1. There are two types of scrapers: rhombus and drag skirt, drag skirt is divided into polyethylene (or similar) material and metal.

1) Rhombus

This form is very uncommon now, although it is still used, especially in the United States and Japan. It consists of squares with a cross-section of approximately 10mmx10mm, clamped by splints, forming an angle of 45° on both sides:

This squeegee works in both directions, skipping strips of solder paste at the end of each stroke, so only one squeegee is needed. However, this is easy to get dirty because the solder paste will run up instead of just staying in the little exposed part of the polyethylene. Insufficient flexibility means that it cannot fit a distorted PCB, which may cause missed print areas. Not adjustable.

2) Drag skirt

This form is very common, consisting of polyethylene with a rectangular cross-section, supported by a splint, and requires two scrapers, one in the direction of the silk screen stroke. No need to skip solder paste strips, since the solder paste is between the two scrapers, the angle of each stroke can be determined individually.

About 40mm scraper is exposed, and the solder paste only goes up 15~20mm, so this form is cleaner.

The scraper is distinguished by hardness range and color code, for example:

60~65shorevery soft red

70~75shoresoft green

80~85shorehard blue

90 + shorevery hard white

Before use, the scraper must be adjusted so that its leading edge is in a straight line and parallel, first check whether its edge is in a straight line, if not, adjust the fixing screw of the splint.

2. Scraper function

Compared with the metal laser stencil, the dynamic requirements of the scraper are different for the latex mesh.

On the latex screen, the squeegee needs to push the solder paste in front of it, pump the solder paste through the screen and print it to the screen printing area. To achieve this effect, a soft squeegee (70~75shore, green) needs to be used. Deformation occurs where it comes into contact with the wire mesh.

Even softer squeegees (60~65shore, red) can be used to screen print inks on thick hybrid ceramic substrates.

When using the laser stencil, the scraper rolls the solder paste in front, and it can flow into the wire hole without pumping, and then scrapes off the excess solder paste, leaving the solder paste as thick as the template on the PCB pad. Deflection of the blade is neither required nor expected, so a harder (ie: 80-85 shore, blue) or metal blade can be used.

The hardness of the squeegee and the pressure must be coordinated. If the pressure is too small, the squeegee will not clean the solder paste on the SMT stencil. If the pressure is too high, or the squeegee is too soft, the squeegee will sink into the larger holes on the SMT stencil. Dig out the solder paste.