Squeegee pressure in the printing process
A simple and understandable application note, developed by AIM, indicates selected problems related to the pressing force of the squeegees.
Definition of squeegee pressure
It is a vertically directed compressive force, measured in PSI (pounds per square inch), exerted by sharp squeegees on the surface of the template during the print cycle.
Principle of operation
During the printing cycle, it is necessary to control the pressure force so that it is evenly distributed over the entire length of the squeegee. The pressure is applied to provide the force necessary to push the solder paste across the entire width of the print area through the apertures, filling all holes in the template and simultaneously wiping the template surface leaving only a very thin layer of solder paste.
Problems related to the squeegee pressure
Due to their lower hardness, polymer doctor blades generally require a higher vertical compressive force to provide the desired degree of abrasion to the template surface. There is a problem here: the greater the pressure exerted by the squeegee blade, the more the phenomenon of removing solder paste from the stencil holes increases. There are two basic solutions to this problem: you can try to print at a slower speed to reduce the overall pressure of the squeegee or switch to a harder blade material.
A less known problem with some types of plastic squeegees is that the greater the vertical pressure on the blade becomes, the more the plastic material will deflect (i.e., much more than a metal blade), which alters the rake angle between the blade and the blade. the surface of the template. This can cause problems with spreading the paste and/or filling the holes, as well as with solder leakage.
Another common problem with the use of plastic squeegees is that they wear out very quickly, increasing the level of defects as they deteriorate.
A harder metal doctor blade will require less pressure than a plastic blade and, consequently, it will be much easier to control the deposition of solder paste along the entire length of the blade. In addition, the metal blade doctor blade does not scrape the solder paste out of the apertures, allowing better control of the volume of paste left behind and a better-defined deposit.
Benefits of proper squeegee pressure
The key advantages of proper squeegee pressure are uniform paste height, stable deposit, and reduced wear on squeegees and templates.
To set the correct squeegee pressure for a given process (assuming that the squeegee blade is properly installed and adjusted), perform the following steps:
1. Choose a good, perfectly flat substrate.
2. Place the substrate under the template.
3. Adjust the squeegee pressure so that the excess paste remains on the surface of the template after the printing cycle.
4. After applying the solder paste to the template, complete the printing cycle.
5. Observe the amount of solder paste remaining on the stencil surface
6. Add a little pressure to the squeegees and perform another print cycle.
7. Observe the amount of solder paste remaining on the stencil surface again. It should be slightly less than the first pass.
8. Continue repeating steps 6 and 7 until only a very thin layer of solder paste is left on the surface of the template.
When using polymer squeegees, it is essential to use a material with a sufficiently high hardness to minimize defects in the standard process. Always keep the edges of the polymer squeegee straight and sharp.
Some machine manufacturers offer kits to replace polymer squeegees with metal ones.
Typical squeegee pressure should be between 1 and 1.5 lbs (pounds per inch).
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