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Selective Soldering With Added Flexibility
The SelectLine (left) and SelectLine-C (right) are selective soldering systems that can be reconfigured by means of add-on function modules.

Selective soldering systems provide a great deal of capability for electronic manufacturing facilities, but they also require some education to learn how to gain the maximum benefits from those systems. For a typical printed-circuit board (PCB), components to be selectively soldered are usually surrounded by parts that have been attached by more traditional soldering methods, such as reflow solder or even hand soldering. As a result, any selective soldering system must provide the precision and control to properly attach the desired components without causing damage to any previously soldered components. Fortunately, the SelectLine of selective soldering systems from SEHO Systems GmbH ( provide the soldering precision and repeatability needed for controlled selective soldering without danger of damaging previously attached components or portions of PCBs. In addition, these selective soldering systems offer a great deal of flexibility for working on different types of PCB assemblies without having to completely reconfigure the system.
The Twin-Select concept is based on the use of independently controlled EM solder units installed on separate Z axes.

Effective selective soldering systems should provide the control of soldering materials needed to avoid any waste of those materials. In addition, such systems should help minimize production time, in order to also minimize associated production costs. The SelectLine soldering systems employ micro-drop jet fluxers capable of high precision and repeatability along with minimum consumption of soldering materials. Well-defined flux applications can be controlled in extremely small areas by as many as three nozzle heads on each SelectLine XY axis portion. These selective soldering systems are constructed with a modular design, allowing selection of various preheat and soldering modules to achieve a desired configuration. In fact, depending on production requirements, these selective soldering systems can be equipped with as much or as little capability as needed for a particular application or set of applications. With the micro-drop jet fluxers available for the SelectLine soldering systems, multiple similar fluxers can be used for high-throughput applications. When greater flexibility is needed, two different flux types can be used simultaneously with these multiple fluxers, running under software control.

For any selective soldering system, process control starts with flux deposition. In the SelectLine selective soldering systems, both the filling level in the flux container and the function of the drop jet nozzles are continuously monitored under computer and software control. The flux quantity control in these systems actually measures the quantity of each drop of flux/solder that is applied to a printed circuit board (PCB).

Flexible Preheating
A selective soldering system requires a reproducible and effective preheating process to activate the flux and achieve optimum wetting activity. For flexibility, the preheat area of each SelectLine selective soldering system can be configured individually and by length and type, with excellent energy efficiency. Effective transfer of heat to the bottom side of a target PCB is achieved by quartz heating elements that can be individually activated. If necessary, additional infrared (IR) heaters can be installed in one of the selective soldering systems to provide topside PCB heating. Both the bottom-side and top-side heating subsystems are controlled by means of the system software, for highly precise and reproducible soldering temperature profiles.

For large-mass PCB assemblies, where a greater amount of heating may be required, the preheat module on these selective soldering systems can be equipped with convection heating capability. Such capability ensures complete and effective heat transfer with homogeneous temperature distribution within the preheat zone even for large-mass PCB assemblies.

One concern for selective soldering systems during long soldering cycles is maintaining a PCB assembly at the same required temperature. For this purpose, a topside heating unit can be included in the soldering area of a modular SelectLine selective soldering system. As with the other modular heating units, this additional heating unit can be monitored and controlled by means of the computer software, with a contactless pyrometer installed for precise control of the preheat temperature profile.

The SelectLine selective soldering systems feature extremely flexible and precise modular soldering areas. The electromagnetic (EM) soldering units employed in these systems provide extremely stable and accurate wave height, combined with precise solder nozzles that effectively control solder flow for the amount of energy consumed. These systems incorporate Twin-Select dual solder pots for selective soldering flexibility. With this concept, two EM soldering units are installed on separate Z axes and can be programmed and controlled independently. This eliminates changeover times and allows the use of two completely different solder alloys in the production of a PCB assembly. In addition, if these soldering units are equipped with soldering nozzles with different diameters, this Twin-Select dual-solder-pot concept can significantly reduce cycle times. For example, connectors on a PCB assembly can be soldered in one pass using a larger solder nozzle while finer structures on the same assembly are soldered by means of a smaller-diameter solder nozzle, in contrast to changing over solder nozzles for different passes on the different-sized structures to be soldered.

Innovative Solutions
For demanding throughput requirements, the SelectLine selective soldering systems employ the Synchro concept (patented by SEHO) which essentially allows synchronous production of two PCB assembly lines using one selective soldering system. The modular design of these systems allows them to be equipped with additional soldering modules or with a dual conveyor belt for parallel processing of PCB assemblies. The Synchro concept is an intelligent software feature that coordinates the soldering process for two PCB assemblies in such a way that total throughput is nearly doubled without the need for significant investment. The same soldering program is loaded for both soldering units synchronously, working in an infinite loop. The assemblies can enter the soldering area independently and are not linked to the same cycle. With the new Synchro concept, cycle times can be reduced by nearly 50 percent.

Soiled solder nozzles can greatly impact the repeatability of any selective soldering system. In the SelectLine systems, the negative effects of soiled or clogged solder nozzles are avoided by means of automatic ultrasonic cleaning of the wetted miniwave solder nozzles. What had been done previously through the manual application of aggressive cleaning chemicals is automatically achieved in the SelectLine selective soldering systems in an environmentally friendly way by means of automated ultrasonic cleaning through the machine. As part of this process, solder nozzles receive a gentle cleaning and then are completely rewetted. In addition to saving time and contributing to high soldering process repeatability, this cleaning approach also results in longer solder nozzle operating lifetimes.
Solder nozzles on SelectLine soldering systems are kept clean and clear through the use of automatic ultrasonic cleaning.

Reducing production costs while maintaining high quality is critical for any electronic production process. Because manually controlled repair soldering processes can be expensive and time-consuming, while also providing inconsistent soldering results, many production facilities look to automated selective soldering systems as an important step towards achieving a zero-fault production process. By integration of the required hardware and software as in the SelectLine selective soldering systems, a high degree of process control is possible. Precise positioning of the soldering modules for each PCB assembly is an important part of achieving repeatable and reproducible soldering results, and in the SelectLine systems, automatic position correction is achieved by means of fiducial recognition.

Each system's software automatically compensates for various types of misalignment, such as offset or linear shrinkage within a PCB assembly. These automated systems also help to control correct loading of the soldering program and appropriate feeding of PCB assemblies for each project. The automatic Z-height correction recognizes warped assemblies caused by previous thermal or mechanical loads, and automatically calculates correct Z values for all points of the soldering program. Stable wave heights are ensured using a measuring needle or a high-precision laser micrometer. Because the solder level in the solder pot can affect the stability of the wave, it is controlled and solder wire is supplied automatically if needed.

These systems also use computer-controlled flux quantity control to ensure accurate flux deposition in terms of amount and location. Contactless pyrometer control in the preheat area monitors the temperatures across a PCB assembly and helps provide reproducible temperature profiles.

Cameras provide additional safety and process control in the SelectLine systems and can even can be integrated into the offline teach program for fast parameter optimization. The cameras enable solder-joint inspection immediately following the soldering process, so that faulty PCB assemblies can be removed from the inline production path and moved t a repair workplace. These compact selective soldering systems provide benefits not only in cost but in terms of floor-space requirements and process optimization.

Each SelectLine selective soldering system is run under the command of mcServer machine communication software. The software, which meets Industry 4.0 system requirements, provides comprehensive control of the soldering process with real-time access to all connected machines that are installed in different production sites worldwide. Additionally, it is possible to directly link to cameras integrated in a machine. Using a straightforward user interface, the mcServer software collects, analyzes and archives all information about the machine and production process. With a specific serial number, the entire process for a single PCB assembly can be traced. Production statistics, user statistics or AOI statistics can be generated as easily as comprehensive reports for documentation. Using appropriate interfaces, every machine can be integrated into nearly each specific MES/ERP system for superordinate control of the process.


Contact: Seho North America, Inc., 1420 Jamike Drive, Erlanger, KY 41018 859-371-7346 fax: 859-282-6718 E-mail: Web: or

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