Stencil Manufacturing

1, 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, one mesh frame can be used repeatedly. Fixed mesh frame is covered with glue screen yarn glued on the mesh frame, the latter is fixed by glue. Fixed mesh frame is easier to obtain uniform steel sheet tension, tension size is generally 35~48N/cm2. (The allowable tension of the normal fixed mesh frame is 35 Newton-42 Newton)

2, Net

▪ Mesh is used to fix steel sheets and mesh frames. It can be divided into stainless steel mesh and polymer mesh. Stainless steel wire mesh commonly used about 100 mesh, can provide more stable enough tension, but after the use of too long, stainless steel wire deformation loss tension; Polyester netfly organic matter is often used 100 mesh, it is not easy to deformation, long service life.

3, wafer

▪ Copper sheet, stainless steel sheet, nickel alloy, polyester, etc. The template is generally made of foreign high quality 301/304 stainless steel sheet. The foreign steel sheet greatly improves the service life of the template with its excellent mechanical properties.

4, glue

▪ The glue used to paste mesh frame and steel sheet plays a large role in the template, can be used for different customers, specially used glue, this glue can maintain firm adhesion, and can resist the complex cleaning of various template cleaning agents.

Production method

1.chemical etching method

▪ Process flow: data file PCB→ film production → exposure → development → etching → steel sheet cleaning → mesh

▪ Features: one molding, faster speed; The price is cheap.

▪ Disadvantages: Easy to form an hourglass shape (not enough etching) or the opening size becomes larger (over etching); The influence of objective factors (experience, agents, film) is great, the production process is too many, the cumulative error is large, not suitable for fine pitch steel mesh production method; There is pollution in the production process, which is not conducive to environmental protection.

2. laser cutting method

▪ Process: film production of PCB, coordinate, data file, data processing, laser cutting, grinding, mesh

▪ Features: High precision data production, little influence of objective factors; Trapezoidal opening is conducive to demoulding; Can do precision cutting; The price is moderate.

▪ Cons: Cutting one by one, slow production.

3. Electroforming method

▪ Process: substrate coated with photographic film → exposure → development → electroforming nickel → steel sheet cleaning → mesh

▪ Features: Smooth hole wall, especially suitable for ultra-fine spacing steel mesh.

▪ Disadvantages: the process is difficult to control, the production process has pollution, is not conducive to environmental protection; The production cycle is long and the price is high.

Opening Design

▪ The opening design of the steel mesh should consider the release of the solder paste, which is determined by three factors:

(1) the width/area ratio of the opening; ② the geometry of the opening side wall; ③ The finish of the hole wall.

▪ Of the three factors, the latter two are determined by the manufacturing technology of the steel mesh, and the former one is more important to us.

▪ Because laser steel mesh is very cost-effective, so here we focus on the laser steel mesh opening design.

▪ First, we know the width ratio and area ratio: width ratio: the ratio of the opening width to the thickness of the steel mesh. Area ratio: The ratio of the opening area to the cross-sectional area of the hole wall

▪ Generally speaking, for good demoulding results, the width ratio should be greater than 1.5 and the area ratio should be greater than 0.66.

Of course, the steel mesh opening design, can not blindly pursue the width ratio or area ratio and ignore other process problems, such as tin, tin, etc.. In addition, for sheet elements above 0603(1608), we should pay more attention to how to prevent tin beads.

Steel mesh (SMT template) opening design tips:

1, fine spacing IC/QFP, in order to prevent stress concentration, it is best to round both ends; BGA, 0402 and 0201 with square holes are similar.

2, sheet element anti tin bead opening method is the best choice of concave opening method, which can effectively prevent components tombstone.

3. When the steel mesh is designed, the opening width should ensure that at least 4 maximum tin balls can pass smoothly.

Post-Processing

▪ Etching and electrocasting steel mesh generally do not do post-processing, the steel mesh post-processing is mainly for laser steel mesh.

▪ Because the laser cutting will produce metal slag attached to the wall and the opening, so the general surface grinding; Of course, grinding is not only to remove slag (burr), but also to roughen the surface of the steel sheet, increase the surface friction, roll with solder paste, and achieve good tin effect. If necessary, you can also choose "electric polishing" to completely remove the slag (burr) and improve the hole wall.

▪ Different manufacturing processes, raw materials used and opening design will have an important impact on the quality of steel mesh, so it is very necessary to understand the production process and specification of steel mesh.