The main factors affecting the printing of solder paste

cThe main influencing factors of solder paste printing can be summarised as follows:

Tin paste quality:

1. Viscosity: viscosity is an important parameter indicator of solder paste, directly affecting the printing performance. Viscosity is too large if the paste is not easy to wear out the template leakage holes, affecting the printing effect. The main factors affecting viscosity include the percentage content of tin powder, powder particle size, powder spherical surface finish and temperature.

2. Thixotropy: solder paste is a non-Newtonian fluid, when subjected to shear, the viscosity of the paste is rapidly reduced, when the shear force stops, the viscosity of the paste will rise rapidly, has achieved the filling of the stencil openings and release, to achieve the effect of the transfer of solder paste.

3. Adhesion: an important parameter of the solder paste, the adhesion of the paste to ensure bonding of the chip, fixed chip on the pad does not move, as well as in the solidification of the chip will stick. Adhesion and flux in the paste has a direct relationship with the formulation of the flux, solder paste with a high, sustained adhesion is very important.

4. The composition of the tin powder, the composition of the flux and the matching of the tin powder and flux, which play a decisive role in the melting point of the solder paste, printing performance, solderability and solder joint quality. Generally speaking, it is hoped that the alloy composition of tin powder is as close as possible to the eutectic or is the eutectic state. For example, the composition of tin powder must be appropriate, the composition of the flux should be appropriate, the ratio between them should also be well coordinated, so that the paste will perform well in all aspects.

5. Tin powder particle size, shape and particle size distribution of electronic packaging is critical. Size fine particles of tin powder is more suitable for high-density, narrow-pitch products to ensure uniform printing and fine solder joints. Typically, the particle diameter of tin powder is about one-fifth of the stencil opening. Selecting the right type of solder paste is important to achieve a quality solder joint.

Storage of solder paste:

Storage conditions for solder paste, such as temperature and humidity, have a significant impact on its quality. Excessive temperatures can reduce the viscosity of solder paste, and too much humidity can lead to deterioration. Recycled solder paste should be stored separately from freshly made solder paste and, if necessary, used separately.

Stencil stencils:

The quality of the stencil will directly determine the quality of solder paste printing. Parameters such as the thickness and opening size of the stencil need to be confirmed to ensure the printing quality of the solder paste.

Printing equipment:

Printing machine is the key equipment for printing solder paste onto PCB samples, and its type (e.g., manual, semi-automatic, full-automatic) and performance have a significant impact on the process and quality.

Printing method and speed:

1. Printing methods are divided into contact and non-contact, each with its own advantages and disadvantages, need to choose according to actual needs.

2. Printing speed: scraper speed is conducive to the rebound of the stencil, but it will hinder the transfer of solder paste to the pads of the printed circuit board; too slow will cause poor resolution of the solder paste printed on the pads.

Squeegee parameters:

The speed and pressure of the scraper have a direct impact on the printing quality of the solder paste. The faster the scraper speed, the faster the rolling speed of the paste, the less viscous, in favour of the filling of the paste; but the filling time is also shorter. Squeegee pressure should be appropriate, too large may damage the squeegee and stencil, too small may lead to insufficient amount of solder paste.

Other factors:

The setting of printing parameters, such as temperature, humidity, air pressure, etc., will also have an impact on the quality of solder paste printing.

In summary, the main factors affecting solder paste printing include solder paste quality, storage conditions, stencil templates, printing equipment, printing methods and speeds, squeegee parameters and other factors. In practice, these factors need to be considered according to the specific circumstances to ensure the quality and efficiency of solder paste printing.

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