As is well known, PCBA manufacturing is the process of soldering components onto the surface of a circuit board, involving steps such as solder paste application, component placement, and high-temperature soldering. When dealing with complex circuit boards, installing hundreds of components may be necessary, meaning there is a need for repeating the process numerous times. To enhance production efficiency, PCB stencils are commonly used in the process—a tool with precise hole positions, supported by a sturdy stainless steel structure. It simplifies the solder paste printing process by applying solder paste to all designated soldering points on the board in one go.
What Is the Stencil in a PCB
The concept of a stencil is quite straightforward—it is essentially a stainless steel sheet with holes corresponding to the soldering points on a PCB. PCB stencils are commonly used in the SMT process during the solder paste printing stage. In this step, the stencil is placed over the PCB, aligning the holes in the stencil with the soldering points on the PCB. Solder paste is then applied to the stencil, and as a squeegee or scraper passes over the stencil’s surface, the pressure causes the solder paste to pass through the holes and onto the PCB. Therefore, PCB stencils are also known as SMT stencils or solder paste stencils.
So, why use it?
Imagine you need to assemble 1000 PCB, each with 100 soldering points. This would require applying solder paste 100,000 times—a colossal task. However, with a stencil, you simply align it with the PCB and push the solder paste through, significantly reducing the workload. Moreover, in modern electronic assembly processes, automation typically handles the entire production flow. Stencils are integrated into SMT equipment, allowing you to input design information, and the equipment autonomously completes the alignment process. After printing, you only need to perform SPI to check the results.
Apart from meeting automation requirements, using stencils helps ensure the consistency of solder paste printing. Stencils are manufactured based on the PCB design, with each hole’s depth corresponding to the amount of solder paste deposition. This ensures a consistent amount of solder paste for each soldering point, mitigating risks of soldering defects and component offset.
Manufacturing Technologies of PCB Stencils
There are various manufacturing technologies for PCB stencils, with three widely used methods in modern electronic manufacturing: chemical etching, laser cutting, and electroforming. Each of these techniques has its advantages and is suitable for specific scenarios.
Chemical Etching:
This technique involves using corrosive chemical solutions to selectively remove unwanted metal portions from a stainless steel sheet, forming openings corresponding to the PCB pads. While it is cost-effective, precision control can be challenging, and the resulting profiles often exhibit a “water leaking” shape, which is not conducive to high-quality solder paste printing. Therefore, it is not suitable for high-precision designs.
Laser Cutting:
Laser cutting utilizes a controlled machine to use a high-energy laser beam for cutting and perforating. The laser directly ablates the stainless steel sheet, resulting in smooth hole walls with roughness less than 3um. Laser cutting technology is highly advantageous for manufacturing high-precision stencils at a moderate cost.
Electroforming:
Electroforming is a more complex technique involving the gradual deposition of metal material in the holes of a mold through an electroplating process. This forms a steel stencil with the desired aperture and shape. Its major advantage lies in high precision without the need for subsequent compensation processes. However, it comes with a higher cost and is typically used in the manufacturing of micro BGAs, ultra-fine pitch QFPs, and small chip components.
Stencil Opening Design
0201 Chips: Ensure the spacing between adjacent components is between 0.23 to 0.25 millimeters, while maintaining a 1:1 ratio between stencil aperture and spacing.
0603 and above Chips: Typically, there are four common design forms. To effectively prevent solder ball generation, it is recommended to use an Arc design or a stencil openings ratio of 0.9.
Diodes: These components require more solder paste. It is generally recommended to use a 1:1 stencil aperture ratio. For diodes with larger inner distances and smaller pads, to maintain the inner distance, the outer three sides of the pads can be increased by 10-15% based on the area.
SOT23-1: Due to the small size of the pads and components, it is recommended to use a 1:1 stencil aperture ratio.
SOT89: As the pads and components are relatively large with small pad spacing, potential printing quality issues can arise. To address this, it is suggested to use the stencil opening method shown in the diagram and perform intermittent processing.
Large Pads: When pads are large (usually one side larger than 4mm, the other side not less than 2.5mm), to prevent solder ball generation, it is recommended to use a segmented stencil opening with grid lines. The grid line width is 0.4mm, and the grid size is approximately 3mm, which can be evenly distributed based on the pad size.
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