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SMT Printing Process And Precautions
Apr 27, 2018

1, the graphics alignment


The substrate on the work table is aligned with the stencil so that the substrate land pattern completely coincides with the open pattern of the steel mesh.


2, solder paste input


Too little solder paste is put in, which may lead to poor filling and printing less. Too much solder paste may cause solder paste to fail to form a rolling motion. Solder paste cannot be scraped clean, causing poor mold release; Solder paste for a long time exposed to the air quality of the solder paste is not appropriate, the amount of solder paste to ∮h = 13-23 more appropriate.


In production, the operator inspects the height of the solder paste on the steel wire every half an hour. Every half hour, the solder paste outside the length of the squeegee on the stencil is moved to the front end of the stencil and the solder paste is evenly distributed.

3, scraper and stencil angle


The smaller the angle between the scraper and the stencil, the greater the downward pressure, the easy to inject the solder paste into the mesh, but it is also easy to make the paste squeezed to the underside of the stencil, causing the solder paste to adhere. The angle between the scraper and the stencil is generally 45-60°. At present, most automatic and semi-automatic presses use 60°.


4, blade pressure


Squeegee pressure is also an important factor affecting the print quality. The squeegee pressure actually refers to the depth at which the squeegee descends, the pressure is too small, the squeegee does not adhere to the surface of the stencil, and the excessively low pressure causes a layer of solder paste on the surface of the stencil, which can easily cause printing defects such as printing and bonding.


5, printing gap


The print gap is the distance between the stencil and the PCB, which is related to the amount of solder paste remaining on the PCB after printing.


6, printing speed


Since the squeegee speed is inversely proportional to the paste's viscosity, the PCB has a narrow pitch, high-density pattern, and the speed is slower. If the speed is too fast, the time for the squeegee to pass through the opening of the steel mesh is relatively short, and the solder paste cannot penetrate into the opening sufficiently, which may cause the solder paste to be insufficiently formed or miss printed marks. There is a certain relationship between the printing speed and the squeegee pressure. The ideal squeegee speed and pressure should be just to scrape the solder paste from the surface of the stencil.


7, steel mesh and PCB separation speed


After the solder paste is printed, the instantaneous speed at which the stencil leaves the PCB is the separation speed, which is related to the parameters of the print quality. It is most important in narrow pitch, high density printing. The advanced printing equipment has one or more tiny staying processes when the stencil leaves the solder paste pattern, that is, multi-stage demoulding, so that the best print forming can be ensured.


8, cleaning mode and cleaning frequency


The stencil contamination is mainly caused by the overflow of the solder paste from the opening edge. If it is not cleaned in time, it will contaminate the PCB surface. The residual solder paste around the opening of the steel mesh will become hard, and in severe cases, it will plug the opening of the steel mesh.


Cleaning the bottom of the steel net is also a factor that guarantees the printing quality. The cleaning mode and cleaning frequency should be determined according to the conditions such as solder paste, stencil material, thickness, and opening size.