Process characteristics and process of PCB selective soldering

Process characteristics of selective welding

The process characteristics of selective soldering can be understood by comparison with wave soldering. The most obvious difference between the two is that the lower part of the pcb in the wave soldering is completely immersed in the liquid solder, while in the selective soldering, only a certain specific area is in contact with the solder wave. Since pcb itself is a poor heat transfer medium, it does not heat the solder joints that melt adjacent components and pcb areas during soldering.

The flux must also be pre-coated before soldering. Compared to wave soldering, the flux is applied only to the portion of the PCB to be soldered, rather than the entire pcb. In addition, selective soldering is only suitable for soldering of the interposing components. Selective soldering is a completely new approach to understanding the selective soldering process and equipment required for successful soldering.

Selective welding process

Typical selective soldering processes include: flux coating, pcb preheating, dip soldering, and drag soldering.

Flux coating process

In the selective soldering, the flux coating process plays an important role. At the end of solder heating and soldering, the flux should be sufficiently active to prevent bridging and prevent oxidation of the pcb. The flux is sprayed by the x/y robot carrying the pcb through the flux nozzle and the flux is sprayed onto the pcb to be soldered. Fluxes are available in single-nozzle spray, micro-hole spray, and simultaneous multi-point/pattern spray.

The microwave peak after the reflow soldering process, the most important thing is the accurate spraying of the flux. The micro-hole spray type will never contaminate the area outside the solder joint. The minimum flux point pattern diameter of micro-spraying is greater than 2mm, so the positional accuracy of the solder deposited on the pcb is ±0.5mm, so that the flux can always be covered on the part to be welded. The tolerance of the sprayed soldering dose is provided by the supplier. The flux usage is specified and a 100% safety tolerance range is usually recommended.

Preheating process

The main purpose of preheating in the selective soldering process is not to reduce the thermal stress, but to remove the solvent pre-drying flux, so that the solder has the correct viscosity before entering the solder wave. During soldering, the effect of preheating the heat on the soldering quality is not a critical factor. The thickness of the pcb material, the package size of the device, and the type of flux determine the preheating temperature setting. In selective welding, there are different theoretical explanations for preheating: Some process engineers believe that pcb should be preheated before flux is sprayed; another point of view is that direct soldering is not required without preheating. The user can arrange the selective soldering process according to the specific situation.

Welding process

The selective soldering process has two different processes: a drag welding process and a dip soldering process.

The selective drag welding process is performed on a single small tip solder wave. The drag welding process is suitable for welding on very tight spaces on the pcb. For example: individual solder joints or pins, single row pins can be dragged. The pcb moves on the solder wave of the tip at different speeds and angles to achieve the best soldering quality.

In order to ensure the stability of the welding process, the inner diameter of the tip is less than 6mm.

After the flow direction of the solder solution is determined, the tip is installed and optimized in different directions for different soldering needs. The robot can approach the solder wave from different angles from 0° to 12°, so the user can solder various components on the electronic components. For most devices, the tilt angle is recommended to be 10°.

Compared with the dip soldering process, the soldering solution of the soldering process and the movement of the pcb board make the heat conversion efficiency during soldering better than the dip soldering process.

However, the heat required to form the weld joint is transmitted by the solder wave, but the solder wave quality of the single tip is small, and only the solder wave temperature is relatively high to meet the requirements of the drag welding process. For example: solder temperature is 275 ° C ~ 300 ° C, drag speed 10mm / s ~ 25mm / s is generally acceptable. Nitrogen is supplied to the soldering area to prevent solder wave oxidation, and the solder wave eliminates oxidation, so that the drag welding process avoids the occurrence of bridging defects, which increases the stability and reliability of the soldering process.

The machine is characterized by high precision and flexibility.

The modular design system can be customized according to the customer’s special production requirements, and can be upgraded to meet the needs of future production development. The robot’s radius of motion covers the flux nozzles, preheating and soldering tips, so the same equipment can be used to perform different welding processes. The machine-specific synchronization process can greatly shorten the board process cycle.

The ability of the robot to make this selective weld has the characteristics of high precision and high quality welding.

The first is the highly stable and precise positioning capability of the robot (±0.05mm), which ensures that the parameters of each plate are highly repeatable.

Secondly, the 5-dimensional movement of the robot enables the pcb to contact the tin surface at any optimized angle and orientation. Good welding quality. The tin wave height stylus mounted on the manipulator splint device is made of titanium alloy. The height of the tin wave can be measured periodically under program control. The height of the tin wave can be controlled by adjusting the speed of the tin pump to ensure process stability.

Despite these advantages, the single-nozzle solder wave dragging process is also inadequate: the soldering time is the longest in the three processes of flux spraying, preheating and soldering.

And because the solder joints are one by one, the soldering time will increase greatly as the number of solder joints increases, and the soldering efficiency cannot be compared with the conventional wave soldering process. However, the situation is changing and the multi-tip design maximizes throughput. For example, double-welded nozzles can double the throughput and flux can be designed as dual nozzles.