The manufacturing process of solder balls largely determines their final quality. As common manufacturing processes, atomization and dripping methods have unique effects on the surface finish and internal density of solder balls, which will further affect the actual application effect of solder balls in the fields of electronic packaging.
When preparing solder balls by atomization, the molten metal liquid flow is impacted and broken into fine droplets by high-pressure airflow. In this process, there is a strong interaction between the high-speed airflow and the metal droplets. On the one hand, the high-speed impact of the airflow will cause irregular disturbances on the surface of the droplets. Just as the turbulent water impacting the river bank will make the river bank bumpy, the impact of the airflow will also cause tiny wrinkles and protrusions on the surface of the droplets. On the other hand, during the rapid cooling process of the droplets, the surface tension has not yet fully exerted its effect to make the surface reach the ideal smooth state, and the droplets have solidified, which results in some micron-level concave and convex structures remaining on the surface of the final solder ball, and the surface appears not smooth enough.
From the perspective of internal density, during the atomization process, due to the very fast formation and cooling speed of the droplets, the gas is easily wrapped inside the droplets. These encapsulated gases cannot escape in time when the droplets solidify, forming tiny pores or loose structures inside the solder ball. Moreover, the cooling speeds of droplets of different sizes are inconsistent. Large droplets cool relatively slowly, and the diffusion and arrangement time of the atoms inside them is more abundant, which is easy to form local coarse crystals, making the internal density uneven, which poses a potential threat to the reliability of the solder ball in subsequent welding applications.
The preparation of solder balls by the dripping method has a different mechanism of action. In the dripping method, the molten metal liquid drips regularly through a precision nozzle. In an inert gas environment, the surface tension of the droplets can fully play its role during the falling process, so that the droplets naturally shrink into a more regular spherical shape. Compared with the atomization method, there is no impact interference of high-speed airflow, and the surface of the droplets is relatively calmer. At the same time, the step-by-step cooling method adopted by the dripping method allows the droplets to have enough time to repair surface defects. During the slow cooling process, the atoms on the surface of the droplets have the opportunity to rearrange and fill in tiny defects, so that the solder ball can have a better surface finish and relatively low surface roughness after solidification.
In terms of internal density, the slow cooling process experienced by the droplets in the drip method is of great significance. During the long cooling time, the metal atoms can fully diffuse and arrange into a compact and orderly crystal structure, and the gaps between the grain boundaries are reduced. In addition, the low cooling rate allows the gas dissolved in the liquid to have enough time to escape from the surface, greatly reducing the probability of internal pores. The resulting solder ball has a higher internal density and a more compact structure, providing a good foundation for its application in high-precision welding scenarios.
Although there are obvious differences in the effects of the atomization method and the drip method on the surface finish and internal density of the solder ball, the quality of the solder ball can be improved to a certain extent by optimizing the process parameters. For example, the atomization method can reduce surface defects and internal pores by adjusting the air flow pressure and the temperature of the molten metal; the drip method can optimize the nozzle design, control the dripping frequency and cooling conditions, and further improve the sphericity of the solder ball and the uniformity of the internal structure.
In practical applications, the appropriate manufacturing process should be selected according to the specific use requirements of the solder ball. For some microelectronic packaging fields that have extremely high requirements for surface finish and internal density, the solder balls prepared by the droplet method are more popular due to their excellent surface quality and high density; and in some scenarios that are more sensitive to cost and have relatively less stringent requirements for surface quality, the atomization method still has a wide range of applications due to its higher production efficiency.
