“Even the best solder paste, equipment, and application methods do not necessarily guarantee adequate results. The user must control the process and equipment variables to achieve good print quality.†Backflow in surface mount assembly In soldering, solder paste is used for the connection of pins or terminals of a surface mount component to a 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.
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.
The chemically etched template 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 other steps of the template are made.
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.
Source: SMT Information Network
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.
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.
The chemically etched template 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 other steps of the template are made.
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.
Source: SMT Information Network
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