PCBA Tech - SMT printing process technology control points

SMT automatic printing machine

1) Graphic alignment:

Align the optical positioning point (MARK point) of the substrate and the stencil on the worktable through the printer camera, and then fine-tune the X, Y, Θ of the substrate and the stencil to make the substrate land pattern and the stencil opening The graphics completely overlap.

2) The angle between the scraper and the steel mesh:

The smaller the angle between the squeegee and the stencil, the greater the downward pressure, and it is easier to inject the solder paste into the mesh, but it is also easier for the solder paste to be squeezed onto the bottom surface of the stencil, causing the solder paste to stick. Generally 45~60 °. At present, automatic and semi-automatic printing machines mostly use the angle of 60 ° scraper and steel mesh

3) The amount of solder paste input (rolling diameter):

The rolling diameter of the solder paste ∮h ≈13~23mm is more appropriate.

∮h is too small to cause the solder paste to miss printing and the amount of tin is small.

∮h is too large, too much solder paste at a certain printing speed, it is easy to cause the solder paste to fail to form a rolling motion, the solder paste cannot be scraped cleanly, resulting in poor printing and demolding, thick solder paste after printing, etc.; and Excessive solder paste exposed to the air for a long time is detrimental to the quality of the solder paste.

During production, the operator checks the height of the solder paste strips on the screen every half an hour, and every half an hour, the solder paste on the screen beyond the length of the squeegee is moved to the front end of the stencil with a bakelite scraper and the solder paste is evenly distributed. .

4) Scraper pressure:

Squeegee pressure is also an important factor affecting printing quality. The pressure of the squeegee actually refers to the depth of the descent of the squeegee. If the pressure is too small, the squeegee is not close to the surface of the steel mesh, which is equivalent to increasing the printing thickness. In addition, if the pressure is too small, a layer of solder paste will remain on the surface of the stencil, which is likely to cause printing defects such as printing and forming adhesion.

5) Printing speed:

Since the squeegee speed is inversely proportional to the viscosity of the solder paste, the speed will be slower when there are narrow spacing and high-density graphics. If the speed is too fast, the time for the squeegee to pass through the opening of the stencil is relatively too short, and the solder paste cannot fully penetrate into the opening, which may easily cause printing defects such as insufficient solder paste molding or missing printing. There is a certain relationship between printing speed and squeegee pressure. Decreasing the speed is equivalent to increasing the pressure. Appropriately reducing the pressure can increase the printing speed.

The ideal squeegee speed and pressure should be just to scrape the solder paste from the surface of the stencil. The influence of squeegee pressure and speed on printing:

The influence of squeegee pressure and speed on printing

6) Printing gap:

The printing 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.

7) Separation speed of stencil and PCB:

After the solder paste is printed, the instantaneous speed at which the stencil leaves the PCB is the separation speed, which is a parameter related to printing quality, and is the most important in fine-pitch and high-density printing. In advanced printing machines, when the stencil leaves the solder paste pattern, there is 1 (or more) minute staying process, that is, multi-stage demolding, which can ensure the best printing and forming.

Separation speed of stencil and PCB

When the separation speed is too high, the adhesive force of the solder paste decreases, and the cohesion between the solder paste and the pad is small, so that part of the solder paste sticks to the bottom surface of the stencil and the wall of the opening, causing printing defects such as less printing and tin collapse. When the separation speed slows down, the viscosity and cohesive force of the solder paste are large, so that the solder paste is easily separated from the stencil opening wall, and the printing state is good.

8) Cleaning mode and cleaning frequency:

Cleaning the bottom surface of smt stencil is also a factor to ensure printing quality. The cleaning mode and cleaning frequency should be determined according to the solder paste, stencil material, thickness, and opening size. (Set dry cleaning, wet cleaning, one-time reciprocating, wiping speed, etc., the cleaning frequency can refer to the use of steel mesh), the pollution of the steel mesh is mainly caused by the overflow of solder paste from the edge of the opening. If it is not cleaned in time, it will contaminate the PCB surface, and the residual solder paste around the openings of the stencil will become hard, and in severe cases, the openings of the stencil will be blocked.