PCBA Tech - Method of appraising the quality of SMT solder paste

The solder paste applied in the production and processing of SMT patches should be uniformly weighed, with good consistency, clear graphics, and adjacent graphics as far as possible without adhesion; graphics and pad graphics should be as good as possible; the total area of solder paste on the pad is about 8mg/mm3, the amount of finely spaced components is about 0.5mg/mm3. The solder paste should cover more than 75% of the total land area. The solder paste should be packaged and printed without serious concrete slump, the edges are neat, and the displacement cannot exceed 0.2mm; the spacing of the prefabricated component protective layer pads is fine, and the displacement cannot exceed 0.1 mm; the base steel plate is not allowed to be polluted by the solder paste environment.

The factors that affect the printing quality of SMT solder paste processing include viscosity, printability (rolling, transfer), thixotropy, and room temperature service life. The quality of the patch will affect the printing quality. If the printing performance of the solder paste is not good, in severe cases, the solder paste will only slide on the template. In this case, the solder paste cannot be printed at all.

The viscosity of the solder paste for SMT chip processing is an important factor affecting the printing performance.

The viscosity is too high, the solder paste cannot easily pass through the opening of the template, and the printed lines are incomplete. If the viscosity is too small, it is easy to flow and collapse, which will affect the printing resolution and smoothness of lines. The viscosity of the solder paste can be measured with an accurate viscometer, but in actual work, the following method can be used: stir the solder paste with a spatula for 8-10 minutes, and then stir a small amount of solder paste with a spatula to let the solder paste fall naturally. If the solder paste decreases gradually, the viscosity will be moderate. If the solder paste does not slide at all, the viscosity is too high; if the solder paste slides down at a faster speed, it means that the solder paste is too thin and the viscosity is too small.

The viscosity of the solder paste for SMT chip processing is not enough, and the solder paste will not roll on the template during printing. The direct consequence is that the solder paste cannot completely fill the opening of the template, resulting in insufficient solder paste deposition. If the viscosity of the solder paste is too high, the solder paste will hang on the wall of the template hole and cannot be completely printed on the pad. The adhesive selection of solder paste usually requires its self-adhesive ability to be greater than its adhesion to the template, and its adhesion to the template hole wall is less than its adhesion to the PCB pad.

The shape, diameter and uniformity of the solder particles in the solder paste for SMT chip processing also affect its printing performance. Generally, the diameter of the solder particles is about 1/5 of the size of the template opening. For pads with a pitch of 0.5 mm, the size of the template opening is 0.25 mm, and the larger diameter of the solder particles does not exceed 0.05 mm. Otherwise, it is easy to cause blockage during the PCB printing process. The specific relationship between lead pitch and solder particles. Generally speaking, fine-grain solder paste will have better printing clarity, but it is prone to edge collapse and has a high chance of oxidation. Generally, considering performance and price, the lead pitch is regarded as one of the important selection factors.