The characteristics of selective soldering can be understood by comparing it to wave soldering. The most obvious difference between the two is that in wave soldering, the lower portion of the PCB is completely immersed in liquid solder, while in selective soldering, only specific areas are exposed to the solder wave. Because the PCB itself is a poor heat conductor, it does not heat and melt the solder joints of adjacent components and PCB areas during soldering. Flux must also be applied before soldering. Unlike wave soldering, flux is applied only to the lower portion of the PCB to be soldered, not the entire PCB. Furthermore, selective soldering is only suitable for soldering plug-in components. Selective soldering is a new method, and a thorough understanding of the selective soldering process and equipment is essential for successful soldering.

The typical selective soldering process includes: flux spraying, PCB preheating, dip soldering and drag soldering.

The flux application process plays a crucial role in selective soldering. During soldering heating and at the end of soldering, the flux must be sufficiently active to prevent bridging and oxidation of the PCB. Flux spraying is performed by an X/Y robot, which carries the PCB over the flux nozzle, where it is sprayed onto the PCB surface to be soldered. Flux can be applied in a variety of ways: single-nozzle spray, micro-hole spray, and synchronized multi-point/pattern spray. Accurate flux application is crucial for microwave peak selective soldering after reflow. Micro-hole spraying ensures zero contamination outside the solder joint. The minimum flux dot pattern diameter for micro-hole spraying is greater than 2mm, so the flux deposited on the PCB must have a positional accuracy of ±0.5mm to ensure consistent coverage of the solder joint. The flux dosage tolerance is provided by the supplier, and the technical specifications should specify the amount of flux to be used. A 100% safety tolerance is generally recommended.

Selective soldering processes include two different processes: drag soldering and dip soldering.

Selective drag soldering is performed using a single, small solder nozzle with a solder wave. The drag soldering process is suitable for soldering in very tight spaces on PCB. For example, individual solder joints or pins, or even single-row pins, can be drag-soldered. The PCB is moved along the nozzle's solder wave at varying speeds and angles to achieve optimal soldering quality. To ensure a stable soldering process, the nozzle's inner diameter is less than 6mm. After the solder solution flow direction is determined, the nozzle is mounted and optimized in different orientations for different soldering needs. The robot can approach the solder wave from various angles, ranging from 0° to 12°, allowing users to solder a variety of components onto electronic assemblies. A 10° tilt angle is recommended for most components.

Compared to dip soldering, the movement of the solder solution and PCB during drag soldering results in more efficient heat transfer during soldering. However, the heat required to form the weld is transferred by the solder wave, but the solder wave mass of a single nozzle is low. Only a relatively high solder wave temperature can meet the requirements of the drag soldering process. For example, a soldering temperature of 275°C to 300°C and a drag speed of 10 mm/s to 25 mm/s are generally acceptable. Nitrogen is supplied to the soldering area to prevent oxidation of the solder wave. The solder wave eliminates oxidation, allowing the drag soldering process to avoid bridging defects. This advantage increases the stability and reliability of the drag soldering process.

The machine boasts high precision and flexibility. Its modular design allows for complete customization to meet specific customer production requirements and is upgradeable to meet future production growth needs. The robot's range of motion covers the flux nozzle, preheating nozzle, and solder nozzle, allowing the same machine to perform various soldering processes. The machine's unique synchronized process significantly reduces single-board production cycle times. The robot's capabilities contribute to the high precision and high-quality nature of this selective soldering process. Firstly, the robot's highly stable and precise positioning capability (±0.05mm) ensures highly repeatable parameters for each board produced. Secondly, the robot's five-dimensional motion allows the PCB to contact the solder surface at any optimized angle and orientation, achieving optimal soldering quality. A solder wave height probe, made of titanium alloy, mounted on the robot's clamping mechanism regularly measures solder wave height under program control. This height is then controlled by adjusting the solder pump speed to ensure process stability.

Despite these advantages, the single-nozzle solder wave drag soldering process also has drawbacks: soldering time is the longest of the three steps: flux spraying, preheating, and soldering. Furthermore, because solder joints are drag-soldered one by one, soldering time increases significantly as the number of joints increases, making it incomparable to traditional wave soldering processes in terms of soldering efficiency. However, this situation is changing. Multi-nozzle designs can maximize throughput. For example, using dual soldering nozzles can double throughput. Dual nozzles can also be used for flux.

Immersion selective soldering systems use multiple solder nozzles, each designed to correspond to the PCB joints to be soldered. While less flexible than robotic systems, they offer comparable throughput and are less expensive than robotic systems. Depending on the size of the PCB, single or multiple boards can be transported in parallel, with all joints undergoing fluxing, preheating, and soldering simultaneously. However, due to the varying distribution of solder joints on different PCB, specialized solder nozzles are required for each PCB. Keeping the nozzles as large as possible ensures a stable soldering process and avoids impacting adjacent components on the PCB. This is crucial for design engineers, yet also a challenge, as process stability may depend on it.

Immersion selective soldering allows for solder joints between 0.7mm and 10mm in diameter. The soldering process is more stable for short leads and small pads, minimizing the risk of bridging. The distance between adjacent solder joint edges, components, and soldering tips should be greater than 5mm. The primary purpose of preheating in selective soldering is not to reduce thermal stress, but to remove solvents and pre-dry the flux, ensuring the flux has the correct viscosity before entering the solder wave. During soldering, the impact of preheating heat on soldering quality is not a critical factor. The preheat temperature setting is determined by the PCB material thickness, component package specifications, and flux type. There are differing theories regarding preheating in selective soldering: some process engineers believe the PCB should be preheated before flux application; others suggest soldering can proceed directly without preheating. Users can tailor their selective soldering process to their specific needs.