Factors Affecting Quality in Solder Paste Printing

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 of the 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.

Pin spacing (mm)
0.8 or more 0.65
0.5
0.4

Particle diameter (um)
75 or more 60 or less 50 or less 40 or less

Table 1 Pin pitch and solder particle relationship

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.

Metal content%
Thickness (inches)

Wet solder paste Reflow soldering
90
0.009
0.0045

85
0.009
0.0035

80
0.009
0.0025

75
0.009
0.0020

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

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.

2 Template Factors

2.1 stencil materials and engraving

Two methods, chemical etching and laser cutting, are commonly used. For high-precision screens, laser cutting methods should be used because laser-cut holes have straight walls with small roughness (less than 3 microns) and a taper.

2.2 Relationship between various parts of stencil and solder paste printing

(1) The size of the hole

The shape of the openings in the stencil and the shape of the pads on the printed board are very important for the precise printing of the solder paste. The openings in the stencil are mainly determined by the size of the corresponding pads on the printed board. In general, the size of openings in the stencil should be 10% smaller than the corresponding pads.

(2) Thickness of stencil

The thickness of the stencil and the size of the opening have a great relationship with the printing of the solder paste and the subsequent reflow. Specifically, the thinner the hole is, the more favorable the solder paste is released. It has been demonstrated that good print quality requires that the ratio of the opening size to the thickness of the stencil be greater than 1.5. Otherwise, the solder paste is not completely printed. In general, for a lead pitch of 0.5mm, use a 0.12∽0.15mm stencil thickness, a lead pitch of 0.3∽0.4mm, and a thickness of 0.1mm stencil.

(3) Opening direction and size of mesh plate

Solder paste release in the direction of the length of the pad is consistent with the direction of printing, printing effect is better than the direction perpendicular to the two.

The specific screen design process can be implemented according to Table 3.

Unit: mm

Component Type Pin Spacing Pad Width Pad Length Opening Width Opening Length Template Thickness
PLCC
1.27
0.65
2.00
0.60
1.95
0.15-0.25

QFP
0.635
0.35
1.50
0.32
1.45
0.15-0.18

QFP
0.50
0.254
1.25
0.22
1.20
0.12-0.15

QFP
0.40
0.25
1.25
0.20
1.20
0.10-0.12

QFP
0.30
0.20
1.00
0.15
0.95
0.07-0.12

0402

0.50
0.65
0.45
0.60
0.12-0.15

0201

0.25
0.40
0.23
0.35
0.07-0.12

BGA
1.27
?0.80
?0.75
0.15-0.20

BGA
1.00
?0.63
0.56
0.10-0.12

μBGA
0.50
?0.30
0.28
0.07-0.12

Flip chip
0.25
0.12
0.12
0.12
0.12
0.08-0.10

Flip chip
0.20
0.10
0.10
0.10
0.10
0.05-0.10

Flip chip
0.15
0.08
0.08
0.08
0.08
0.03-0.08

Table 3 Relationship between mesh opening size, mesh thickness, pad width, and lead pitch

3 Process control factors in the solder paste printing process

Solder paste printing is a very crafty process, and it involves a lot of process parameters. Improper adjustment of each parameter will have a very big impact on the quality of placement products.

3.1 screen printer printing parameter setting adjustment

This article will discuss the process control factors that affect solder paste printing quality in several ways:

(1) Squeegee pressure

The change of the squeegee pressure has a great influence on the printing. Too little pressure will make the solder paste unable to effectively reach the bottom of the template. The cut cannot be well deposited on the pad; too much pressure will cause the solder paste to be printed too thin and may even damage the form. The ideal state is just to scrape the solder paste from the surface of the stencil. In addition, the hardness of the scraper also affects the thickness of the solder paste. A too soft blade will cause the solder paste to dent, so it is recommended to use a harder scraper or metal scraper.

(2) Print thickness

The thickness of printing is determined by the thickness of the template. Of course, the settings of the machine and the properties of the solder paste also have a certain relationship. The slight adjustment of the printing thickness is often achieved by adjusting the speed of the doctor blade and the pressure of the doctor blade. By appropriately reducing the printing speed of the doctor blade, the amount of solder paste printed on the printed board can be increased. It is clear that reducing the speed of the blade is equivalent to increasing the pressure of the blade; on the contrary, increasing the speed of the blade is equivalent to reducing the pressure of the blade.

(3) Printing speed

Squeegee speed is conducive to the rebound of the template, but at the same time will hinder the transfer of solder paste to the PCB pad, and the speed is too slow will cause the pad printed on the solder paste resolution is poor. On the other hand, the speed of the doctor blade has a great relationship with the viscosity of the solder paste. The slower the doctor blade speed, the greater the viscosity of the solder paste. Similarly, the faster the doctor blade speed, the lower the paste paste consistency. Usually for fine-pitch printing speed range 12mm/s∽40mm/s

(4) Printing method

The stencil printing method can be divided into on-contact and off-contact printing. Printing with a gap between the template and the printed board is called non-contact printing. In the machine setting, this distance is adjustable, the general gap is 0∽1.27mm; and the stencil printing without printing gap (zero gap) is called contact printing. The vertical printing of the contact printing template minimizes the impact on the printing quality.