## 2 Materials and Methods
### Selective Soldering  
Selective soldering is an advanced technology used in electronics assembly that enables the precise soldering of specific points on printed circuit boards (PCBs). This process is particularly useful for devices with complex designs, where soldering all connections simultaneously, as in wave soldering, could damage sensitive components or lead to defects. Selective soldering allows for localized soldering without affecting adjacent elements.  
Stages of the Selective Soldering Process [27]:  
1. Flux application: Flux is applied to selected points on the printed circuit board (PCB) to remove oxides from the soldered surfaces and improve wettability. The application is performed using precisely controlled nozzles to avoid contamination in other areas of the board.
2. Preheating: The board undergoes controlled preheating to reduce the risk of thermal stress. Preheating prepares the PCB for the actual soldering process while ensuring that its components are not damaged.
3. Soldering: The soldering process is carried out using soldering nozzles that apply molten solder alloy to the selected points on the board. The use of a protective atmosphere, typically nitrogen, minimizes solder oxidation, resulting in high-quality joints. The process is highly precise, making it suitable for working with complex and densely populated PCBs.
4. Cooling: After soldering is completed, the board is cooled in a controlled manner, allowing the solder to solidify quickly and the joints to stabilize [28].  
Applications of Selective Soldering:  
Selective soldering is primarily used for boards with mixed assembly of THT (Through-Hole Technology) and SMT (Surface Mount Technology) components, where it is necessary to protect sensitive elements. This technology is also employed in the production of devices that require high precision and reliability, such as medical, telecommunications, and automotive equipment [29].  
Advantages of Selective Soldering:  
1. Precision: This technology enables soldering of only selected points, helping to avoid defects in other areas of the PCB.
2. Component protection: The process minimizes the risk of thermal damage to sensitive components on the board.
3. Flexibility: Selective soldering is well-suited for complex board layouts and various types of components.
4. Joint quality: A protective atmosphere and precise control of process parameters ensure high-quality solder joints.
5. Material savings: Accurate dosing of flux and solder reduces material consumption, thereby lowering production costs [30].  
Recent research demonstrates that integrating AI-based optimization algorithms with simulation models can significantly improve the efficiency and quality of selective soldering processes, enabling more robust and adaptive manufacturing systems [31].  
Limitations of Selective Soldering:  
1. Longer process time: Selective soldering is slower compared to wave soldering, as only specific points are soldered.
2. Initial costs: Implementing the technology requires the purchase of advanced soldering equipment and training for operators.  
In summary, selective soldering is precise and versatile but slower and more expensive than other soldering techniques. Nevertheless, it proves especially valuable in projects with a high level of complexity.