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SMT surface mount welding "tin bead defect" solution

Date: 2013/4/11 21:26


The existence of solder beads not only affects the appearance of electronic products, but also lays hidden dangers to product quality. The reason is that the density of modern printed circuit board components is high, the pitch is small, and the solder beads may fall off during use, which will cause short circuit of the components and affect the quality of electronic products.

The solder bead phenomenon is one of the main defects in the surface mount process. Its generation is a complex process and the most annoying problem. It is very difficult to completely eliminate it.

The diameter of the solder beads is approximately between 0.2mm and 0.4mm, and there are also those exceeding this range, which are mainly concentrated around the chip resistance-capacitance components. Generally speaking, the causes of solder beads are multifaceted and comprehensive. Printing thickness of solder paste, composition and oxidation degree of solder paste, stencil making and opening, whether solder paste has absorbed moisture, component mounting pressure, solderability of components and pads, reflow temperature setting, outside Environmental influences may be the cause of solder beads.

The following are the causes and solutions of solder beads from various aspects.

The choice of solder paste directly affects the welding quality. The metal content in the solder paste, the oxidation degree of the solder paste, the particle size of the alloy solder powder in the solder paste, and the thickness of the solder paste printed on the printed board can all affect the generation of solder beads.

A. Metal content of solder paste. The metal content of the solder paste is about 88% to 92% by mass and about 50% by volume. When the metal content increases, the viscosity of the solder paste increases, which can effectively resist the force generated by vaporization during preheating. In addition, the increase in the metal content makes the metal powders more densely packed, making them more capable of binding and not being blown apart during melting. In addition, the increase in metal content may also reduce the "slump" after printing the solder paste, so it is not easy to generate solder beads.


B, the degree of metal oxidation of the solder paste. In 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 solder beads is directly proportional to the degree of oxidation of the metal powder. Generally, the degree of solder oxidation in the solder paste should be controlled below 0.005%, with a maximum limit of 0.15%.


C. Particle size of metal powder in solder paste. The smaller the particle size of the powder in the solder paste, the larger the total surface area of the solder paste, which results in a higher degree of oxidation of the finer powder, which in turn increases the solder bead phenomenon. Our experiments show that solder powder is more likely to be generated when using a finer-grained solder paste.


D, the printing thickness of the solder paste on the printed board. The thickness of the solder paste after printing is an important parameter for stencil printing, usually between 0.12mm and 0.20mm. Thick solder paste will cause the solder paste to "fall" and promote the generation of solder beads.

E. The amount of flux in the solder paste and the activity of the flux. Too much solder volume will cause the local collapse of the solder paste, which will lead to the occurrence of solder beads. In addition, when the activity of the flux is small, the deoxidizing ability of the flux is weak, and tin beads are also easily generated. No-clean solder pastes are less active than rosin-based and water-soluble solder pastes, so they are more likely to produce solder beads.


F. In addition, the solder paste is generally refrigerated in a refrigerator before use. After taking it out, it should be returned to room temperature and opened for use (or stirred back with LT-180A solder paste mixer), otherwise, the solder paste easily absorbs moisture. The reflow solder spatters and produces solder beads.

2. Template making and opening. We generally make templates based on the pads on the printed board, so the opening of the template is the size of the pads. When printing solder paste, it is easy to print the solder paste on the solder resist layer, so that solder beads are generated during reflow soldering. Therefore, we can make a template in this way, reducing the opening of the template by 10% compared to the actual size of the pad. In addition, the shape of the opening can be changed to achieve the desired effect. Here are a few recommended pad designs:


The thickness of the template determines the printing thickness of the solder paste, so appropriately reducing the thickness of the template can also significantly improve the solder bead phenomenon. We once performed such an experiment: at first, a template with a thickness of 0.18 mm was used, and after soldering, it was found that the solder beads next to the resistance-capacitance component were more serious. Later, a template was remade with a thickness of 0.15 mm and an open form For the former design in the figure above, reflow soldering essentially eliminates solder beads.


Component mounting pressure and solderability of components. If the pressure is too high during placement, the solder paste is easily squeezed onto the solder resist layer under the component, and the solder melts and runs around the component to form solder beads during reflow soldering. The solution can reduce the pressure when mounting, and use the template opening form recommended above to prevent the solder paste from being squeezed outside the pad. In addition, the solderability of components and pads has a direct impact. If the oxidation of components and pads is serious, it will also cause the generation of solder beads. After the hot-air leveling of the solder pads, the solder-to-flux ratio is changed after the solder paste is printed, so that the solder ratio is reduced. The smaller the pads, the more serious the proportion misalignment. This is also a cause of solder beads.

Setting of reflow temperature. Solder beads are generated when the printed board is reflow soldered. Reflow soldering can be divided into four stages: preheating, thermal insulation, reflow, and cooling. During the preheating phase, the temperature of the solder paste and the components and the pads is raised to 120C-150C, which reduces the thermal shock of the components during reflow. At this stage, the flux in the solder paste begins to vaporize, which may make small The particulate metal runs to the bottom of the component separately, and runs around the component to form solder beads during reflow. At this stage, the temperature rise cannot be too fast, generally it should be less than 1.5C / s, too fast can easily cause solder splashes and form solder beads. Therefore, the temperature curve of reflow soldering should be adjusted, and a moderate preheating temperature and preheating speed should be adopted to control the generation of solder beads.


Influence of external factors. Generally, the optimal temperature for solder paste printing is 25C + 3C, humidity is 60% relative humidity, and the temperature is too high, which reduces the viscosity of the solder paste and easily causes "slump". When the humidity is too high, the solder paste easily absorbs moisture and easily Spatter, this is the cause of solder beads. In addition, the printed board will be exposed to the air for a long time to absorb moisture, and the pads will oxidize, and the solderability will be poor. You can bake it in a 120C-150C drying box for 12-14h to remove water vapor.

In summary, it can be seen that the generation of solder beads is a very complicated process. We should comprehensively consider when adjusting the parameters, and explore experience in production to achieve the best control of solder beads.

summary

Solder bead (SOLDER BALL) phenomenon is the main defect in the surface mount (SMT) process, which mainly occurs around the chip resistance capacitor (CHIP), caused by many factors. In this paper, through analysis of various reasons that may cause solder beads, corresponding solutions are proposed.

Key equipment to solve related issues:


Solder Paste / Red Offset Printing Machine-Precision and Reliable

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Product name: LT-180A Full automatic solder paste mixer

Product model: LT-180A

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