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Design and Quality Control of SMT Steel Mesh

Source: Linchuan Precision  Article author:Linchuan Precision  Time:2019-10-08 10:03  Read: 614

What is SMT steel mesh
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SMT steel mesh, also known as steel plate, its main function is to help the deposition of solder paste, transfer the exact amount of solder paste to the exact position on the bare board. The steel mesh consists of a mesh frame, a wire mesh and a steel sheet. There are a number of holes in the steel sheet that correspond to the locations on the PCB that need to be printed. In use, the PCB is placed under the steel mesh, and the solder paste is printed on the PCB through a small hole in a fixed position on the steel mesh. The less the solder paste is blocked on the steel grid, the more it deposits on the PCB. Therefore, when a bad situation occurs during the printing process, the condition of the steel plate should be checked first. SMT steel mesh can be divided into seven types: laser template, electropolishing template, electroforming template, step template, bonding template, nickel plating template and etching template. The SMT stencil manufacturing process has three methods: chemical etching, laser cutting and electroforming.


SMT steel mesh design and quality control

General technical requirements, stretch net: red rubber + aluminum tape method, in the aluminum frame and glue bonding, must be evenly scraped a layer of protective paint. At the same time, in order to ensure sufficient tension and good flatness of the stencil, it is recommended that the stainless steel plate retain 25mm-50mm from the inner side of the frame.


General technical requirements

1, stretch net

Use red rubber + aluminum tape method, in the aluminum frame and the glue joint, it must be evenly scraped with a protective paint. At the same time, in order to ensure sufficient tension and good flatness of the stencil, it is recommended that the stainless steel plate retain 25mm-50mm from the inner side of the frame.


2, the frame

The frame size is determined by the requirements of the printing press. For example, the DEK265 and MPM UP 3000 models have a frame size of 29ˊ 29ˊ and are made of aluminum alloy. The frame profile is 1.5ˊ 1.5ˊ.


3. Reference point

According to the size and shape provided by the PCB data, it is opened in a 1:1 manner and is semi-transparent on the reverse side of the printing. At the corresponding coordinates, the entire PCB is opened at least two reference points.


4, opening requirements

(1) Position and size ensure high opening accuracy and open in strict accordance with the prescribed opening method.

(2) The size of the independent opening should not be too large, the width should not be larger than 2mm, and the bridge with a pad size larger than 2mm should be 0.4mm in the middle to avoid affecting the strength of the stencil.

(3) The open area must be centered.


5, characters

For the convenience of production, it is recommended to engrave the following characters in the lower left or lower right corner of the stencil: Model; T; Date; stencil production company name.


6, stencil thickness

In order to ensure the amount of solder paste printed and the quality of soldering, the surface of the stencil is smooth and uniform, and the thickness is uniform. The thickness of the stencil is referred to the above table, and the thickness of the stencil should be based on the premise of the finest pitch QFP BGA.

For example, there are 0.5mm QFP and CHIP 0402 components on the PCB, and the thickness of the stencil is 0.12mm;

For example, there are 0.5mmQFP and CHIP 0603 components on the PCB, and the thickness of the stencil is 0.15mm;


Steel mesh opening shape and size requirements

1. General principles

According to the IPC-7525 steel mesh design guideline, in order to ensure that the solder paste can be smoothly released from the stencil opening to the PCB pad, the opening of the stencil mainly depends on three factors:

1,) area ratio / width to thickness ratio area ratio > 0.66

2.) The mesh hole wall is smooth. Especially for QFP and CSP with a pitch of less than 0.5mm, the supplier is required to perform electropolishing during the manufacturing process.

3)) With the printed surface as the top, the opening under the mesh should be 0.01mm or 0.02mm wider than the upper opening, that is, the opening is inverted to facilitate the release of the solder paste, and the number of cleaning of the stencil can be reduced.


Normally, the SMT component has a stencil opening size and shape that is consistent with the pad and opens in a 1:1 manner.


In special cases, some special SMT components have special provisions for the size and shape of the stencil opening.


2, special SMT component stencil opening

CHIP components:

The CHIP component above 0603 is effective in preventing the generation of tin beads.

SOT89 component: due to larger pads and components

The pad pitch is small, and it is easy to cause soldering quality problems such as solder beads.

SOT252 component: Since the SOT252 has a large pad, it is easy to produce tin beads, and the reflow soldering tension causes a large displacement.


IC:

A. For a standard pad design, an IC with a PITCH of = 0.65 mm has an opening width of 90% of the pad width and a constant length.

B. For the standard pad design, PITCH "= 005mm IC, because of its small PITCH, is easy to bridge, the stencil opening mode has the same length direction, the opening width is 0.5PITCH, and the opening width is 0.25mm.


Other situations:

If a pad is too large, usually one side is larger than 4mm and the other side is not less than 2.5mm, in order to prevent the generation of tin beads and the displacement caused by tension, the mesh opening is recommended to be grid line segmentation, and the grid line width is 0.5mm, the grid size is 2mm, and can be equally divided by the pad size.


Printing stencil opening shape and size requirements:

For simple PCB assembly, the glue process is preferred. The dispensing is preferred. The CHIP, MELF, and SOT components are printed through the stencil. The IC uses glue to avoid stenciling. Here, only the CHIP, MELF, SOT printing stencils are recommended for the opening size and opening shape.

1. Two diagonal positioning holes must be opened at the diagonal of the stencil, and the FIDUCIAL MARK point opening is selected.

2. The openings are all long strips.


Testing method

Steel mesh detection method

1) Check the opening and center the stretched net by visual inspection.

2) Check the correctness of the stencil opening through the PCB entity.

3) Use a graduated high power microscope to examine the length and width of the opening of the stencil and the smoothness of the walls of the holes and the surface of the steel sheet.

4) The thickness of the steel sheet is verified by detecting the thickness of the solder paste after printing, that is, the result is verified.


Last article:What is SMT steel net?
Next article:What are the factors that lead to the shortage of solder paste in SMT steel mesh
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Linchuan Precision specializes in the production and development of SMT steel mesh
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producing over 200,000 high-quality steel meshes.
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