PCBA Tech - About the connection between PCBA welding and steel mesh

This article mainly elaborates on the association and difference betweenPCBA circuit board welding and steel mesh

The relationship between stencil and SMT patch processing is inseparable. Many SMT patch processing practitioners must have a good grasp of the relationship between PCBA welding and stencil. Today, this topic will give you an explanation of those things about PCBA welding and stencil.

The so-called Stencil is a thin piece of steel. The size of the stencil is usually fixed to match the solder paste printer, but the thickness of the stencil ranges from 0.08mm, 0.10mm, 0.12mm, 0.15mm, 0.18 mm, etc. are used by people as needed.

The purpose of the stencil is to allow solder paste to be printed on the circuit board in the SMT chip processing process. Therefore, there will be many openings on the stencil. When printing the solder paste, apply The solder paste is above the stencil, and the circuit board is placed under the stencil, and then a scraper (usually a scraper, because the solder paste is similar to a toothpaste-like viscous substance) is used to brush the stencil with the solder paste When the solder paste is squeezed, it will flow down from the openings of the stencil and stick to the top of the circuit board. After removing the stencil, you will find that the solder paste has been printed on the circuit board. Simply put, the stencil is like a cover that needs to be prepared when spraying paint, and the solder paste is equivalent to paint. The cover is engraved with the graphics you want, and spraying the paint on the cover will show the desired graphics.

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)

Features: 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.

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.

Features:

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

Reduce the number of wiping SMT templates, greatly improving 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.

Features: 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 to 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 demolding, and further improve the release effect of the solder paste, 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.

Basically, the classification of SMT steel mesh is divided into three categories according to the production and processing methods, that is: chemical etching process, laser cutting process, electroforming process and other processes.

Chemical etching

The pattern is exposed on both sides of the metal foil by coating the anti-corrosion protective agent on the metal foil, positioning the photosensitive tool with a pin, and then using a double-sided process to corrode the metal foil from both sides at the same time to form an opening on the rigid mesh.

Features: one-time molding, faster speed and cheaper price

Disadvantages: It is easy to form an hourglass shape (not enough etching); or the size of the opening becomes larger (over etching); objective factors (experience, medicine, film) have a large influence, there are many production links, and the cumulative error is large, which is not suitable for fine pitch template production; The production process is polluted, which is not conducive to environmental protection.

Comparison of chemical etching (left) and laser cutting (right)

Laser cutting

Generated directly from the customer's original Gerber data, the template production has good position accuracy and reproducibility. Laser technology is the only process that allows current templates to be reworked.

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.

Electroforming

By developing the photoresist on a substrate (or core mold) to be formed with openings, and then electroplating the template around the photoresist atom by atom and layer by layer, an incremental rather than decremental process.

Features: The hole wall is smooth, especially suitable for the production of ultra-fine pitch templates

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.

With the continuous development of PCB, the requirements for reliability are getting higher and higher, and 3D steel mesh will appear.

For the application of step printing on circuit boards or substrates, the VectorGuard3D stencil made of electroformed nickel can not only print two heights at the same time, but also print different high and low surfaces with a difference of up to 3 mm. Prior to this, if components such as power transistors need step printing support, they must be applied after stencil printing. VectorGuard3D eliminates the point coating step, streamlines the printing process, and can effectively improve productivity.