PCBA Tech - What is SMT steel mesh? And what does it do?

Stencils, also known as SMT Stencils, are special molds for SMT. Its main function is to help the deposition of solder paste; the purpose is to transfer an accurate amount of solder paste to an accurate position on the empty PCB.

1. Template material

1. Screen frame

The net frame is divided into movable net and fixed net frame. The movable net frame directly installs the steel sheet on the frame. A net frame can be used repeatedly. To fix the screen frame is to stick the screen yarn on the screen frame with glue, and the latter is fixed by glue. The fixed screen frame is easier to obtain uniform steel sheet tension, and the tension is generally 35~48N/cm2. (The allowable tension of the normal fixed screen frame is 35 Newton-42 Newton.

2. Mesh

The mesh yarn is used to fix the steel sheet and the screen frame, and can be divided into stainless steel wire mesh and polymer polyester mesh. Stainless steel wire mesh is usually about 100 meshes, which can provide stable and sufficient tension. Only after a long time of use, the stainless steel wire mesh is easily deformed and loses its tension. Polyester mesh net fly organic matter is often used 100 mesh, it is not easy to deform. Long service life.

3. Thin slices

That is, copper sheets, stainless steel sheets, nickel alloys, polyesters, etc. used to open holes.Technology's template uniformly uses American high-quality 304 stainless steel sheet, which greatly improves the service life of the template with its excellent mechanical properties.

4. Glue

The glue used to stick the screen frame and the steel sheet has a greater effect in the template.

2. Etching template

The metal template and the flexible metal template are etched by chemical polishing from both sides using two positive patterns. In this process, etching is not only performed in the desired vertical direction but also in the lateral direction. This is called an undercut, and the opening 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-shaped narrowing in the middle.

Because the hole wall of the electro-etching template may not be smooth. Electropolishing, that is, a micro-etching process for post-process hole wall treatment, is a method to smooth the hole wall. Another way to achieve a smoother hole wall is nickel plating. The smooth surface after polishing is good for the release of the solder paste, but it may cause the solder paste to cross the surface of the template without rolling in front of the squeegee. This problem can be achieved by selectively polishing the hole walls instead of processing the entire template surface. Nickel plating further improves smoothness and printing performance.

3. Laser cutting template

Laser cutting is a subtractive process, but it does not have the problem of undercutting. The template is made directly from Gerber data, so the accuracy of the hole opening is improved. The data can be adjusted to change the size as needed, and better process control will also improve the accuracy of the hole opening. The vertical of the hole wall of the laser cutting template. Laser cut templates will produce rough edges. Because the metal vaporized during cutting becomes slag. This may cause blockage of the solder paste. Smoother hole walls can be post-treated by electropolishing. The laser-cut template cannot be made into a stepped multi-level template if the thinner area is not chemically etched in advance.

Four, electro-polishing template

Polishing is an electrolytic post-treatment process that "polishing" the hole wall, resulting in reduced surface friction and good solder paste release and reduced voids. It can also greatly reduce the cleaning of the bottom surface of the template. Electropolishing is achieved by attaching a metal foil to the electrode and immersing it in an acid solution to react. The current causes the corrosive to erode the rougher surface of the hole first, and the effect on the hole wall is greater than the effect on the top and bottom surfaces of the platinum metal, resulting in a "polishing" effect. Then, before the corrosive agent acts on the top and bottom surfaces, the platinum metal is removed. In this way, the surface of the hole wall is polished. Therefore, the solder paste will be effectively rolled on the surface of the template by the scraper (instead of pushing and filling the holes.

Five, electroforming template

The third process for making templates is an additive process, most commonly called electroforming. In this process, nickel is deposited on the copper cathode core to form openings. A photosensitive dry film is laminated on copper foil with a thickness of about 0.25. The film is polymerized by ultraviolet light through a light-shielding film with a template pattern. After development, a cathode pattern is produced on the copper center of mass. Only the openings of the template remain covered with photoresist. Then, a template is formed by nickel plating around the photoresist. After reaching the desired template thickness, the photoresist is removed from the opening, and the electroformed nickel foil is separated from the copper core by bending. Electroforming has unique sealing characteristics, reducing the need for tin bridges to clean the bottom surface of the template. The process has no geometric restrictions, and has an inherent trapezoidal smooth hole wall and low surface friction to facilitate the release of solder paste. The process is as follows: by developing the photoresist on a substrate (or core mold) where the opening is to be formed, and then electroplating the template around the photoresist atom by atom and layer by layer. The nickel atoms are deflected by the photoresist, creating a trapezoidal structure. Then, when the template is removed from the substrate, the top surface becomes the contact surface to produce a sealing effect. The continuous nickel thickness in the range of 0.001-0.012// can be selected. This process is suitable for ultra-dense spacing. For example, 0.008-0.16 or other applications. It can reach an aspect ratio of 1:1.

Six, electroforming template characteristics

1. The surface adhesion of the nickel material template is small, which is good for the solder paste demoulding.

2. The surface of the template and the wall of the tapered hole are easy to control to facilitate the rolling and demolding of the solder balls.

3. The high position accuracy and the hole opening error at the very bottom are especially suitable for ultra-fine pitch pads.

4. The hole wall is smooth without the need for post-burr processing.

5. Compared with stainless steel plate, the hardness is increased by 30%, and the service life can reach more than 500,000 times.

6. The electroformed plate has no tin balls, which greatly reduces the time and frequency of cleaning the stencil.

7. Nickel hardness>500VH.

8. The small opening size is 1mil.

9. The hole size tolerance is ±0.1mil.

10. The core difference of the hole position is ±0.1mil.

Seven, template cleaning

Template cleaning has played an increasingly important role in surface mount and through-hole technology. Fine-pitch and ultra-fine-pitch parts, together with other advanced packaging, bring new and important requirements for template cleaning. In order to achieve a sustainable level of high quality and precision during the printing of fine pitches, there must be no solder paste residue on the template.

8. Cleaning agent requirements The cleaning agent must be practical, effective, and safe for the worker's working environment. They must be able to remove various solder paste and flux residues, uncured glue and other impurities on both sides of the misprinted assembly A- and B0. The template 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 to the frame by epoxy resin. Temperatures over 130°C will cause the resin layer to soften and cause template defects. In addition, if the temperature expansion coefficient between the aluminum frame and the polyester fiber of the stainless steel sheet is subjected to a long-term high-temperature cleaning process, the close-spaced opening may be deformed.

Nine, wipe under the SMT template

An effective bath agent is a solvent that may dissolve the flux and adhesive in the solder paste and has a flash point higher than 110 degree Celsius. The solvent bar applies a certain amount of solvent to the entire width of the paper. It is important that the characteristics of the paper and the solvent are matched to reduce the absorption and consumption of the solvent on the paper. Once the solvent is applied, the vacuum system helps remove residual solder paste from the openings in the template. Wiping frequency is generally determined by the following factors: including template type, solder paste, PCB substrate coplanarity and printer settings. The fine-pitch, high-density templates provide a slippery surface after most electropolishing, but bottom wiping is required to maintain a high pass rate. The cleanliness of the mold SMT board is critical to the ball planting process. Adhesive solder paste containing small particles and tiny template openings can reduce the transfer rate of extruded solder paste. After a printing stroke, a lot of solder paste residues may accumulate in the inner layer of the stencil opening, which may dry quickly and contaminate the solder paste deposits in the printing stroke below.

For this reason, the template is clean between each PCB printing. It is recommended to use a burr-free cloth and solvent to wipe the bottom of the template.