**PCB Circuit Board Industry Process Development**
1. An obvious trend in the PCB industry is the evolution of reflow soldering technology. In principle, traditional through-hole components can also be reflow soldered, a method commonly known as through-hole reflow soldering. The key advantage of this approach is that it allows for all solder joints to be completed in a single step, thereby reducing production costs. However, the application of reflow soldering is limited by temperature-sensitive components, whether they are interposers or SMDs. This limitation led to the development of selective soldering.
2. Selective soldering can often be used in conjunction with reflow soldering in most applications. This combination presents an economical and effective solution for completing the soldering of remaining through-hole components, and it is fully compatible with future lead-free soldering processes.
**Process Characteristics of Selective Soldering**
3. The characteristics of selective soldering can be better understood by comparing it with wave soldering. The most obvious distinction between the two is that, in wave soldering, the entire underside of the PCB is immersed in liquid solder. In contrast, selective soldering targets only specific areas of the PCB for solder wave contact.
4. Since the PCB itself has relatively poor thermal conductivity, the soldering process does not affect or damage the solder joints of nearby components or other areas of the board. Additionally, flux must be applied before soldering. Unlike wave soldering, where flux is applied to the entire PCB, in selective soldering, flux is only applied to the areas that require soldering.
5. Moreover, selective soldering is specifically designed for the soldering of through-hole components. It represents a new approach to the process, and a thorough understanding of the selective soldering technology and equipment is essential for achieving successful results.
—
This version maintains the original content while making it flow more naturally, with slight rephrasing to enhance clarity and readability.
**Selective Soldering Process**
The typical selective soldering process consists of the following stages: flux spraying, PCB preheating, dip soldering, and drag soldering.
**Flux Coating Process**
In selective soldering, the flux coating process is critical to the overall performance. By the end of the soldering heating phase, the flux must retain sufficient activity to prevent bridging and avoid PCB oxidation. Flux spraying is performed by an X/Y manipulator, which moves the PCB through the flux nozzle, applying flux onto the areas of the PCB to be soldered. There are several methods for flux spraying, such as single-nozzle spray, micro-hole spray, and synchronous multi-point/pattern spray. For selective soldering, particularly after the reflow soldering process, precise flux spraying is essential. The micro-hole jet ensures that only the solder joint areas are covered, preventing contamination of surrounding regions. The minimum flux point pattern diameter with micro-point spraying is greater than 2mm, so the flux deposited on the PCB is within a position accuracy of ±0.5mm, ensuring it is always applied to the intended solder areas. The flux supplier specifies the spray tolerance, and the technical specification should define the exact flux amount used. A safety tolerance range of 100% is typically recommended.
**Preheating Process**
The main objective of preheating in the selective soldering process is not to reduce thermal stress but to evaporate the solvent and pre-dry the flux, ensuring it achieves the correct viscosity before it reaches the solder wave. The impact of preheating on soldering quality is not the primary concern; instead, it is the PCB material thickness, component packaging, and flux type that determine the appropriate preheating temperature. There are differing opinions on preheating in selective soldering: some engineers believe the PCB should be preheated before flux spraying, while others suggest that preheating may not be necessary, allowing for direct soldering. The user can determine the selective soldering process based on the specific requirements of the application.
**Welding Process**
There are two primary types of selective soldering processes: drag soldering and dip soldering.
The selective drag soldering process uses a single small solder tip on a solder wave. Drag soldering is ideal for soldering in tight spaces on the PCB, such as individual joints or pins, or single-row pins. The PCB moves across the solder wave at varying speeds and angles to achieve optimal soldering quality. To ensure process stability, the inner diameter of the soldering tip is kept below 6mm. Once the direction of the solder solution flow is determined, the solder tips are installed and adjusted in different orientations to suit various soldering requirements. The manipulator can approach the solder wave from multiple directions, at angles ranging from 0° to 12°, allowing users to solder various devices on electronic components. For most applications, a tilt angle of 10° is recommended.
Compared to dip soldering, drag soldering offers better heat conversion efficiency due to the movement of the PCB and the solder solution. However, the heat required to form the solder joints is transferred by the solder wave. The solder wave of a single solder tip has a limited capacity, and its higher temperature is necessary to meet the demands of the drag soldering process. For instance, the solder temperature is typically between 275°C and 300°C, with a pulling speed of 10mm/s to 25mm/s. Nitrogen is introduced in the soldering area to prevent oxidation of the solder wave. This controlled environment eliminates oxidation, helping to avoid bridging defects, thus enhancing the stability and reliability of the drag soldering process.
