Solder Paste Printing Process Discussion (I)

In solder reflow for surface mount assembly, solder paste is used to connect the pins of the surface mount component or the terminals to the pad. There are many variables, such as solder pastes, screen printers, paste application methods, and printing processes. In the process of printing solder paste, the substrate is placed on a work table, mechanically or vacuum-clamped, aligned with a positioning pin or visually. Or a screen or stencil is used for solder paste printing. This article will focus on several key solder paste printing issues such as stencil design and printing process.
Printing process and equipment

In the solder paste printing process, the printer is the key to achieve the desired print quality. Screen printers available today are divided into two main types: laboratory and production. Each category is further categorized because each company wants to obtain different levels of performance from lab and production type presses. For example, a company's research and development department (R&D) uses a lab type to make a product prototype, while production uses another type. Also, production requirements may vary greatly, depending on production. Because laser cutting equipment is impossible to classify, it is best to choose a screen printer that is compatible with the desired application.

In manual or semi-automatic printing presses, the solder paste is manually placed on the stencil/web, with the squeegee at the other end of the stencil. In automatic presses, solder paste is dispensed automatically. During the printing process, the printing squeegee is pressed down on the stencil so that the bottom surface of the stencil contacts the top surface of the circuit board. As the squeegee passes the length of the entire patterned area that is eroded, solder paste is printed onto the pad through openings in the stencil/web.

After the solder paste has been deposited, the screen is snapped off after the squeegee and returned to the place. This interval or distance is set by the equipment design, approximately 0.020" to 0.040". The disengagement distance and the squeegee pressure are two important variables related to the equipment that achieve good print quality.

If not, this process is called on-contact printing. Contact printing is used when using all-metal stencils and squeegees. Off-contact printing is used for flexible wire mesh.

Squeegee type

The wear, pressure, and hardness of the squeegee determine the print quality and should be carefully monitored. For acceptable print quality, the squeegee edges should be sharp and straight. Low squeegee pressure causes omissions and rough edges, while high squeegee pressure or a very soft squeegee will cause smeared printing and may even damage the squeegee and stencil or screen. Excessive pressure also tends to scoop solder paste from wide openings, causing solder fillets to be insufficient.

There are two types of scraper that are common: rubber or polyurethane scrapers and metal scrapers. When a rubber squeegee is used, a squeegee having a hardness of 70-90 durometer is used. When excessive pressure is used, solder paste that penetrates into the bottom of the stencil may cause bridging, requiring frequent bottom wipes. In order to prevent bottom penetration, pad openings must provide a gasketing effect when printing. This depends on the roughness of the template opening wall.

Metal scrapers are also commonly used. With the use of more closely spaced components, the amount of metal scrapers is increasing. They are made of stainless steel or brass and have a flat blade shape with a printing angle of 30 to 45°. Some scrapers are coated with a lubricating material. Because of the lower pressure, they do not scoop solder paste out of the openings, but also because they are metallic, they are not as easily worn as the rubber squeegee and therefore do not require sharpness. They are much more expensive than rubber squeegees and can cause stencil wear.

Different types of squeegee are used in printed circuit assembly (PCA) using standard components and clinch elements. The amount of solder paste required is very different for each type of component. Fine pitch components require much less solder than standard surface mount components. Pad area and thickness control the amount of solder paste.

Some engineers use double-thickness stencils to apply the proper amount of solder paste to dense-footed components and standard surface mount pads. Other engineers use a different approach - they use more economical metal scrapers that do not need to be sharp. The use of a metal spatula can more easily prevent changes in the amount of solder paste deposited, but this method requires a modified template aperture design to prevent excessive solder paste deposition on the fine pitch pad. This method has become more popular in the industry, but the use of double-thickness printed rubber scrapers has not yet disappeared.

(to be continued)