In the evolution of PCB technology, a notable trend has been the adoption of reflow soldering. Traditionally, reflow soldering was used for surface-mount devices (SMDs), but it has now been extended to through-hole components, known as through-hole reflow soldering. This method allows all solder joints to be completed simultaneously, minimizing production costs. However, the application of reflow soldering is limited by temperature-sensitive components, whether through-hole or SMD. As a result, selective soldering has gained attention. In many applications, selective soldering is employed after reflow soldering, offering an effective solution.
Process Characteristics of Selective Soldering
The characteristics of selective soldering can be understood by comparing it with wave soldering. The most significant difference is that in wave soldering, the entire bottom side of the PCB is immersed in molten solder, whereas in selective soldering, only specific areas come into contact with the solder wave. Since PCBs are poor conductors of heat, selective soldering does not heat or melt solder joints in adjacent components or PCB areas. Flux must also be applied before soldering, but unlike wave soldering, it is only applied to the specific areas to be soldered, not the entire PCB. Additionally, selective soldering is primarily used for through-hole components. It is a novel method, and a thorough understanding of the process and equipment is essential for successful soldering.
Workflow of Selective Soldering
The typical selective soldering process includes the following steps: flux spraying, PCB preheating, dip soldering, and drag soldering.
Detailed Process Steps
1. Preheating Process
Preheating in selective soldering is not primarily aimed at reducing thermal stress but at removing solvents and pre-drying the flux. This ensures the flux has the correct viscosity before entering the solder wave. The heat from preheating is not a critical factor in soldering quality, which is instead determined by PCB material thickness, component packaging specifications, and flux type. There are differing opinions on preheating: some engineers recommend preheating before flux application, while others believe it is unnecessary. The workflow can be adjusted based on specific requirements.
2. Flux Application Process
The flux application step plays a crucial role in selective soldering. The flux must remain active during heating and cooling to prevent bridging and PCB oxidation. Flux is sprayed onto the PCB using an X/Y robotic arm that moves the PCB over a flux nozzle. Flux application methods include single-nozzle spray, micro-spray, and synchronized multi-point/graphic spray. For post-reflow selective soldering, precise flux application is critical. Micro-spray methods ensure no contamination of areas outside the solder points, with a minimum spray pattern diameter of 2mm and a positional accuracy of ±0.5mm. The flux volume tolerance is provided by the supplier, and technical specifications should define the recommended usage, typically with a 100% safety margin.
3. Soldering Process
Selective soldering employs two main techniques: drag soldering and dip soldering.
- Drag Soldering: This process uses a single small nozzle to create a solder wave. It is suitable for soldering in tight spaces on the PCB, such as individual solder points or single rows of pins. The PCB moves at varying speeds and angles over the solder wave to achieve optimal soldering quality. The nozzle’s inner diameter is typically less than 6mm, and its orientation can be adjusted to meet different soldering needs. The robotic arm can approach the solder wave from angles between 0° and 12°, with a recommended tilt of 10° for most components. Drag soldering offers better heat transfer efficiency than dip soldering due to the movement of the solder solution and PCB. However, the small solder wave requires higher temperatures (275°C–300°C) and slower speeds (10mm/s–25mm/s) to ensure quality. Nitrogen is often used to prevent solder wave oxidation, enhancing process stability and reliability. Despite its advantages, drag soldering has limitations. It is the most time-consuming step in the process (after flux application and preheating), and as the number of solder points increases, so does the soldering time. This makes it less efficient than traditional wave soldering. However, multi-nozzle designs can improve throughput. For example, dual nozzles can double production rates, and dual flux nozzles can also be implemented.
- Dip Soldering: This system uses multiple solder nozzles, each designed for specific solder points on the PCB. While less flexible than robotic systems, dip soldering offers higher throughput, comparable to traditional wave soldering, at a lower cost. Depending on the PCB size, single or multiple boards can be processed in parallel, with all solder points undergoing flux application, preheating, and soldering simultaneously. However, custom nozzles must be designed for different PCBs due to variations in solder point distribution. Larger nozzles are preferred for process stability, but designing them to avoid interference with adjacent components can be challenging. Dip soldering is suitable for solder points ranging from 0.7mm to 10mm. It provides stable soldering for short pins and small pads, with a lower risk of bridging. The distance between adjacent solder points, components, and nozzles should exceed 5mm.
Equipment Features
Selective soldering machines are characterized by high precision and flexibility. Their modular design allows customization to meet specific production requirements and future upgrades. The robotic arm’s movement range covers the flux nozzle, preheating zone, and solder nozzle, enabling multiple soldering processes on a single machine. Synchronized processes significantly reduce cycle times per board.
The robotic arm’s high-precision positioning capability (±0.05mm) ensures consistent production parameters for each board. Its 5-axis movement allows the PCB to contact the solder wave at optimized angles, ensuring superior soldering quality. A titanium alloy solder wave height probe, mounted on the robotic arm, periodically measures and adjusts the solder wave height by controlling the solder pump speed, ensuring process stability.
