In laser welding, controlling the geometry of the solder ball is crucial to weld quality, as its formation involves complex physicochemical changes resulting from the interaction between laser energy and the material. The solder ball's morphological characteristics, such as diameter, height, roundness, and surface quality, are directly influenced by process parameters including laser power, pulse width, scanning speed, and defocusing amount. These parameters, by controlling energy input density, interaction time, and heat conduction path, ultimately determine the solder ball's morphology.
Laser power is the primary parameter affecting the solder ball's morphology, regulating its melting and solidification behavior by altering the energy input density. At lower laser power, insufficient energy melts the solder, leading to incompletely melted spherical solder joints with rough surfaces and small sizes. As power increases, the solder melts and spreads more fully, increasing the solder ball's diameter and decreasing its height, resulting in a smoother surface. However, excessively high power can cause excessive solder evaporation, leading to spatter or dents and compromising the integrity of the solder ball's morphology. Therefore, an appropriate power range must be selected based on the solder composition and substrate characteristics to achieve uniform spreading and metallurgical bonding of the solder ball.
Pulse width affects the heat conduction process of the solder ball by controlling the laser's interaction time, thereby regulating its morphology. Short-pulse lasers have concentrated energy and short interaction times, resulting in rapid melting and solidification of the solder, easily forming small, high-height spherical solder joints. However, insufficient wetting may lead to low bonding strength. Long-pulse lasers provide continuous energy input, allowing the solder to fully melt and spread, increasing the solder ball diameter, decreasing its height, and improving surface smoothness. However, excessively long pulse widths may cause excessive solder flow, leading to solder ball shape instability or bridging short circuits. Therefore, pulse width must be optimized in conjunction with scanning speed to balance the spreading properties and forming accuracy of the solder ball.
Scanning speed affects the heat accumulation and cooling rate of the solder ball by controlling the relative motion between the laser and the material, thus determining its morphology. During high-speed scanning, the short laser action time leads to insufficient solder melting, easily forming incompletely melted spherical solder joints with rough surfaces and small dimensions. During low-speed scanning, continuous laser heating causes excessive solder melting, increasing the solder ball diameter and decreasing its height, but this may lead to solder spatter or substrate deformation due to heat accumulation. Therefore, the scanning speed needs to be adjusted according to the solder thickness and substrate thermal conductivity to control the heat input and cooling rate of the solder ball, achieving precise morphological control.
Defocusing affects the size and shape of the solder ball's molten pool by changing the laser spot size and energy distribution, thereby regulating its morphological characteristics. Positive defocusing increases the spot diameter and decreases the energy density, expanding the solder melting range but decreasing its depth, resulting in an increased solder ball diameter and decreased height, improving surface smoothness. Negative defocusing decreases the spot diameter and concentrates the energy density, increasing the solder melting depth but shrinking its range, resulting in a decreased solder ball diameter and increased height, easily forming conical or irregular shapes. Therefore, the defocusing amount needs to be selected based on the solder joint spacing and substrate flatness to optimize the geometry and electrical properties of the solder ball.
The shielding gas affects the surface quality and morphological stability of the solder ball by suppressing plasma and oxidation reactions. During laser welding, the metallic plasma generated by high-temperature evaporation absorbs laser energy, reducing welding efficiency and causing spatter. Simultaneously, the solder and substrate are easily oxidized at high temperatures, forming a brittle compound layer that damages the metallurgical bond of the solder ball. By blowing inert shielding gas, plasma can be effectively dispersed, oxygen isolated, spatter and oxidation reduced, and the smoothness and morphological stability of the solder ball surface improved. Furthermore, the flow rate and angle of the shielding gas need to be precisely controlled to avoid airflow disturbances causing solder flow instability.
Laser welding process parameters have a significant quantitative relationship with the control of the solder ball's geometry. Systematic optimization of parameters such as laser power, pulse width, scanning speed, defocusing amount, and shielding gas is necessary to achieve precise control of the solder ball's size, shape, and surface quality. This process requires combining solder composition, substrate characteristics, and welding requirements, and determining the optimal parameter combination through experimental design and numerical simulation to meet the high standards required for solder ball morphology in fields such as electronic packaging and precision manufacturing.
