Solder Paste Printing Optimization: From Stencil Design to Solder Ball Prevention

The Impact of Stencil Aperture Design on Soldering Quality

The aperture size is generally adjusted based on the actual size of the PCB pads to precisely control the solder paste volume in the solder paste printing process.

Solder Ball Prevention Techniques in Solder Paste Printing

The primary goal of solder ball prevention is to regulate the amount of solder paste beneath the stencil, particularly in areas prone to solder ball formation. By increasing stencil coverage and reducing solder paste volume, minimizing the likelihood of paste dispersing during component placement is feasible, effectively mitigating solder ball risks.

Manufacturers mainly apply this technique to larger pads. According to SMT process specifications, components with package sizes of 0805 (2mm × 1.25mm) and above utilize solder ball prevention designs.

stencil aperture
stencil aperture (Image source: online)

For 0805 and larger components, reducing stencil apertures by about one-third of the pad size can result in semi-circular or rectangular recesses. This design ensures precise and stable solder paste deposition, additionally enhancing soldering reliability and product consistency for larger pads or chip components.

stencil apertures are reduced by about one-third of the pad size, forming semi-circular or rectangular recesses
stencil apertures are reduced by about one-third of the pad size, forming semi-circular or rectangular recesses (Image source: online)

Optimized Stencil Aperture Design for Large Ground Pads and Solder-Filled Traces

For large ground pads or solder-filled traces, adopting an optimized stencil aperture design, often referred to as “window pane,” “matrix,” or “cross-hatching,” is highly recommended. This method involves adjusting the shape and size of the stencil apertures to precisely control the volume of solder paste, thereby effectively mitigating common soldering defects such as cold solder joints, voids, or pin misalignment.

In standard manufacturing practices, stencil apertures for ground pads or solder-filled traces are typically designed to match the pad dimensions in a 1:1 ratio. However, excessive solder paste on large ground pads often leads to issues such as insufficient wetting of component leads, an increased void rate, or solder joint instability. To overcome these challenges, the “window pane” approach refines stencil cutting patterns by segmenting the aperture area. Specifically, aperture dimensions are maintained at approximately 2mm for each segment, with a gap of around 0.25mm between adjacent sections.

This design ensures precise solder paste deposition by reducing excess accumulation, promoting uniform reflow, and preventing common soldering issues. It enhances solder joint reliability and ensures consistency during the entire soldering process, particularly for high-density or thermally demanding assemblies.

stencil window pane aperture
stencil window pane aperture (Image source: online)

Stencil Fiducial Mark and Printing Accuracy

Fiducial Marks are critical for ensuring precise alignment during solder paste printing. SMT stencil printers use these reference points to detect PCB positioning and direction, enabling accurate solder paste deposition onto the pads.

Fiducial Marks are available in two types:

  • Etched-Through Fiducial Marks: Simplified production, suitable for applications with moderate precision requirements.
  • Etched-Half Fiducial Marks: Provide higher accuracy and stability, ideal for manual or semi-automated printing in high-end electronic manufacturing.
Etched-Through Fiducial Marks and Etched-Half Fiducial Marks
Etched-Through Fiducial Marks and Etched-Half Fiducial Marks (Image source: online)

Stencil Thickness Selection and Solder Paste Volume Control

The thickness of a stencil directly influences the volume of solder paste transferred to the pads. Thicker stencils enable higher paste volumes for components requiring substantial solder, while fine-pitch components prefer thinner stencils to prevent bridging.

For fine-pitch components (pin pitch < 0.4mm), a 0.08mm thin stencil is recommended to avoid solder bridging. Conversely, for Wi-Fi modules and similar components requiring more paste, stencil thickness typically ranges between 0.12–0.15mm, with slightly enlarged apertures for optimal soldering results.

Formula of aspect ratio and that of area ratio
Formula of aspect ratio and that of area ratio (Image source: online)
  1. Aspect Ratio: Aperture width (W) / stencil foil thickness (T) > 1.5.
Pitch (mm) Pad Width (mm) Stencil Thickness (mm)
1.27 0.635 0.42
0.80 0.40 0.27
0.55 0.275 0.18
0.40 0.20 0.13
  1. Area Ratio: Aperture area (L×W) / aperture wall area [2×(L+W)×T] > 0.66.

For PBGA components, the printed solder paste diameter should be 0.05mm less than pad size to prevent bridging.

Step Stencil Technology in Solder Paste Printing

Step stencil technology addresses complex soldering needs by offering varying thickness regions within the same stencil to accommodate components with different solder requirements. This approach enhances productivity and soldering quality.

Stencil aperture
Stencil aperture (Image source: online)

Step stencils are particularly advantageous for PCBs integrating small precision components and large power devices. However, step designs can affect the scraper’s longevity, adjustments in scraper parameters and usage frequency is necessary. Additionally, combining adhesive dispensing processes with stencil printing for expanded applications is feasible.

Step stencils
Step stencils (Image source: online)

Stencil Structure and Manufacturing Processes

A stencil consists of three primary components:

  • Aluminum frame: Provides structural support, ensuring overall stability and durability.
  • Polyester mesh: Secures the foil in place, further contributing to the integrity of the stencil.
  • Foil: The critical part that determines the aperture shapes and sizes. Typically made of stainless steel, known for its excellent wear resistance and corrosion resistance, ensuring high performance and longevity in demanding applications.

The high-precision laser cutting technology used in manufacturing ensures consistent aperture dimensions, in order to meet the rigorous requirements of modern PCB assembly. Surface treatments such as polishing and nano-coating reduce surface roughness, minimizing friction between the solder paste and foil. This, hence, results in smoother paste flow through the apertures, which ultimately improves quality and stability in the solder paste printing process.

LCSC primarily uses 304TA stainless steel for the foil material due to its exceptional durability and resistance to corrosion and wear. The aluminum frame provides a lightweight yet sturdy structure, and AB glue is employed to secure the foil to the frame, ensuring a reliable and long-lasting stencil. This combination of materials and advanced manufacturing techniques guarantees stable performance and high-quality results in various production environments.

Some images are sourced online. Please contact us for removal if any copyright concerns arise.


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