Factors Affecting Quality in Solder Paste Printing (I)

Abstract: Solder paste printing is one of the key processes in SMT production. Its control directly affects the quality of the assembled board. This article describes many factors that affect the quality of solder paste printing and analyzes the causes. Some corrective measures and recommendations are also proposed.

Keywords: solder paste; template; printing; surface mount technology; defect

With the rapid development of surface mount technology, in the production process, the influence and role of solder paste printing on the entire production process are increasingly valued by production engineers and process engineers. Grasp and use the solder paste printing technology, analyze the cause of the problem, and apply the improvement measures in the production practice to obtain better solder paste printing quality, which is everyone's dream. This article will discuss some factors that affect the quality of solder paste printing.

1 Solder paste factors

Solder paste is much more complex than pure tin-lead alloys. The main components are as follows: solder alloy particles, fluxes, rheological modifiers/viscosity control agents, and solvents. Different types of solder pastes have different compositions and different application ranges. Therefore, you must be careful when selecting solder pastes, and really grasp relevant factors to ensure good quality. The following factors must be taken into consideration when selecting the solder paste:

1.1 Viscosity of solder paste

The viscosity of the solder paste is an important factor affecting the printing performance. The viscosity is too high. The solder paste is not easy to pass through the opening of the stencil. The printed lines are incomplete, the viscosity is too low, and they easily flow and collapse, affecting the print resolution and lines. The flatness.

The viscosity of the solder paste can be measured with an accurate viscometer. However, the following methods can be used in practice: Stir the solder paste with a spatula for about 3-5 minutes. Then use a spatula to stir up a little of the solder paste so that the solder paste naturally falls. If the solder paste is slow, Slowly step by step, indicating that the viscosity is moderate; if the solder paste does not slide off at all, then the viscosity is too large; if the solder paste constantly slides down at a faster rate, the solder paste is too thin and the viscosity is too low.

1.2 Tackiness of Solder Paste

The stickiness of the solder paste is insufficient, and the solder paste does not roll on the template during printing. The direct result is that the solder paste cannot completely fill the opening of the template, resulting in insufficient deposition of the solder paste. If the viscosity of the solder paste is too high, the solder paste will hang on the wall of the template hole and not all of the solder paste will be printed on the pad.

The sticky selection of solder paste generally requires that the self-adhesive capacity is greater than its bonding ability with the template, and its adhesion to the template hole wall is less than its bonding ability with the pad.

1.3 Uniformity and Size of Solder Paste Particles

Solder paste solder particle shape, diameter size and uniformity also affect the printing performance, the general solder particle diameter is about 1/5 of the template opening size, for fine pitch 0.5mm pad, the template opening size in 0.25mm, the maximum diameter of the solder particles does not exceed 0.05mm, otherwise it is easy to cause blockage during printing. The relationship between the specific pin pitch and solder particles is shown in Table 1. Usually fine-grained solder paste will have better solder paste resolution, but it is prone to edge collapse, while being oxidized and have high chances. Pin spacing is usually one of the important selection factors, taking into account both performance and price.

Table 1 Pin pitch and solder particle relationship

Pin spacing (mm)

0.8 or more

0.65

0.5

0.4

Particle diameter (um)

75 or more

60 or less

Less than 50

40 or less

1.4 Metal Content of Solder Paste

The content of metal in the solder paste determines the thickness of the solder after soldering. As the percentage of metal content increases, the solder thickness also increases. However, with a given viscosity, as the metal content increases, the tendency of the solder to bridge increases.

Table 2 Effect of Metal Content on Reflow Thickness When Solder Paste Has a Certain Thickness

Metal content%

Thickness (inches)

Wet solder paste

Reflow

90

0.009

0.0045

85

0.009

0.0035

80

0.009

0.0025

75

0.009

0.0020

After reflow, the device pins are required to be soldered securely, with a full and smooth solder fillet and a 1/3 to 2/3 height climb in the height direction of the tip of the device (resistance container). From the above table, it can be seen that as the content of metal decreases, the thickness of solder after reflow is reduced. In order to meet the solder paste amount requirements for solder joints, solder paste of 85%--92% metal content is generally used. Manufacturers generally control the metal content at 89% or 90%, the better the effect.