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The Production Process and Specifications of PCB SMT stencil

With the rapid development of electronic products, components have become smaller and smaller, and the requirements for PCB stencil have become higher and higher. According to different production processes, stencil mainly includes chemical etching, laser cutting, and electroforming. The SMT stencil is mainly composed of a screen frame, wire mesh, steel sheet, and bonding glue.

Solder Paste Stencil

1. Materials for Making SMT stencil

a) The Screen Frame

The screen frame is divided into the movable and fixed frame. The movable frame directly installs the steel sheet on the frame, and the frame can be used repeatedly. The fixed frame is covered with glue on the screen frame, and later, it is fixed by glue. The fixed screen frame is easier to obtain uniform steel sheet tension, and the tension is generally 35~48N/cm2. (Normal fixed network allows tension frame 35 Newtons-42 Newtons)

Frame

b) Wire Mesh

The mesh screen frame is used for fixing the steel sheet and net frame and can be divided into stainless steel wire mesh and high molecular polyester net. Stainless steel wire mesh is usually about 100 meshes, which can provide a relatively stable and sufficient tension, but after a long time of use, stainless steel wire mesh is easy to deform and lose tension. The polyester mesh net is often used for an organic matter of 100 mesh, which is not easy to deform and has a long service life.

c) Steel Sheet

Copper sheets, stainless steel sheets, nickel alloys, polyesters, etc. are used to make holes. The template generally adopts a foreign high-quality 301/304 stainless steel sheet, and the foreign steel sheet greatly improves the service life of the template with its excellent mechanical properties.

d) Glue 

The glue is used to stick the screen frame, and the steel sheet has a greater effect on the template. It can be specially used for different customers’ use conditions. This glue can maintain a firm adhesion and can resist various template cleaning agents during complex cleaning.

2. SMT Stencil Production Method

a)  Chemical Etching

Process: 

PCB Data file→Film production→Exposure→Development→Etching→Steel Cleaning→Netting.

Features: 

  • One-time molding
  • Faster speed
  • Lower price

Disadvantages: 

  • Easy to form an hourglass shape (not enough etching) or larger opening size (over etching). 
  • Objective factors (experience, medicament, film) have a large impact, many production links, and large cumulative errors. 
  • It is not suitable for the fine pitch steel mesh production method. 
  • The production process is polluted, which is not conducive to environmental protection.

b)  Laser Cutting Method

Process Flow: 

Film→Coordinates→Data file→Data processing→Laser cutting→Polishing→Encapsulation

Features: 

  • High precision of data production
  • Small influence of objective factors 
  • The trapezoidal opening is good for demoulding. 
  • It can be used for precision cutting.
  • Fair price

Disadvantages: Cutting is one by one, so the production speed is slow.

c) Electroforming Method

Process flow: 

Coating photosensitive film on substrate → Exposure → Developing → Electroforming nickel → Forming → Steel sheet cleaning → Packaging

Features:  Smooth hole wall, especially suitable for 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
  • Price is too high

3. Opening design

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

  1. The width to thickness ratio/area ratio of the opening 
  2. The geometric shape of the sidewall of the opening
  3. The smoothness of the hole wall.

Width to Thickness Ratio: It is the ratio of the opening width to the thickness of the stencil.

Area Ratio: The ratio of the opening area to the cross-sectional area of ​​the hole wall. Generally speaking, to obtain a good demoulding effect, the width to thickness ratio and the area ratio should be greater than 1.5 and 0.66 respectively. Never ignore the process issues, such as continuous tin and multiple tins, to achieve aspect ratio and area ratio.

SMT Stencil Opening Design Tips:

  • Fine-pitch IC/QFP: The best way to prevent stress concentration is to have rounded corners at both ends. The same is true for BGA, 0402, and 0201 with square holes.
  • The anti-tin bead opening method of chip components is best to choose the concave opening method, which can effectively prevent component tombstones.
  • When designing the solder paste stencil, the opening width should at least ensure that the 4 largest solder balls can pass smoothly.

4.   Post-processing

Etching and electroforming SMT solder paste stencils generally do not require post-processing. The circuit board stencil post-processing mentioned here is mainly for laser cutting SMT stencil.

Because the metal slag will adhere to the hole wall and opening after laser cutting, the surface is generally polished. Polishing removes the slag, and it creates friction on the surface. It is done to increase the surface friction for better rolling of the solder paste and to obtain an excellent soldering effect. If necessary, you can also choose “Electropolishing,” which has a very good effect on removing slag (burrs) and improving the hole wall.

Different production processes, raw materials used, and opening design will have an important impact on the quality of the stencil, so it is very necessary to master the production process and specifications of the solder paste stencil.

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