Solder Paste Printing Process Discussion (2)

Stencil type

Important print quality variables include the accuracy and finish of the template hole wall. It is important to preserve the appropriate aspect ratio of the template width and thickness. The recommended aspect ratio is 1.5. This is important to prevent blocking of the template. In general, if the aspect ratio is less than 1.5, the solder paste will remain in the openings. In addition to the aspect ratio, an area ratio greater than 0.66 (pad area divided by hole wall area) is also recommended as recommended by the IPC-7525 "Template Design Guide." The IPC-7525 can serve as a good starting point for a template design.

The process of making the opening controls the smoothness and accuracy of the opening wall. There are three common processes for making templates: chemical etching, laser cutting, and additive processes.

Chemically etched template

The metal template and the flexible metal template are etched using chemical etching from both sides using two positive patterns. In this process, etching is performed not only in the desired vertical direction but also in the transverse direction. This is called undercutting - the opening is larger than desired, resulting in additional solder deposition. Because 50/50 is etched from both sides, the result is a nearly straight hole wall with a narrow, hourglass-shaped narrowing in the middle.

Because the walls of the electroetched template walls may not be smooth, electropolishing, a micro-etching process, is one way to achieve smooth cell walls. Another way to achieve a smoother hole wall is nickel plating. A polished or smooth surface is good for solder paste release but may cause the solder paste to cross the surface of the stencil without rolling in front of the squeegee. This problem can be avoided by selectively polishing the hole walls instead of the entire template surface. Nickel plating further improves smoothness and printability. However, it reduces the opening and requires a graphic adjustment

Laser-cut template

Laser cutting is another subtractive process, but it does not have undercutting issues. Templates are made directly from Gerber data, so the hole opening accuracy is improved. The data can be adjusted as needed to change the size. Better process control also improves hole opening accuracy. Another advantage of laser cutting templates is that the walls of the holes can be tapered. Chemically etched stencils can also be tapered, and if only etched from one side, the opening size may be too large. The opening of the plate is slightly larger than the larger, tapered opening (0.001" to 0.002" of the squeegee face, creating an angle of about 2°), making release of the paste easier.

Laser cutting can create aperture widths as small as 0.004" with an accuracy of 0.0005", making it ideal for printing ultra-fine-pitch devices. Laser-cut stencils also produce rough edges because the vaporized metal becomes metal dross during cutting. This may cause solder paste to clog. Smoother cell walls can be created by microetching. Laser-cut stencils cannot be fabricated into stepped multi-stage stencils without chemically corroding areas that require thinner areas. The laser cuts each hole one by one, so the cost of the template depends on the number of holes to be cut.

Electroformed template

The third process for making a template is an additive process, most commonly called electroforming. In this process, nickel is deposited on a copper cathode core to form openings. A photo-resist film is laminated on a copper foil (about 0.25" thick). The film is polymerized with ultraviolet light through a light-shielding film with a template pattern. After development, a cathode pattern is created on the center of the copper, and only the template opening is retained. Photoresist is covered and then a template is formed around the photoresist by nickel plating.After the desired stencil thickness is reached, the photoresist is removed from the opening. Electroformed nickel foil is bent from The copper heart is separated - a key process step. Now the foil is ready to be framed and the template is made in other steps.

Electroforming step templates can be made but the cost increases. Due to the close tolerances that can be achieved, the electroformed stencil provides a good seal and reduces solder paste leakage on the underside of the stencil. This means that the frequency of the bottom surface of the template is significantly reduced, reducing the potential for bridging.

in conclusion

Chemical etching and laser cutting are subtraction processes for making templates. The chemical etching process is the oldest and most widely used. Laser cutting is relatively new, and electroformed formwork is the latest fashionable thing.

In order to achieve good printing results, it is necessary to have the correct paste material (viscosity, metal content, maximum powder size, and the lowest possible flux activity), the right tools (printers, stencils, and squeegees) and the right process ( Good combination of positioning, cleaning and rubbing.