Abstract: for many years, in the SMT assembly, the welding of the through hole element has been using the selective welding method, and has been in use until now. Are introduced in this paper two kinds of selective soldering methods: drag soldering and welding, and describes the various characteristics of the two methods, and use different.
Are you going to use selective soldering in manufacturing? If you use it, you must discard two main misunderstandings. The first misunderstanding is the use of selective soldering usually choose suitable welding equipment, with the kind of "bells and whistles" equipment. If not used, the welding process is more important than the welding equipment.
Each selective soldering method has its own characteristics, and it needs to have its own unique processing methods and welding tools. In order to meet these requirements, the user must fully understand the welding process, in agile communications on the basis and equipment supply manufacturers keep good relationship, and a for arbitrary demand and problems can make rapid response to global logistics support system.
The second misconception is that selective soldering is only using the wave soldering of some different waves. Selective soldering is completely different from wave soldering. The most striking difference may be in the wave soldering, PCB's reverse side is completely immersed in the flow of solder. In selective soldering, the solder can only be exposed to a specific area. Because the board's own material is not an effective thermal conductivity of the media, the characteristics of the adjacent components and should not be heated into a liquid PCB area is very useful.
The selective heating in a disadvantage, for example, outside the connector pin transfer to the board heat to pin more than the internal, and low temperature outside the area is larger, therefore, connector pins will appear temperature difference (delta T). Therefore, it is necessary to optimize the welding process in order to obtain a better welding effect.
Successful selective soldering requires a completely new approach. Fully understand how the welding process and equipment to meet the needs of the user and equipment suppliers to establish a cooperative relationship between the key factors.
What is selective soldering? In selective soldering, the specific device on the PCB is carried out from the following "selective" welding. Selective soldering is different from wave soldering. In wave soldering, PCB is delivered to a fixed wave crest in a straight line. And selective welding is to use the robot to move the fixed welding nozzle and processing tools on each board.
Selective soldering using two different techniques: drag soldering or welding, the welding area and component. The mobile PCB can move in different directions. The solder is positioned on the jet, while the board moves on the sprayed solder.
Machining tools are based on PCB and components, and each is not the same. According to the cost, the catalogue and the change of the processing tools, the larger change of the required processing tools is considered as a kind of shortcoming. It has a lot of flexibility, so that the structure of the board is competitive. However, the processing tool does not explain everything. Must observe a certain PCB design, to see whether it is suitable for this process, so manufacturers and designers of the board must discuss the countermeasures.
Another advantage of selective soldering is the ability to program each board, or even each component, in a special way. In addition, a board, a series of board or the shift in the production of various kinds of comprehensive application of method can completely achieve repeatability and consistency of automated production.
Construction method
In selective soldering, only through hole element. The key purpose of selective soldering is to reduce the impact of the mounting elements and other adjacent materials on the surface of the board. Because each PCB design layout and materials are not the same, the design of selective welding equipment using the software, which is suitable for different structure.
There are two kinds of welding process and welding selectivity, dip drag. In brief, drag welding is carried out on the crest of a small, separate nozzle. A very dense area, a separate point, or a lead wire. For example, a single row of leads can be welded with a drag. Can be different speed, different angles of transmission and welding of PCB, to achieve the purpose of optimizing the welding effect.
By comparing, dip including will the whole PCB leaching in the common processing nozzle plate by a welding operation is completed all solder joint welding. Although a minor finish a lot of solder joint, but each type of PCB separate tool board is required. This requirement is different from the wave soldering method is an important point.
Of course, the flux must be applied to the lead wire to be welded. Once again, the PCB does not need to apply flux to any part of the weld. So, according to the type of welding process, flux coating method using point-to-point. When using selective flux point coating machine, single head or double head flux coating machine, PCB runs on top of it. Using the multi spot welding flux coating, PCB can be positioned and installed, at the same time, the flux coating machine running under the board, spray liquid drop pre programmed positioning.