In surface mount technology (SMT), different components require varying amounts of solder paste for optimal soldering. Traditional laser stencils regulate paste deposition by adjusting aperture sizes. However, some PCB designs demand precise thickness variations in different areas. Step stencils address this challenge by enabling controlled solder paste thickness adjustments, ensuring reliable solder joints across the board.
How Are Step Stencils Made?
Step stencils are fabricated using a combination of laser engraving and chemical etching. This process allows for localized thickness adjustments while maintaining precision in stencil apertures.
Key Manufacturing Processes:
- Step-Up Design (Localized Thickening): Chemical etching reduces most stencil areas to a thinner profile, while selected regions retain the original thickness to increase solder paste deposition.
- Step-Down Design (Localized Thinning): Certain chemically-etched stencil sections serve to remove excess material, reducing the solder paste volume in those areas.
Step Stencil Production Workflow
Each step stencil undergoes several stages to meet high-quality standards:
- Surface Preparation: The stencil foil needs to clean to remove contaminants.
- Coating: A layer of photosensitive ink is applied for pattern transfer (exposure and etching).
- Drying: Curing the coating ensures stability.
- Exposure: UV light creates the required pattern.
- Developing: Unexposed areas are removed, revealing the etching layout.
- Chemical Etching: Selective etching creates the step features.
- Laser Cutting: Apertures formed by precisely using laser technology.
- Finalization: The stencil undergoes a final cleaning and inspection, ensuring it is ready for printing applications.
To enhance durability and smoothness, an additional etching and polishing step removes burrs from aperture walls. This ensures even solder paste release while maintaining stencil longevity.
Types of Step Stencils
1. Step-Down Stencils (Reduced Thickness Sections)
By reducing the thickness of specific areas of the steel sheet, it is necessary to minimize the amount of solder paste in these regions, and make it suitable for areas with lower solder paste requirements. A 0.1mm-thick stencil etched down from 0.12mm in certain regions is to reduce the solder paste volume. This design allows the stencil to provide different amounts of solder paste deposition in different areas on the same PCB, catering to the soldering needs of various components.
2. Step-Up Stencils (Increased Thickness Sections)
In some areas, the increase in stencil thickness enhances the solder paste deposition. The other areas remain at the original thickness to control solder paste volume. This is ideal for regions requiring a higher solder paste volume.
Chemical etching reduces non-thickened areas in a 0.12mm thick stencil down to 0.1mm, while the thickened regions retain the 0.12mm thickness, thus increasing the solder paste volume in those localized areas. The diagram below shows the original thickness of the steel sheet indicated by the red arrows, while the grayish-white areas, etched down, are marked by the blue arrows.
3. Dual-Process Stencils (Solder Paste & Adhesive Application)
Dual-process Stencils are used to apply both solder paste and surface mount adhesive (SMA) in different regions of the same PCB. To prevent interference between the solder paste and adhesive, the dual-process stencil employs a stepped design. The reduction of the thickness of the steel sheet in the solder paste application area ensures separation between the paste and adhesive.
The diagram below shows the reverse view of the Surface Mount Adhesive (SMA) stencil with a stepped design: grooves formed by etching in the solder paste areas, while through-holes made by laser-drilling are in the adhesive application regions.
Step Stencil Design Considerations
Step Location: Top vs. Bottom
- Step-Up Stencils (Top-Side Thickening): Provide precise solder volume control but may cause blade deformation over time.
- Step-Down Stencils (Bottom-Side Thinning): Prevent blade wear but may create gaps between the PCB and stencil, increasing the risk of solder shorts—especially for 0402 components or 0.2mm pitch parts.
Thickness Control
For step stencil design, thickness differences should not exceed 0.025mm. This ensures smooth printing and prevents defects in high-precision PCB assembly.
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