The stencil was originally made of wire mesh, so it was called a mask at that time. It started with nylon (polyester) mesh, later due to durability, there were wire meshes, copper meshes, and finally stainless steel meshes. However, no matter what kind of wire mesh is, it has the disadvantages of poor molding and low precision.

With the development of SMT, the requirements for stencils increase, and stencils are produced. Affected by the cost of materials and the difficulty and ease of production procedures, the original steel mesh was made of iron/copper plate, but also because of easy corrosion, stainless steel mesh replaced them, which is the current steel mesh (SMT Stencil).

According to the production process of SMT steel mesh, it can be divided into: laser template, electropolishing template, electroforming template, step template, bonding template, nickel-plating template, and etching template.

 

 

 

 

 

Laser template (LaserStencil)

Laser steel mesh template is currently the most commonly used template in the SMT steel mesh industry. Its characteristics are:

Directly use data files for production, reducing production errors;

The opening position of the SMT template is extremely accurate: the overall error is ≤±4μm;

The opening of the SMT template has a geometric pattern, which is conducive to the printing and forming of the solder paste.

Let's focus on the GERBER file below;

GERBER file is a data format proposed by the American GERBER company; it converts PCB information into electronic data that can be recognized by a variety of light drawing machines, also known as light drawing files. GERBER file is a software structure with X, Y coordinates and additional commands. The official scientific name of GERBER format is "RS274 format". It has become a standard format file in the PCB industry.

The GERBER file combined with the Aperture list (also known as D-Code) file defines the starting point of the graph and the shape and size of the graph. D-Code defines the size and shape of lines, holes, pads or other patterns in the circuit.

There are two types of GERBER files: RS274D and RS274X

RS274D contains X, Y DATA, but does not contain D-Code files;

RS274X contains X, Y DATA, D-Code file is also defined in this file.

 

Electropolishing template (E.P.Stencil)

The electro-polishing template is after laser cutting, through electrochemical methods, the steel sheet is post-processed to improve the opening hole wall. Its characteristics are:

1. The hole wall is smooth, which is especially suitable for ultra-fine pitch QFP/BGA/CSP.

2. Reduce the number of wiping of the SMT template and greatly improve work efficiency.

Electroforming template (E.F.Stencil)

In order to comply with the requirements of short, small, light and thin electronic products, ultra-fine volume (such as 0201) and ultra-fine pitch (such as ūBGA, CSP) are widely used. As a result, the SMT stencil industry also proposes more High requirements, electroforming template came into being.

The electroforming template produced by our company is characterized by: different thicknesses can be made on the same template.

Step template (StepStencil)

Due to the different requirements for the amount of solder paste when soldering various components on the same PCB, the thickness of some areas of the same SMT template is required to be different, which leads to the STEP-DOWN&STEP-UP process template.

STEP-DOWN template

Partially thin the template to reduce the amount of tin when soldering specific components.

Bonding template (BondingStencil)

The COB device has been fixed on the PCB, but the patch process of tin printing is still required, which requires the use of a bonding template.

The bonding template is to add a small cover at the PCB bonding position corresponding to the template to avoid the COB device and achieve the purpose of flat printing.

Nickel-plated template (Ni.P.Stencil)

In order to reduce the friction between the solder paste and the hole wall, facilitate demoulding, and further improve the release effect of the solder paste, at the beginning of 2004, Wei Chuangxin Company added a special post-processing template based on the traditional subtractive process "Electropolishing" template. Processing addition process-"nickel plating", and obtained a patent. The nickel-plated template combines the advantages of the laser template and the electroforming template.

Etching template

It is made of 301 type steel sheet imported from the United States. The etched steel mesh is suitable for PCB printing with corners and spacing greater than or equal to 0.4MM. It is suitable for copying and film use. CAD/CAM and exposure methods can be used at the same time, depending on different parts. For zooming, there is no need to calculate the price based on the number of parts. The production time is fast. The price is cheaper than the laser template. It is convenient for customers to file the film.

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The production process of steel mesh includes: chemical etch, laser cutting, and electroform.

1. Chemical etch

Process flow: data file PCB→film production→exposure→development→etching→steel cleaning→zhang net

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

Disadvantages: easy to form an hourglass shape (not enough etching) or larger opening size (excessive etching); objective factors (experience, medicine, film) have a large impact, many production links, large cumulative errors, not suitable for fine pitch steel mesh production method ; The production process is polluted, which is not conducive to environmental protection.

2. Laser cutting

Technological process: film making PCB→acquisition of coordinates→data file→data processing→laser cutting→polishing→zhang net

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

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

3. Electroform

Process flow: coating photosensitive film on the substrate→exposure→development→electroforming nickel→forming→steel cleaning→screening

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

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

The opening design of the stencil should consider the releasability of the solder paste, which is determined by three factors:

①The width to thickness ratio/area ratio of the opening; ②The geometry of the side wall of the opening; ③The smoothness of the hole wall.

Among the three factors, the latter two are determined by the manufacturing technology of the steel mesh, and we consider the first one more.

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

First, we understand the aspect ratio and area ratio:

Width to thickness ratio: the ratio of the width of the opening to the thickness of the steel mesh.

Area ratio: The ratio of the opening area to the cross-sectional area of ​​the hole wall, as shown in the following figure:

Generally speaking, to obtain a good demolding effect, the width to thickness ratio should be greater than 1.5, and the area ratio should be greater than 0.66.

 

The following factors will affect the quality of steel mesh:

1. Production process

We have discussed the production process of steel mesh before, we can know that the best process should be electropolishing after laser cutting. Both chemical etching and electroforming have processes that are prone to errors such as film making, exposure, and development, and electroforming is also affected by the unevenness of the substrate.

2. Materials used

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

3. Opening design

The quality of the opening design has the greatest impact on the quality of the steel mesh. As discussed earlier, the design of the opening should consider the manufacturing process, the aspect ratio, the area ratio, and the empirical value.

4. Production materials

The completeness of the production data will also affect the quality of the steel mesh. The more information, the better. At the same time, when data coexist, it should be clear which shall prevail. Also, generally speaking, making stencils with data files can reduce errors as much as possible.

5. How to use

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

6. Cleaning

The solder paste (glue) is relatively easy to solidify. If it is not cleaned in time, it will block the opening of the stencil, and the next printing will be difficult. Therefore, after the stencil is removed from the machine or the solder paste is not printed on the printing machine for 1 hour, it should be cleaned in time.

7. Storage

The steel mesh is used in a specific storage place and cannot be placed randomly, so as to avoid accidental damage to the steel mesh. At the same time, the steel mesh should not be stacked together, so that it is difficult to handle and may bend the mesh frame.