reserch shows:

Volume ratio> 0.66, solder paste release volume percentage> 80%

The volume ratio is less than 0.5, and the volume percentage of solder paste released is less than 60%

Three factors that affect the ability of solder paste to remove the film

Area ratio/width-to-thickness ratio, the geometry of the side wall of the opening, and the finish of the hole wall

The size [width (W) and length (L)] and template thickness (T) determine the volume of solder paste

Ideally, after the solder paste is released from the hole wall (demolition), a complete tin brick (solder paste pattern) is formed on the pad

 

a. Opening ratio

In order to increase the "process window" and reduce PCB and stencil manufacturing errors that may cause printing offset and other defects, generally the stencil opening will be smaller than the size of the pad on the PCB. For discrete components, the four sides will be retracted by 5%-10%, and there will be "V"-shaped openings on the inside. For IC, the shrinking method is internal cutting or side cutting, and the side cutting method is generally used.

 

b. Hole wall shape/roughness

 

The more common processing methods for steel mesh openings are photochemical corrosion and laser cutting.

Generally speaking, the photochemical corrosion is double-sided corrosion. Due to the manufacturing process, a convex shape is formed in the middle of the inner wall, which will affect the detachment of the solder paste and the generation of printing burrs.

 

 

Laser cutting is currently a more common form. Its advantage is that the opening will naturally form a bell mouth shape with a small top and a large bottom, which is conducive to the detachment of the solder paste. The disadvantage is that the inner wall is relatively rough (cutting burrs), which can be improved by plating nickel after cutting, depositing a 7um-12um nickel layer on the sidewall of the opening, or removing the burrs by chemical polishing or electrolytic polishing. Release performance to eliminate printing burrs.

c. Step-by-step processing (half etching)

 

 

Sometimes there are components with a large amount of tin and a small amount of tin on a PCB at the same time. When the thickness of the stencil cannot be selected, a thicker stencil is used to meet the components with a larger amount of tin. The position where the amount of tin is small is half-etched, that is, the thin steel mesh is locally etched by chemical corrosion at this position to achieve the purpose of locally reducing the thickness of the steel mesh. At present, the steel mesh made by the company does not have this form.