Selective soldering is a soldering technique used to connect electronic components to circuit boards, primarily to achieve precise solder joints, often in complex circuit designs. This method of soldering is characterised by the fact that only specific parts are soldered, rather than the entire assembly being fully heated, thus minimising the effect of heat on the surrounding components.
Selective soldering is divided into two forms: spray soldering and immersion soldering. Spray soldering operates with a single nozzle fixed underneath the PCB and is suitable for soldering small areas such as individual points or pins. By adjusting the rate of movement of the PCB and the angle of approximately 10° between the PCB and the nozzle, the soldering result can be optimised. Dip soldering, on the other hand,involves immersing the area to be soldered on the PCB into a special nozzle disc, thereby completing the soldering of multiple solder joints in a single pass.However,since the layout of the solder joints varies from PCB to PCB, a customised nozzle tray is required for each PCB. The standard process for selective soldering usually involves flux application,PCB preheating, immersion soldering and spraying. In some cases, the preheating step can be omitted and the soldering can be done by spraying only,or the PCB can be preheated before flux application. Users can flexibly adjust the selective soldering process according to the actual situation.
Selective soldering process characteristics
The concept of selective soldering is relatively intuitive, its operating speed compared to wave soldering is slower, because it adopts a component-by-component local wave soldering method, rather than wave soldering as a one-time impact on all the soldering points.
By comparing with wave soldering, we can grasp the characteristics of selective soldering more clearly. Wave soldering is to completely immerse the entire lower part of the PCB in liquid solder, while selective soldering allows only part of the PCB to come into contact with the solder wave. Since the heat transfer performance of PCBs is not ideal, selective soldering requires flux to be applied to the part of the PCB in contact with the solder to aid heat transfer and the smoothness of the soldering process, which is different from wave soldering that requires flux to be applied to the entire PCB. Selective soldering is mainly applied to the soldering of plug-in components on PCBs to meet the demand for soldering accuracy and localised soldering.
Selective Soldering Process Flow
Flux Application: This is a critical first step to activate the flux and prevent oxidisation and bridging of the PCB after the solder has been heated and the soldering is complete. This process usually involves a robot holding the PCB and applying the flux through a flux nozzle, which can be applied in a variety of ways, including single nozzle and microvia spraying. Microvia spraying is known for its precision, ensuring that the flux is applied only to the area of the solder joint, avoiding contamination of the surrounding area, which is particularly important during the microwave peak soldering phase after reflow.
Preheating stage: the main purpose of this step is to remove excess flux in the non-soldering area, while adjusting the viscosity of the solder before entering the wave to the appropriate state. The preheating temperature is determined by the thickness of the PCB, the size of the device package and the type of flux. On the preheating and flux application order, there are different views in the industry, some people advocate the first preheating and then apply flux, there are some people believe that preheating before the soldering preparation is not necessary.
Drag Soldering Operation: This is performed using a small, single-nozzle waveform and is suitable for soldering in confined areas. Due to the relative motion of the board and the solder, the heat transfer efficiency of drag soldering is better than that of dip soldering, which can effectively remove oxides, reduce the bridging phenomenon, and improve the robustness and reliability of the soldering. However, drag soldering has some limitations on the pin length and longer cycle times, and does not have some of the capabilities of dip soldering. But according to the specific characteristics of the circuit board, drag soldering and dip soldering may have a different effect.
Dip Soldering Procedure: Fully immerse the PCB into the customised nozzle plate and complete the soldering of all solder joints in one pass. Different PCBs require different tooling boards. There is a difference in capability between dip and drag soldering, but when used in combination, they create a solid soldering process with short cycle times.
Selection of welding process and its materials
Flux Functions and Selection:
Fluxes play multiple roles in the soldering process, including removing oxides, reducing the surface tension of the solder to facilitate wetting, and covering the joint prior to soldering to prevent secondary oxidation. The choice of flux is based on the cleaning and electrical performance needs of the product. According to the cleaning requirements, flux can be divided into no cleaning, water cleaning, half-water cleaning and solvent cleaning of four; and according to the degree of rosin activity, can be divided into inactive (R), medium activity (RMA) and full activity (RA) three. For high requirements of electronic products, such as military and life support equipment, must be clean flux; communications, industrial equipment, office equipment and computers and other electronic products can be used free of cleaning or cleaning flux; and general household electronic products can be used free of cleaning flux or RMA type rosin flux, and the latter can also be used in the case of non-cleaning.
Flux spraying technology:
With the wave soldering of the comprehensive spraying is different, selective soldering using selective spraying, that is, according to the position and shape of the PCB solder joints flexible choice of spraying methods, including point spraying and line spraying.
Importance of preheating session:
Preheating is critical in the soldering process, and its role includes evaporating solvents in the flux to reduce gases generated during soldering, activating the flux to remove oxides and contaminants, protecting the metal surface from re-oxidation, and preheating the printed circuit board and components to avoid thermal stress damage. Preheating method is divided into the bottom preheating and top preheating, the former using short-wave infrared, preheating speed and flux effective components of the destruction of small; the latter is the use of hot air, applicable to fully preheat the PCB and heat dissipation of faster components, but need to pay attention to avoid OSP treatment of PCB pads after the secondary oxidation of copper foil.
Preheating parameter setting principles:
Preheating parameters are set mainly based on the performance parameters of the flux to ensure that the preheating is completed after the temperature of the PCB solder joints in the flux activation temperature range (such as alcohol-based flux commonly used in the market, the activation temperature range is generally 90-120 ° C). For the PCB containing heat-sensitive components, preheating parameters need to be set to take into account the temperature requirements of heat-sensitive components; and for faster heat dissipation PCB, in order to achieve the requirements of the fill rate of the solder joints, and sometimes need to appropriately increase the temperature of the preheated solder joints. In the case of flux performance parameters can not be taken into account, should give priority to the requirements of the welding fill rate.
Difference between selective soldering and conventional soldering.
Welding method
Selective soldering involves precise soldering of specific weld points, often with automated equipment, enabling the soldering of single or multiple specific areas. This method reduces the thermal impact on the entire component and thus avoids thermal damage.
In contrast, conventional welding methods typically heat the entire joint, meaning that most of the area is exposed to heat during the welding process. This can lead to more pronounced thermal distortion and other accompanying problems.
Thermal management
In selective soldering,the soldering takes place only at specific locations, allowing for effective control of heat input.This means that less heat is applied to the surrounding components, reducing the risk of thermal damage.
In contrast, in conventional soldering, the greater heat input tends to cause thermal deformation of the surrounding material, which may affect the quality of the solder and the performance of the electronic components.
Welding Accuracy
Selective soldering offers very high soldering accuracy and is suitable for use in dense circuit designs, enabling high quality soldering of small solder joints. Most selective soldering techniques ensure consistent and accurate soldering, reducing soldering defects.
Conventional soldering is relatively less accurate and may struggle to achieve the required accuracy, especially when dealing with complex and dense cloth boards.
Level of automation
Selective welding is usually performed using fully or semi-automated equipment, offering the advantages of high efficiency and repeatability. High levels of automation are designed to make the production process more stable and reduce the errors associated with manual handling.
In contrast, conventional welding often relies on manual welding, which is less efficient and the quality of the weld is affected by the skill level of the operator.
Selective soldering technology has taken its place in electronics manufacturing with its unique advantages. Its precise operation for specific welding points effectively reduces the thermal impact on the whole assembly, thus reducing the risk of thermal damage to a certain extent. At the same time, the high level of precision and automation of selective soldering improves soldering consistency and productivity. However, it is worth noting that every welding technology has its applicable scenarios and limitations, and selective welding is no exception. In practical applications, it is necessary to select the appropriate welding technology according to the specific needs of the product and process conditions to ensure the best welding results and product quality.
