1) Design of printed steel mesh

The design and manufacturing process of printed steel mesh affects the quality of solder paste printing, assembly yield and reliability. Considering the compatibility with current processes, the choice of stencil thickness should take into account the wafer level CSP requirements for solder paste volume, while ensuring that the solder paste content of other components on the board is met. Steel slabs of 6 mil or 5 mil thickness are currently common in the factory, but may not be suitable for wafer level CSPs of 0.4 mm, sometimes requiring a 4 mil thick stencil. Printing on thicker stencils may result in too much tin to bridge, or plugging may result in incomplete printing. Thinner stencil printing is easier to demold, and solder paste transfer efficiency is higher, but it may cause less tin. A suitable opening design, an ideal width to depth (thickness) ratio, or an open area ratio can achieve satisfactory printing results. So, what is the appropriate opening area ratio? We say that the area ratio for achieving higher solder paste transfer efficiency is the optimum open area ratio. The transfer efficiency of the solder paste F = the amount of solder paste actually obtained V / 1 theoretical solder paste amount × 100%. Through the standard deviation of the amount of solder paste, you can know whether the amount of solder paste on each pad is uniform. The smaller the standard deviation, the more uniform the amount of solder paste. Comparing the relationship between the standard deviation of the amount of solder paste and the area ratio of the opening area, we can see in Fig. 2 that the larger the area ratio of the opening area, the smaller the standard deviation of the amount of solder paste, and the better the consistency of printing.

For wafer level CSPs with a ball pitch of 0.4 mm, the following stencil design is recommended:

1 steel mesh thickness of 4 mil, can use 9 mil square hole or 10 mil round hole;

2 The thickness of the steel mesh is 5 mil. It is recommended to use a 10 mil square hole or a 11 mil round hole.

For wafer level CSPs with a ball pitch of 0.5 mm, the following stencil design is recommended:

The thickness of the steel mesh is 4 to 6 mils, and 11 mil square holes are recommended.

2) Production of printed steel mesh

The material of the printed steel mesh is generally stainless steel foil, and other materials, such as nickel, etc., depending on the process used. The opening of the steel mesh hole can be applied by chemical etching, laser cutting, chemical etching and laser cutting, and electroforming. The quality of the openings obtained by these methods is different, and the cost is also large. difference. The quality of the openings obtained by laser and electroforming is good, but the price will be higher. For high-precision applications with dense pitch, such as wafer-level CSP, laser cutting is used, and even electroforming is used. However, in the fabrication of the printed back reference point, a half-etching method is required. After the hole is cut by the laser, it is generally necessary to electrolytically polish the wall of the hole and the surface of the steel mesh to remove burrs and metal slag and some oxides to ensure satisfactory printing quality. This process is very important, the mold release is not good, or the solder paste is pulled, and the plug hole is often related to the rough wall or burr. Sometimes it is necessary to plate nickel on the surface of the steel mesh to prevent wear and improve the release effect.