Manufacturing process and material selection of SMT steel mesh

What is the role of SMT steel mesh?
Stencils, also known as SMT Stencil, is a special mould for SMT. Its main function is to help the deposition of solder paste; The purpose is to transfer the exact amount of solder paste to the exact position on the empty PCB.

Steel mesh
1、 Formwork material
1. Net frame
The mesh frame is divided into movable mesh and fixed mesh frame. The movable mesh frame directly installs the steel sheet on the frame. A mesh frame can be used repeatedly. Fixed screen frame is to glue the screen yarn onto the screen frame, and the latter is fixed by glue. It is easy to obtain uniform steel sheet tension for fixed mesh frame, and the tension is generally 35~48N/cm2. (The allowable tension of the normal fixed mesh frame is 35 Newtons – 42 Newtons. The fixed mesh frame is adopted by Jialichuang Company, and the normal tension is 40 Newtons.)

2. Gauze
Mesh is used to fix steel sheet and mesh frame, which can be divided into stainless steel wire mesh and polymer polyester mesh. Stainless steel wire mesh is usually about 100 mesh, which can provide relatively stable and sufficient tension. However, the stainless steel wire mesh is easy to deform and lose tension after being used for a long time. The organic matter of the polyester-net fly is usually 100 mesh, which is not easy to deform. Long service life.

3. Flake
That is, copper sheet, stainless steel sheet, nickel alloy, polyester, etc. The formwork of Jialichuang Technology is uniformly made of high-quality 304 stainless steel sheet from the United States, which greatly improves the service life of the formwork with its excellent mechanical properties.

4. Glue
The glue used to paste the mesh frame and steel sheet plays a large role in the formwork.

2、 Etching template
The metal template and flexible metal template are etched by using two positive patterns through chemical grinding from both sides. In this process, etching is not only carried out in the desired vertical direction, but also in the horizontal direction. This is called undercut, and the hole is slightly larger than expected. Because 50/50 is etched from both sides, the result is an almost straight hole wall with a slight hourglass narrowing in the middle.
Because the hole wall of the electroetching template may not be smooth. Electropolishing, a micro-etching process for hole wall treatment in the later process, is a method to achieve smooth hole wall. Another method to achieve a smoother hole wall is nickel plating. The smooth surface after polishing is good for the release of solder paste, but it may cause the solder paste to cross the template surface instead of rolling in front of the scraper. This problem can be solved by selectively polishing the hole wall instead of treating the entire template surface. Nickel plating further improves smoothness and printing performance.

3、 Laser cutting template
Laser cutting is a subtraction process, but it has no undercut problem. The template is directly made from Gerber data, so the accuracy of opening is improved. The data can be adjusted as needed to change the size, and better process control will also improve the opening accuracy. The verticality of the hole wall of the laser cutting template. The template cut by laser will produce rough edges. Because the vaporized metal becomes metal slag during cutting. This may cause solder paste blockage. The smoother hole wall can be treated after electropolishing. The template cut by laser can not be made into step-type multi-stage template without chemical corrosion on the area that needs to be thinner in advance.

4、 Electropolished formwork
Polishing is an electrolytic post-treatment process that “polishes” the hole wall, resulting in reduced surface friction, good solder paste release and reduced cavities. It can also greatly reduce the cleaning of the bottom surface of the formwork. Electropolishing is achieved by connecting the metal foil to the electrode and immersing it in the acid solution for reaction. The current causes the etchant to attack the rougher surface of the hole first, and the effect on the hole wall is greater than that on the top and bottom of the metal platinum, resulting in a “polishing” effect. Then, remove the metal platinum before the corrosive agent acts on the top and bottom surfaces. In this way, the hole wall surface is polished. Therefore, the solder paste will be effectively rolled (not pushed and filled) on the surface of the template by the scraper.

5、 Electroforming formwork
The third process for making templates is an addition process, which is most commonly called electroforming. In this process, nickel is deposited on the copper cathode core to form an opening. A photosensitive Ganxiao film is laminated on the copper foil with a thickness of about 0.25. The film is polymerized by ultraviolet light through a light-shielded film with a template pattern. The cathode pattern is generated on the copper core after development. Only the template opening shall be covered with photoresist. Then the template is formed by nickel plating around the photoresist. After reaching the desired template thickness, the photoresist is removed from the opening, and the nickel foil formed by electroforming is separated from the copper core by bending. Electroforming has unique sealing characteristics, reducing the need of tin bridge to clean the bottom surface of the template. This process provides nearly perfect positioning, without geometric constraints, with internal trapezoidal smooth hole wall and low surface friction to facilitate the release of solder paste. The process is as follows: develop photoresist on a substrate (or core mold) to form an opening, and then plate the template around the photoresist atomically and layer by layer. The nickel atom is deflected by the photoresist to produce a trapezoidal structure. Then, when the template is removed from the substrate, the top surface becomes the contact surface to produce a sealing effect. Continuous nickel thickness in the range of 0.001 to 0.012/can be selected. This process is suitable for ultra-dense spacing. For example, 0.008-0.16 or other applications. It can reach 1:1 aspect ratio.

6、 Characteristics of electroforming template
1. The use of nickel materials with low surface adhesion is conducive to the release of solder paste.
2. The formwork surface and conical hole wall are easy to control to facilitate the rolling and demoulding of welding balls.
3. The extremely high position accuracy and the hole opening error at the electrode bottom are especially suitable for ultra-fine spacing pads.
4. The hole wall is smooth, and there is no need for post-burr processing.
5. The hardness is 30% higher than that of stainless steel plate, and the service life can reach more than 500000 times.
6. There is no tin ball in the electroforming plate, which greatly reduces the time and frequency of screen plate cleaning.
7. Nickel hardness>500VH.
8. The minimum opening size is 1mil.
9. Hole size tolerance ± 0.1mil.
10. The check error of opening position is ± 0.1mil.
7、 Cleaning of formwork

Template cleaning has played an increasingly important role in surface mounting and through-hole technology. The parts with dense spacing and ultra-dense spacing, together with other advanced packaging, bring new important requirements to the template cleaning. In order to achieve continuous high quality and precision in the process of printing dense spacing, there must be no solder paste residue on the template.

8、 Cleaning agent requires that the cleaning agent must be practical, effective and safe for the working environment of workers. They must be able to remove all kinds of solder paste and flux residues, uncured adhesive and other impurities on both sides of misprinted assembly A – and B0. The formwork frame must be suitable for cleaning conditions, such as temperature, time, mechanical energy and cleaning chemicals. The frame is composed of ester fiber, which is laminated onto the frame through epoxy resin. Temperatures higher than 130 ℃ will cause the resin layer to soften and cause template defects. In addition, the temperature expansion coefficient between polyester fibers of aluminum frame and stainless steel sheet may cause the deformation of closely spaced openings if subjected to long-term high-temperature cleaning process.

9、 Wiping under the formwork
The effective bath agent is a solvent that can dissolve the flux and adhesive in the solder paste and has a flash point higher than 110 ℃. The solvent bar applies a certain amount of solvent on the whole paper width. It is important to match the characteristics of paper and solvent to reduce the absorption of solvent on paper and the consumption of solvent. Once the solvent is applied, the vacuum system helps remove the residual solder paste from the opening of the template. The wiping frequency is generally determined by the following factors: template type, solder paste, coplanarity of PCB substrate and printer setting. The closely spaced and high-density formwork provides a smooth surface after most electric polishing, but the bottom wiping is required to maintain a high qualification rate. The cleanliness of the formwork is key to the ball planting process. Sticky solder paste containing small particles and small template openings can reduce the transfer rate of extruded solder paste. After a printing stroke, the inner layer of the template opening may accumulate a lot of solder paste residues, which may soon dry and pollute the solder paste deposits in the following printing stroke.
For this reason, it is recommended to thoroughly clean the template between each printing. It is recommended to wipe the bottom of the template with a cloth and solvent without burrs.

10、 Common methods of formwork cleaning
The lint-free rag can be used to remove most stains with a pre-soaked lint-free rag and cleaning solvent. The rag can remove the uncured solder paste and adhesive relatively easily and quickly. Its advantages are low cost, quantitative application of solvent, and easy recovery and utilization. As the pin spacing becomes more dense, the printing quality must be required. The lint-free pre-soaked rag cannot continuously remove the solder paste or adhesive from the closely spaced holes. If the solder paste is dried and filled into the opening before reusing the template, the positioning of the plate will be poor.

Soak. Ultrasonic agitation and water cleaner are feasible for cleaning ultra-fine spacing formwork and misaligned formwork. The impact energy must effectively remove dirt from the etching area of the closely-spaced template with holes using cleaning solvent. Water-soluble detergent can be used at low concentration and low temperature to prevent formwork delamination and expansion.

Air spray formwork cleaning system is designed for solvent, semi aqueous and all aqueous chemical cleaning agents. These systems usually use a single container for washing and flushing. Use a rotating bar to conduct spray impact on the template or assembly surface. The air spray system carries solder balls to filter through the subsystem to prevent re precipitation.

Selection of chemical cleaning agent, such as VOC cleaning agent, which uses the cleaning agent strengthened by inorganic detergent. The recommended concentration is 3-10%. These cleaning agents are effective for most uncured solder paste. This cleaning agent technology moistens the solder paste, dissolves the resin adhesive into the cleaning solvent, and removes the solder ball from the surface. These solvents can work at room temperature of 25 ℃.

The bottom surface SMT of the template with dense spacing and ultra-dense spacing shall be cleaned during the process to prevent a small amount of solder paste from drying and accumulating residues around the opening. These residues can be removed by using lint-free paper rolls and specially designed solvents.

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