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Stencil requirements for the SMT stencil printing process

Stencil requirements for the SMT stencil printing process

Currently, the electronics industry uses many different types of laser stencils from many different SMT stencil suppliers, however, not all stencils are created equal. As the stencil industry is required to be able to deliver quickly, in many cases. On-time delivery is as important as the performance of the stencil.

It is believed that about 50% of defects in SMT stencil assembly lines are caused by the printing process. SMT stencil stencils have become a commodity, and many stencil SMT stencil manufacturers are selling them, sometimes, just based on their price, and not focusing on quality and experienced technical support. Does it look very unusual? At the same time, we are seeing increasingly challenging requirements for stencil performance due to the continued miniaturization of packages and components. Currently. The many different types of stencils used by the electronics industry come from many different SMT stencil manufacturing, however, not all SMD stencils are created equal. As the stencil industry is required to be able to deliver quickly, in many cases on-time delivery is as important as the performance of the stencil.

There are many types of stencils, including those manufactured by laser cutting technology directly on stainless steel materials to high-end all-nickel electroformed stencils, with many other stencils and variations between the two. The choice of stencil depends on the specific technical requirements of the application. It depends on the following variables.

The minimum component pitch to be printed;
The type of components to be placed on the board;

The minimum component area ratio calculated;

Currently, laser-cut stencils are the common stencil. The technology behind laser cutting equipment continues to improve, and this, coupled with extremely rapid improvements in core materials, has led to a significant expansion in the capabilities of laser direct stencils, and thus their increasing popularity and the number of laser direct cut SMT stencil contractors. Each stencil behaves differently in printing, and the results of printing are highly variable. The cumulative time and performance of the laser equipment used to cut the stencil may affect the printing process and ultimately the stencil's area ratio (AR), which has become a distinction between what stencil is used and what is not.

The AR has become an important factor in distinguishing what type of stencil to use. The AR is the area of the stencil aperture wall divided by the area of the pad under the stencil. The smaller the AR, the more challenging it is to release the solder paste, so the stencil must be perfectly aligned and the aperture walls must be smoother. In the past, it was considered that an area ratio of 0.5 to 0.6 was very small, and now it is desired to reduce the area ratio to 0.42.

SMT stencil

For smaller area ratios, in laser cutting, a way to improve the performance of PCB stencils is to electro-polish the stencil, nickel plating, or by progressive electroforming to produce a 100% nickel mesh. Electrolytic polishing and nickel plating smooth the sidewalls of the stencils with uneven surfaces formed by the laser cutting process, however, these processes require more time and have the adverse effect of delaying the delivery time of the stencil. On the other hand, by laser cutting a 100% nickel plate, you can get the smallest laser cut area ratio, but the delivery time is the same as the delivery time for laser direct cut stencils. Some of the better-performing laser cut stencils can have area ratios of 0.48 to 0.50. Also, laser-cut nickel sheets, or laser-cut stencils that are treated after cutting, can vary greatly from one SMT stencil manufacturer to another.

Speaking of area ratios, for all current applications, even an area ratio of 0.48 is not small enough. In these cases, a fully electroformed stencil is the next step in stencil manufacturing. By forming a piece of 100% nickel stencil by electroforming, the holes of the mesh are formed simultaneously in this process. This method yields a much smoother wall of holes. As with other types of stencils, the electroforming capacity and the quality of the stencil will vary using this method. The best electroforming stencil with an area ratio of 0.42 has the best performance, while other methods of making stencils with an area ratio of 0.49 may not perform well for printing.

As electronics continue to miniaturize, for SMT stencil manufacturers, the more important thing for them right now is to increase investment and improve testing capabilities. Obviously, there are many options for stencils. Knowing what kind of stencil is needed for what kind of application and having a deep understanding of it, while trying to keep the cost of the stencil to a minimum and keep delivery fast, is a subject in itself.

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