Screen printing of solder paste

The advantage of screen printing over stencil printing is its lower cost, and it can print larger areas with the disadvantage of limited printing accuracy and limited thickness of the coating. In the screen printing process, the solder paste rolls over the mesh, and a layer of film is applied to the circuit board where solder paste is not needed near the mesh. The wire mesh may be stainless steel, metal treated polyester, polyester or nylon. The wire mesh is tensioned on a hard metal frame (mostly aluminum) and the metal frame is placed in the frame of the screen printing equipment. Above.

The scraper pushes the solder paste through the hole in the mesh, and at the same time presses the wire mesh to make the solder paste contact with the circuit board. After the scraper passes, the mesh rebounds away from the circuit board, and the circuit board opens at the opening of the screen. The solder paste is left behind, and the separated particles in the solder paste flow together to form a uniform film. Screen printing technology requires highly skilled and well-trained operators to ensure that the displacement of the solder paste relative to the placement is less than 0.15 mm.

1. Classification of wire mesh fabrics

In addition to sorting according to the mesh fabric material used, it can also be classified according to the number of meshes, that is, according to the number of wires (openings) per centimeter, for example, "55-T" means 55 per centimeter. Screen printed fabric, or equivalent to 140 screen printed fabrics per inch. The materials used in the screen include:

1) Stainless steel: It has a long service life and provides excellent aligning and solder paste flow, which avoids the accumulation of static charge and requires a lower overhead height.

2) Metal-treated polyester: It has high abrasion and corrosion resistance, allows good solder paste flow, and the metal surface prevents static charge accumulation.

3) Polyester: It is more elastic and cheaper than stainless steel. If the board is not welded properly, it needs more elastic mesh material.

4) Nylon: It has excellent elasticity.

The number of threads that make up the mesh should be approximately 30 per cm. The minimum distance between the wires should be three times the diameter of the largest solder particles in the solder paste. Table 13-3 shows some options for solder paste printing.

Only a hard frame is used to achieve the desired mesh tension for precise printing, and aluminum is the most commonly used metal for screen printing frames. Steel frames are rarely used because they are prone to rust, and stainless steel frames weigh almost three times as much as aluminum for frames of the same size. However, one must sometimes consider the fact that the linear thermal expansion coefficient of aluminum is as high as twice that of stainless steel.

For example: stainless steel: 1m expansion of 0.06mm when the temperature rises 5 °C;

Aluminum: 1m expansion of 0.13mm when the temperature rises by 5 °C.

The size of the frame should be large enough that the outer edge of the image and the inner edge of the frame are at least 150 mm between all four sides. If the distance is too small, the adjustment will be more difficult, as the overhead height increases and the blade As the distance between the frames decreases, the distortion increases.

3. Overhead height and frame lift off

Overhead height refers to the distance between the screen or stencil and the surface of the board. Higher fabric tensions allow for lower overhead heights. The additional lift of the frame at the edge when the squeegee begins to print is referred to as frame lift. This uplift is provided by advanced printing equipment during the squeegee movement, with this increased height to ensure the same overhead height. Similarly, the angle between the board and the screen will also be maintained by moving the blade over the screen. Typically, the height setting should be as low as possible. With high tension fabrics, one can get a lower overhead. height.

4. Stretching of the fabric

Screen-printed fabrics can be stretched by air or mechanical means. Newly stretched fabrics typically lose 10% - 20% of tension within 1 h of stretching, depending on the type of fabric, stretching equipment, frame The stability and the remaining time before the fabric on the frame is glued are slightly different. Therefore, for more precise work, it is recommended to leave a sleep time of 12 h before the screen molding is performed after the screen stretching. The tensile strength of the fabric depends on the material used and the diameter of the wire, which is proportional to the square of the wire diameter.

5. Remove grease

It is often recommended to degrease the plate before each use, either by commercial silkscreen degreaser or with a 20% caustic pot, but after rinsing, neutralize with 5% acetic acid. .

6. Printing speed

Typical printing speeds are 20 - 80 mm per second, depending on the thixotropy of the solder paste used, which is usually recommended by the solder paste manufacturer. As the solder paste moves and rolls over the screen, the more liquid in the solder paste, the greater the printing speed that can be achieved. At the beginning of printing, the solder paste should have a printing speed of at least 90 mm/s before the solder paste contacts the mesh aperture to achieve the thixotropic properties of the solder paste. The number of meshes refers to the number of openings or the number of wires per inch, and the number of meshes used for solder paste printing is typically between 60 and 200. Typical maximum solder particles should not exceed 113 of the mesh to prevent clogging. For example, an 80 mesh screen has an opening of about 224 μm, so the size of the solder particles should not exceed 75 m. A mesh with a fine mesh can be used for fine solder paste deposition, a mesh with 180 meshes uses 80 meshes, and a fine solder paste powder can be used to produce a deposition thickness of 100 - 150 μm. Hall (1994) proposed the need for screen printers to achieve low defect rates, and Noble and Moore (1992) described methods for determining the accuracy of screen printer printing.