In the manufacture of PCB, solder paste is applied for connecting the surface mount components to pads on the board. The application of solder paste should be even and done with precision. For this purpose, PCB engineers make use of solder stencils. In the surface mount assembly process, solder stencils are the essential equipment for effective and repeatable solder paste deposition.
Solder stencil facilitates an easier, faster, and accurate deposition of solder paste, minimizing the error-prone manual work. These stencils have a foil usually made of stainless steel. First, it is laser cut to match the places where surface mount devices are placed on the board. Next, the solder paste is applied through these cuts with a metal squeegee blade. After this, the manufacturer pulls away stainless steel foil. It enables the solder paste to be placed perfectly on the board, which helps you to receive the desired surface-mounted devices.
The solder stencil’s imperative features are perfectly designed to avoid errors in the PCB fabrication and assembly process. To improve the efficiency of the solder stencils design, the key elements that you need to take care of are:
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You can calculate the necessary thickness of the stencil foil through aspect ratio and area. The aspect ratio of the stencil is the width of the aperture divided by stencil thickness. Generally, an aspect ratio of 1.5 or greater is suitable for acceptable solder paste transfer. The area ratio is the area beneath the stencil aperture opening, which is divided by the size of the inside aperture wall.
Besides adhering to the prescribed aspect ratio and area ratio, the stencil thickness should also satisfy the most fine-pitch QFPs and BGAs. Achieving thickness accuracy is necessary for releasing the exact amount of solder paste through the opening.
The apertures of the stencil should be designed to match the exact land area on the printed circuit boards. As the miniaturization of electronics is becoming more popular, it is pushing the stencil printing process further. However, to achieve the solder paste transfer efficiency, it is essential to regulate the aperture design. For appropriate aperture size, area ratio and aspect ratio are crucial metrics. The right aperture size is the necessary volume, uniformity, and definition of the material that is to be deposited on the surface.
Aperture Positional Accuracy
The positional accuracy of the stencil apertures determines how effectively the solder paste will be placed on the PCB pad. Therefore, the stencil apertures should be aligned to the pads on the PCB. During laser manufacturing, the X/Y movement of the cutting beam should be controlled for accurate positional accuracy of the apertures. Your stencil vendor should take care of it while designing the solder stencil.
Solder Stencil Material
The material type of the stencil also impacts the release of the solder paste from the apertures. The most common material used for solder stencils is stainless steel. Mylar and Kapton are also popular alternatives, and it costs lower than stainless steel stencils. However, stainless steel stencils are the most suitable material for error-free prototyping. In addition, solder stencil material should be capable of withstanding harsh and rough working conditions in the PCB manufacturing environment.
Multi-Level Or Stepped Stencils
Sometimes the PCB design consists of acceptable pitch devices for which it requires a thin stencil, and for the remaining more significant parts, a thicker stencil is required. For such circuit boards, multi-level or ‘stepped’ stencil needs to be designed. Materials are added to the stencils to create step-up areas, which helps increase the volume and height of the solder paste in the required areas.
Removal of materials from the stencils helps create step-down regions, reducing the volume and size of the solder paste in the required areas. The apertures should be designed at a minimum distance from the step edge.
Stencil Manufacturing Process
How the stencil is manufactured also affects the release of the solder paste from the apertures. Two of the most common technologies used for designing the solder stencil are electroformed and laser-cut.
Chemical etching also helps to create steps for step stencils. But the final stencil apertures are generally manufactured through laser-cut. Laser-cut stencils have a much more acceptable gap which is trapezoidal and supports a better paste release. Based on solder paste application, there are mainly three kinds of a stencil, i.e., prototype stencil, framed stencil, and frameless stencils.
PCB Design Specific
While designing the solder stencil, an alignment of the stencil to the PCB is essential. To perfectly align the solder paste on the PCB pads, ‘fiducial marks,’ also known as registration marks, are added to both PCB and stencil. A fiducial is a round solder mask opening, and it has around bare copper in the center. The vision system of the screen printer produces fiducials marks on the stencil, enabling the printer to align the board with the stencil automatically.
These are some of the critical design features that need to take care of while designing the solder stencil. The rightly configured solder stencil ensures the development of neatly and adequately sized and shaped deposits on the printed circuit boards.