What is Steel Mesh?

Stencils, also known as SMT stencils, are SMT-specific molds. 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 location on the empty PCB.

Stencil

Steel mesh (stencils), also known as SMT Stencil, is a

Development of SMT process: SMT stencil (SMT template) is also used in the adhesive process. ^[1]

Steel mesh was originally made of wire mesh, so it was called stencil (

According to the production process of SMT steel mesh, it can be divided into:

The stencil manufacturing process includes chemical etch, laser cutting, and electroform.

1. Chemical etch

Process flow: data file PCB film production exposure development etching steel sheet cleaning Zhang Wang

Features: One-time molding, faster speed; cheaper price.

Disadvantages: easy to form hourglass shape (not enough etching) or larger opening size (over-etching); objective factors (experience, medicament, film) have a great impact, there are many production links, and large cumulative errors are not suitable for fine pitch ; The production process is polluted and not conducive to environmental protection.

2. Laser cutting method

Process flow: film production PCB take coordinates data file data processing laser cutting polishing Zhang Wang

Features: high accuracy of data production, little influence of objective factors; trapezoidal opening is good for demoulding; precision cutting can be done; the price is moderate.

Disadvantages: cutting one by one, the production speed is slower.

3. Electroform

Process flow: coating a photosensitive film on the substrate exposure development electroforming nickel forming steel sheet cleaning Zhang Wang

Features: The hole wall is smooth, especially suitable for the ultra-fine-pitch steel mesh manufacturing method.

Disadvantages: the process is difficult to control, the production process is polluting, and it is not conducive to environmental protection; the production cycle is long and the price is too high.

Stencil opening design should be considered

Etching and electroforming stencils are generally not post-processed. The post-processing of the stencils mentioned here is mainly for laser stencils.

After the laser cutting, metal slag adheres to the wall and the opening, so the surface is generally polished; of course, polishing is not only to remove the slag (burr), but also to roughen the surface of the steel sheet. Increase the surface friction to facilitate the rolling of the solder paste to achieve a good soldering effect.

If necessary, you can also choose "electric polishing" to completely remove the slag (

SMT stencil must be cleaned before, during, after (usually used

SMT stencil is a "delicate" precision mold, so you should pay attention to:

1. Handle gently;

2. The steel mesh should be cleaned (wiped) before use to remove the dirt carried during transportation;

3. Stir the solder paste or red glue evenly to avoid blocking the openings;

4. The printing pressure is adjusted to the best: the scraper can just scrape the solder paste on the steel net (

The following factors will affect the quality of the steel mesh:

1. Production process

Earlier we discussed the manufacturing process of the steel mesh. It can be known that the best process should be electropolishing after laser cutting. Both chemical etching and electroforming have processes that are prone to errors such as Shoufeilin, exposure, and development, and electroforming is also affected by uneven substrates.

2. Materials used

Including screen frame, wire mesh, steel sheet, adhesive, etc. The screen frame must be able to withstand a certain program of relay and have a good level; the screen is best to use a polyester screen, which can maintain the tension and stability for a long time; the steel sheet is best to be 304, and the matte It is more conducive to the rolling of the solder paste (adhesive); the adhesive must be strong enough and resistant to certain corrosion.

3.Opening design

The quality of the opening design has the greatest impact on the quality of the stencil. As discussed earlier, the design of the opening should consider the manufacturing process, width-to-thickness ratio, area ratio, and experience values.

4. Production information

The integrity of the production information will also affect the quality of the stencil. The more complete the information, the better. At the same time, when data coexist, it should be clear which one prevails. Also, in general, making a stencil from a data file can minimize errors.

5. How to use

The correct printing method can maintain the quality of the stencil. Conversely, incorrect printing methods such as excessive pressure, and the stencil or PCB is not level during printing will damage the stencil.

6.Cleaning

Solder paste (adhesive) is relatively easy to cure. If it is not cleaned in time, it will block the stencil opening, and it will be difficult to print next time. Therefore, after the stencil is removed from the machine or the solder paste is not printed on the printer for 1 hour, it should be cleaned in time.

7.Storage

Stencils are applied to specific storage locations and should not be placed randomly. This will prevent accidental injury to the stencil. At the same time, the steel nets should not be stacked together, so that it is difficult to hold and may bend the net frame.