Talking about the Causes of the Formation of Tin Beads and Improvement Methods

1 Introduction

SMD components are more and more widely used in the electronics industry due to their small size, low cost, and high reliability. At present, SMD components are mainly reflow soldering, and the quality of its soldering directly affects the quality of the product. The phenomenon of tin beading is one of the main defects in surface mount technology (SMT) production. The production of tin beads is a complicated process. Because of its many reasons and difficult to control, it often troubles SMT process technicians. Generally, the diameter of tin beads is between 0.2mm and 0.4mm, and some of them exceed this range. They are mainly concentrated on the side of the chip resistor-capacitor component, and sometimes appear near the IC or connector pins. The tin beads not only affect the appearance of electronic products, but more importantly, due to the high density of printed board components and small spacing, the tin beads may fall off when the product is in use, causing short circuits in the components and affecting the quality of electronic products. [1] The acceptability standard of electronic components (IPC-A-610E) requires that the size of the solder beads cannot violate the minimum electrical gap. [2] Therefore, it is necessary to clarify the reasons for its occurrence and effectively control it.

2 The formation mechanism of tin beads

Tin ball refers to some large solder balls before the solder paste is soldered. The solder paste may be out of the printed pad due to various reasons such as collapse or squeeze. When soldering, these are out of the pad. The solder paste fails to fuse with the solder paste on the pad during the soldering process and becomes independent, and is formed on the component body or near the pad. [3] Most tin beads generally occur on both sides of chip components, as shown in Figure 1. If the amount of tin is too much, the pressure when the component is placed will squeeze the solder paste under the component body (insulator), and it will be melted during reflow soldering. Due to the surface energy, the melted solder paste will gather into a ball, which has The component tends to lift up, but this force is extremely small, and it is squeezed to both sides of the component by the gravity of the component, separated from the pad, and formed tin beads when cooled. If the component has high gravity and more solder paste is squeezed out, multiple solder balls will be formed.

3 Reasons for the formation of tin beads

Generally speaking, there are many reasons for the formation of tin beads, as shown in Table 1 below. Such as the printing thickness of the solder paste, the alloy composition and oxidation degree of the solder paste, the quality of the solder paste or the use of the solder paste if it is not stored in accordance with the regulations, the production and opening of the stencil, the cleaning of the stencil, the component placement pressure, the components and the pads The solderability, the setting of the reflow soldering temperature, and the influence of the external environment may all be the cause of the tin bead.

Material reasons

1. The thixotropic coefficient of solder paste is small

2. Solder paste collapses under cold or slightly thermal collapse

3. Too much flux or low activation temperature

4. Tin powder is oxidized or has uneven particles

5. PCB pad pitch is small

6. The material of the scraper is slightly small or deformed

7. The steel mesh hole wall is not smooth and has burrs

8. Poor solderability of pads and components

9. The solder paste is wet or has moisture

Process reasons

1. More tin

2. There is residual solder paste on the contact surface of the stencil and PCB

3. Imbalance of heat or improper setting of furnace temperature

4. Excessive patch pressure

5. The gap between PCB and stencil printing is too large

6. Small scraper angle

7. Small steel mesh spacing or wrong opening ratio

8. The solder paste has not been properly reheated before use

A. The metal content of the solder paste.

The mass ratio of the metal content in the solder paste is about 88% to 92%, and the volume ratio is about 50%. When the metal content increases, the viscosity of the solder paste increases, which can effectively resist the force generated by vaporization during the preheating process. In addition, the increase of the metal content makes the metal powders tightly arranged, so that they can be more easily combined without being blown away when they are melted. In addition, the increase in metal content may also reduce the "sagging" of the solder paste after printing, so it is not easy to produce

Solder beads.

B. The degree of metal oxidation of the solder paste.

In the solder paste, the higher the degree of metal oxidation, the greater the resistance of the metal powder during soldering, and the less wetting between the solder paste and the pads and components, resulting in reduced solderability.

Experiments show that the incidence of tin beads is directly proportional to the degree of oxidation of the metal powder. Generally, the oxidation degree of the solder in the solder paste should be controlled below 0.05%, and the maximum limit is 0.15%

C. The particle size of the metal powder in the solder paste.

The smaller the particle size of the powder in the solder paste, the larger the overall surface area of ​​the solder paste, which leads to a higher degree of oxidation of the finer powders, which intensifies the solder beading phenomenon. Our experiments show that when selecting solder paste with a finer particle size, solder powder is more likely to be produced.

D. The printing thickness of the solder paste on the printed circuit board.

The thickness of the solder paste after printing is an important parameter of the missing plate printing, usually 0.12mm-20mm

between. Too thick solder paste will cause the solder paste to "collapse" and promote the formation of solder beads.

E. The amount of flux in the solder paste and the activity of the flux.

Too much amount of solder will cause partial collapse of the solder paste, which makes solder beads easy to produce. In addition, when the activity of the flux is small, the deoxidation ability of the flux is weak.

Therefore, tin beads are easily produced. The activity of no-clean solder paste is lower than that of rosin and water-soluble solder pastes, so it is more likely to produce tin beads.

F. In addition, the solder paste is generally refrigerated in the refrigerator before use.

After taking it out, it should be restored to room temperature and then opened for use. Otherwise, the solder paste will easily absorb moisture, and the reflow solder will splash and cause solder beads.