Second, template production
Solder paste printing is the first key process in surface mount technology. Especially for fine pitch assemblies, due to the small device lead size and lead line spacing, solder paste printing requires fine process control, while printed solder paste Printed template is one of the key process equipment.
1. It is important that the manufacturer chooses to choose a template maker.
Prior to the selection, a comprehensive evaluation of the manufacturer’s equipment advancement, delivery cycle, price, and after-sales service should be conducted. At the same time, several key points for template production should be noted:
1 Frame materials: In order to meet the strength requirements and facilitate printing operations, hollow aluminum alloy profiles are often used. The production company should have profile materials that are compatible with users' screen printing machines.
2 stencil printing template materials and processing methods: For solder paste printing with fine-pitch components, the template should use stainless steel foil laser cutting process. The manufacturer should have advanced laser cutting equipment to meet the 0.5mm pitch device. Missing printing requirements and the user's requirements for the largest range of printing missed printing.
3 Tensile technology of formwork frames and formworks is also an important link. Tensile webs should be balanced and the thickness of solder paste printed boards must be uniform. Adhesive viscosity requirements are not affected by the stencil cleaning agent and must not appear after several times of printing.
4 template MARK points require the production of fine, according to the requirements of the screen printing machine can be processed into a thorough or semi-feedback penetration, semi-etched MARK points on the black plastic coating, coating requirements smooth, smooth.
2. The choice of stencil thickness The stencil thickness should be determined according to the minimum pitch of the device on the PCB. The usual empirical number is: Device Specifications stencil thickness 1.27mm QFP 0.25mm-0.3mm 0.65mm QFP 0.18mm-0.2mm 0.5mm QFP 0.12 Mm-0.15mm 3. Selection of template opening size: In order to control solder balls or bridging and other quality problems during the welding process, the size of the template opening is usually slightly smaller than the pad pattern size, especially for fine pitch devices below 0.5 mm. In this case, the width of the opening should be reduced by 15% to 20% from the width of the corresponding pad, and the lack of solder volume caused by this can be compensated by appropriately increasing the dimension of the pad length.
When the size of the pad cannot be changed when the design has been finalized or because the circuit requires device modification (such as increased device electrode height), if the amount of solder after reflow is found to be insufficient and the climb is not enough, the opening size can be made at the pad length when the template is fabricated. The direction of increase in the appropriate, but doing so because the solder paste beyond the pad printed on the responsibility of the board solder mask, will lead to the occurrence of solder balls around the tip of the device should be used with caution.
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 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.
Third, process control
According to different products, the corresponding printing process parameters are set in the printing program, such as working temperature, working pressure, squeegee speed, template automatic cleaning cycle, etc. At the same time, it is necessary to establish strict process management and process specification.
1. Solder paste is strictly used according to the specified brand within the validity period. Solder paste is kept in the refrigerator on weekdays. It must be kept at room temperature for more than 6 hours before use. Afterwards, the lid can be opened and used, and the used solder paste can be stored separately. Quality is qualified.
2. Before production, the operator uses a special stainless steel rod to stir the solder paste to make it uniform, and periodically uses a viscosity tester to measure the viscosity of the solder paste.
3. After printing the first piece of printing on-the-spot printing or equipment adjustment, use the solder paste thickness tester to measure the thickness of the solder paste. The test points shall be selected on the test surface of the printed board, such as up and down, left and right, and 5 points in the middle. Record the value. The thickness of the solder paste is required to be between -10% of the template thickness and -15% of the template thickness.
4. In the production process, 100% inspection of solder paste printing quality shall be conducted. The main content is whether the solder paste pattern is complete, whether the thickness is uniform and whether the solder paste is tipped.
5. Clean the template according to the process requirements after the shift is completed.
6. After the printing experiment or printing fails, the solder paste on the printed board requires thorough cleaning and drying with an ultrasonic cleaning device to prevent the occurrence of solder balls after reflow due to residual solder paste on the board.
Conclusion
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. Through practice, through the full control of solder paste printing in the production process, it can ensure that the product obtains good welding quality and is conducive to tracking and analysis of quality problems.
Solder paste printing is the first key process in surface mount technology. Especially for fine pitch assemblies, due to the small device lead size and lead line spacing, solder paste printing requires fine process control, while printed solder paste Printed template is one of the key process equipment.
1. It is important that the manufacturer chooses to choose a template maker.
Prior to the selection, a comprehensive evaluation of the manufacturer’s equipment advancement, delivery cycle, price, and after-sales service should be conducted. At the same time, several key points for template production should be noted:
1 Frame materials: In order to meet the strength requirements and facilitate printing operations, hollow aluminum alloy profiles are often used. The production company should have profile materials that are compatible with users' screen printing machines.
2 stencil printing template materials and processing methods: For solder paste printing with fine-pitch components, the template should use stainless steel foil laser cutting process. The manufacturer should have advanced laser cutting equipment to meet the 0.5mm pitch device. Missing printing requirements and the user's requirements for the largest range of printing missed printing.
3 Tensile technology of formwork frames and formworks is also an important link. Tensile webs should be balanced and the thickness of solder paste printed boards must be uniform. Adhesive viscosity requirements are not affected by the stencil cleaning agent and must not appear after several times of printing.
4 template MARK points require the production of fine, according to the requirements of the screen printing machine can be processed into a thorough or semi-feedback penetration, semi-etched MARK points on the black plastic coating, coating requirements smooth, smooth.
2. The choice of stencil thickness The stencil thickness should be determined according to the minimum pitch of the device on the PCB. The usual empirical number is: Device Specifications stencil thickness 1.27mm QFP 0.25mm-0.3mm 0.65mm QFP 0.18mm-0.2mm 0.5mm QFP 0.12 Mm-0.15mm 3. Selection of template opening size: In order to control solder balls or bridging and other quality problems during the welding process, the size of the template opening is usually slightly smaller than the pad pattern size, especially for fine pitch devices below 0.5 mm. In this case, the width of the opening should be reduced by 15% to 20% from the width of the corresponding pad, and the lack of solder volume caused by this can be compensated by appropriately increasing the dimension of the pad length.
When the size of the pad cannot be changed when the design has been finalized or because the circuit requires device modification (such as increased device electrode height), if the amount of solder after reflow is found to be insufficient and the climb is not enough, the opening size can be made at the pad length when the template is fabricated. The direction of increase in the appropriate, but doing so because the solder paste beyond the pad printed on the responsibility of the board solder mask, will lead to the occurrence of solder balls around the tip of the device should be used with caution.
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 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.
Third, process control
According to different products, the corresponding printing process parameters are set in the printing program, such as working temperature, working pressure, squeegee speed, template automatic cleaning cycle, etc. At the same time, it is necessary to establish strict process management and process specification.
1. Solder paste is strictly used according to the specified brand within the validity period. Solder paste is kept in the refrigerator on weekdays. It must be kept at room temperature for more than 6 hours before use. Afterwards, the lid can be opened and used, and the used solder paste can be stored separately. Quality is qualified.
2. Before production, the operator uses a special stainless steel rod to stir the solder paste to make it uniform, and periodically uses a viscosity tester to measure the viscosity of the solder paste.
3. After printing the first piece of printing on-the-spot printing or equipment adjustment, use the solder paste thickness tester to measure the thickness of the solder paste. The test points shall be selected on the test surface of the printed board, such as up and down, left and right, and 5 points in the middle. Record the value. The thickness of the solder paste is required to be between -10% of the template thickness and -15% of the template thickness.
4. In the production process, 100% inspection of solder paste printing quality shall be conducted. The main content is whether the solder paste pattern is complete, whether the thickness is uniform and whether the solder paste is tipped.
5. Clean the template according to the process requirements after the shift is completed.
6. After the printing experiment or printing fails, the solder paste on the printed board requires thorough cleaning and drying with an ultrasonic cleaning device to prevent the occurrence of solder balls after reflow due to residual solder paste on the board.
Conclusion
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. Through practice, through the full control of solder paste printing in the production process, it can ensure that the product obtains good welding quality and is conducive to tracking and analysis of quality problems.
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