Where do solder beads come from? How to solve it effectively?

For more than ten years, with the continuous development of electronic information products in the direction of lightness, power saving, miniaturization and flatness, surface mount technology (SMT) has become an indispensable process in the manufacture of various electronic products. However, as one of the main defects in SMT production, solder beads pose a serious threat to the quality and reliability of electronic products. Therefore, how to effectively reduce the generation of solder beads has become one of the key control contents of SMT enterprises.

Definition and hazards of tin beads

In SMT production, during the reflow soldering process, tiny spherical solder beads or irregularly shaped solder particles, i.e., solder beads, are easily formed due to the solder paste metal particles spatter. The diameter of the solder bead is usually between 0.2~0.4mm, and it mainly appears on the side of the chip component or between the IC pins. It not only affects the appearance of PCB products, but also may cause short circuits in use, seriously affect the quality and life of electronic products, and may even cause personal injury.

Causes and control methods of solder beads

The generation of solder beads is the result of a combination of factors, involving raw materials, solder paste, templates, mounting, reflow soldering, environment and other links. Therefore, it is particularly important to study the causes in depth and take effective control measures.

1. Raw material problems

1.1 PCB Quality & Components

• Improper pad design: If the component body is pressed too much on the pad, the solder paste will be extruded too much, and solder beads may be generated. When designing a PCB, you should choose the appropriate component package and pad size.
• Poor printing of solder mask:P poor printing of CB solder mask, or rough surface may lead to the appearance of solder beads during reflow. The PCB incoming material must be strictly inspected, and the solder mask should be returned or scrapped when the solder mask is seriously bad.
• Pad contamination: When there is moisture or dirt on the pad, it is easy to produce solder balls. Moisture or dirt on the PCB should be carefully removed before production.
• Device substitution problem: The substitution requirements of devices with different package sizes in the customer's incoming materials may lead to a mismatch between the device and the pad, and it is easy to produce solder beads. Substitution should be avoided as much as possible.

1.2 PCB moisture

• There is too much moisture in the PCB, and when it passes through the reflow oven after mounting, the moisture expands sharply and produces gas, resulting in the formation of solder beads. It is required that the PCB must be dry vacuum packed before being put into SMT production, and if it is wet, it needs to be baked before use. For organic solder pack (OSP) boards, baking is not allowed, and OSP boards that have not been used for more than 3 months need to be redressed.

2. Solder paste selection

The quality of solder paste significantly affects the soldering effect, and its metal content, oxide content, metal powder particle size, activity and other factors may affect the formation of solder beads.

2.1 Metal content and viscosity

• The volume ratio of metal content in solder paste is about 50%, and the mass ratio is about 89%~91%. Too much flux will reduce the viscosity of the solder paste, and the force generated during the vaporization of the flux in the preheating zone will be too large, and solder beads will be easily generated. The viscosity of solder paste is usually between 0.5~1.2 KPa·s, and the optimal viscosity is about 0.8 KPa·s. Increasing the metal content increases viscosity and reduces solder bead formation.

2.2 Oxide content

• The higher the oxide content in the solder paste, the greater the resistance during the bonding process after the metal powder is melted, and the solder beads are easy to form during the reflow soldering stage. Therefore, vacuum operation should be required in the metal powder manufacturing process to prevent oxidation.

2.3 Particle size and uniformity of metal powder

• The particle size, shape and uniformity of metal powder affect the printing performance. The oxide content of finer particles is high, which is easy to produce collapse edges, resulting in the increase of tin beads; Larger particles are more likely to lead to tinning. Poor uniformity also increases the risk of solder beads.

2.4 Solder paste activity

• If the solder paste is not active well or dries too quickly, the force generated by the vaporization of the diluent in the preheating zone is too large, and it is easy to produce solder beads. If you encounter a solder paste with poor activity, it should be stopped and replaced immediately.

3. Template issues

3.1 Template thickness

• If the stencil is too thick, the solder paste will be printed too thickly, and solder beads will easily be generated after reflow soldering. The appropriate thickness of the template should be selected according to the pad size, and the template should be remade if necessary.

3.2 Template opening design

• The opening of the template is not treated with anti-solder beads, which is easy to produce solder beads. The openings of the chip parts should be treated with anti-solder beads. The size of the stencil opening should be less than 10% of the contact area of the pad, and the lead-free template opening design should be larger than that of the leaded to ensure that the solder paste completely covers the pad.

4. Printing parameter issues

4.1 Solder paste collapse

• The collapse of solder paste is related to the pressure, speed and demoulding speed of the scraper. Adjusting these parameters can reduce edge collapse and solder bead generation.

4.2 Printing Offset and Thickness

• If the printing offset or the solder paste is too thick, the excess solder paste overflows after the component is pressed down, and it is easy to form solder beads. The printing thickness is usually the thickness of the stencil (1+10%±15%), and too thick can easily lead to collapse edges and tin beads.

5. Placement pressure problem

• When the pressure of the part is too large during mounting, when the component is pressed on the solder paste, part of the solder paste is squeezed under the component, and it is easy to form solder beads after reflow soldering. The appropriate placement pressure should be selected.

6. Furnace temperature problem

• The reflow soldering curve is divided into four stages: preheating, heat preservation, reflow and cooling. If the preheating temperature is too high or the heating is too fast, the vaporization phenomenon will be exacerbated, causing the solder paste to splash and form solder beads. The furnace temperature and conveyor belt speed should be adjusted to control the preheating temperature and speed.

7. Process control issues

7.1 Precautions for the use of solder paste

• The storage period of solder paste is generally 3~6 months, and it should be stored in a constant temperature refrigerator at 2°C~10°C according to the principle of "first-in, first-out". It needs to be warmed up for 4 hours before use, and it should be stirred evenly after opening, and it should be used within the same day.

7.2 Environmental Controls

• The optimal temperature for printing is 25°C±3°C, and the relative humidity is 45%~65%. If the temperature and humidity are too high, the solder paste is easy to absorb water vapor, and it is easy to produce tin beads during reflux.

7.3 Cleaning and Placement Time

• After the printing fails, the PCB should be thoroughly cleaned and baked. Delayed or incomplete stencil cleaning can lead to residual solder paste contaminating the PCB and producing solder beads. Placement should be completed as soon as possible after printing to prevent flux volatilization.

summary

In SMT production, the generation of solder beads involves many aspects such as raw materials, solder paste, stencils, printing parameters, placing pressure, furnace temperature and process control. It's not enough to focus on one aspect or adjust a single parameter. In order to achieve the best welding results and meet the high quality requirements of electronic connection technology, it is necessary to control all the details of the pre-production preparation phase and the production process, taking into account and controlling the various factors that affect the generation of solder balls.