SMT surface mount, Stencil is the key to accurate repeat printing of solder paste. Because the solder paste is printed through a steel Stencil. The printed solder paste fixes the parts, and the parts are fixed to the substrate during reflow soldering. The stencil design contains the elements - the thickness, size and shape of the stencil. It is a guarantee to ensure production and reduce the defect rate.
Today, a wide variety of materials and fabrication techniques enable suppliers to design stencils that meet the challenges of subtle slopes, small parts and high-density packaged boards. In addition, stencil technology is now a wide range of imaging materials, and stencil designers have gained a wealth of knowledge and experience. They know how big a hole is and what shape affects the deposit. Steel stencil has two basic functions. One is to ensure that materials such as solder and glue are properly placed on the substrate, and the other is to ensure the size and shape of the deposit.
The production of steel stencil raw materials and steel stencil. The most commonly used material for steel mesh is metal, mainly stainless steel. In recent years, a wide variety of plastics have been used. Steel mesh manufacturing techniques include chemical etching, laser cutting and electroforming. Manufacturers can choose from a wide range of stencil types to meet specific application requirements. In the past, the commonly used method of making stencils was low-cost chemical etching, which was an inverse process that used imaging techniques to determine the pore shape and then simultaneously etched into holes from both sides of the stencil. Without a trapezoidal wall (to facilitate the release of solder paste), the hole on the side of the stencil facing the substrate is slightly larger. Double-sided corrosion can cause knife edges on the wall of the hole, which can also cause “not completely corroded” and “excessive corrosion.” After the steel mesh is corroded, the edge of the hole can be smoothed by “electropolishing”. Chemical corrosion is suitable for large/bulk holes, but it does not meet the requirements of pore sizes below 0.5 mm.
In order to reduce the slope and increase the density of the components, laser cutting has been widely used in the stencil making process. The laser-cut stencil is made directly from the GERBER data model. Reduce imaging steps, which in turn reduces the chance of image registration errors. Computational CNC laser cutting is directly driven by GERBER data to form high precision, repeatable holes. The accuracy of this technique is that the aperture tolerance is plus or minus 0.05 mm, and the aperture tolerance does not change even in large area printing.
Laser-cut holes exhibit characteristic strip marks. In the manufacturing process, by adjusting the intensity of the laser, the surface of the stencil can be formed with a sharp contrast without the need for internal filling. These characteristics contribute to the accuracy of the manufacturing process. The intrinsic nature of laser cutting results in the formation of a trapezoidal section in the bore - that is, a funnel shape, which contributes to the release of solder paste, and one may be concerned that laser cutting leaves characteristic stripes on the holes. Recent laser cutting techniques have been able to reduce the generation of streaks, but when specific applications require smooth hole faces, laser polishing can be followed by chemical polishing and physical polishing to achieve a smooth hole wall.
The size and shape of the laser stencil determines the volume, uniformity and precision of the deposited material on the substrate. Therefore, strict control of the quality of the opening is the key to the successful design of the steel mesh. Especially when a small amount of deposited material requires high precision. The measurement of the area ratio (the ratio of the open area to the surface area of the hole wall) and the apparent ratio (the ratio of the width of the hole to the thickness of the stencil) can be used to determine the approximate size of the hole.
A common rule is that for solder paste release, the area ratio should be greater than 0.66 and the apparent ratio should be greater than 1.5. Design holes follow these rules and it is necessary to consider the advantages of each fabrication process. For example, an apparent ratio of less than 1.5 is a major challenge for chemical corrosion, while laser cutting can be made into a steel mesh with an apparent ratio of 1.1. Very important for stencil designers is the area ratio. It is directly related to the release of solder paste. During the printing process, when the stencil is separated from the substrate, the surface tension indicates whether the solder paste is transferred to the pad printed on the substrate or continues to adhere to the wall of the stencil. When the pad area is larger than 66% of the hole wall area, the possibility of effective transfer of the solder paste is increased. When the ratio is less than 66%, the efficiency of solder paste transfer is lowered, and the printing quality is deteriorated. This level has an effect on the completion of the hole wall. The electropolished laser-cut hole wall during the manufacturing process improves the effect of solder paste transfer. Component pitch and hole density also indicate the choice of proper manufacturing techniques. For applications with a slope of less than 0.5 mm, laser cutting or electroforming can only be selected. These two lasers improve the quality of solder paste printing and precisely mesh the steel mesh. Both technologies have advantages and disadvantages.
The development of stencil design and stencil production technology meets the needs of standard and fine-slope SMT assemblies, and will continue to advance. One of the challenges is to increase the amount of mixing of components while requiring large and small amounts of material to deposit. Steel mesh suppliers should look for new ways to overcome current area ratio rules.