickness, or too low a paste viscosity (excessive temperature) causing the printed paste to slump to the sides.
Alignment offset or paste bleed under the stencil: PCB not in tight contact with the stencil (gap present), or alignment error exceeding 20% of pad width.
Improper separation parameters: Stencil separation speed too fast, pulling the paste into peak-shaped protrusions. Optimizing the separation speed (using a slow-fast two-stage separation) can improve this.
Fiducial recognition or mechanical positioning error: Typically the allowable offset is within 50 μm. If it consistently exceeds this, check the printer’s vision system or whether the PCB support is flat.
Quality Inspection Methods
Modern SMT lines typically use SPI (Solder Paste Inspection) for inline control of printing quality. SPI uses 3D measurement technology to inspect solder paste volume, area, height, and offset, promptly intercepting defects and preventing them from flowing into subsequent placement and reflow stages. For fine-pitch components below 0.5 mm, visual inspection alone is unreliable; SPI or AOI must be relied upon.
Need Solder Paste Printing Service?
Looking for expert Solder Paste Printing services? Our skilled team and state-of-the-art equipment ensure precise and reliable solder paste application for your PCBs. Get in touch with Elipcb today to discuss your project and benefit from our top-notch soldering solutions.
Advantages of ELIPCB Solder Paste Printing
- 100% SPI inspection rather than sampling: The line is equipped with 3D SPI to perform inline inspection of volume, area, height, and offset for every board and every pad, so defective products are intercepted before placement and reflow—rather than being cleaned up after reflow by AOI or rework.
- Closed-loop feedback capability: Closed-loop control linking SPI data with printer parameters (e.g., volume deviation automatically triggering fine adjustments to pressure/speed).
- Area ratio control: Stencil aperture design follows IPC-7525, with the area ratio controlled above 0.66—the hard threshold for good release in fine-pitch printing. Capable of consistently achieving pitches of 0.5 mm and below (such as 0.4 mm pitch QFN, 01005 components) with solder starvation and bridging under control.
- Controlled temperature and humidity: Shop-floor temperature of 23±3°C and humidity <70% (45%±5% for fine-pitch printing), preventing the paste from thinning and slumping due to excessive temperature or filling poorly due to low temperature.
- Full lifecycle management of solder paste: Refrigerated at 2–10°C, warmed for ≥2 hours, and used within 24 hours of opening, preventing condensation ingress and flux evaporation that cause viscosity drift.