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Selective Soldering — Options and Opportunities
December 31, 1969 |Estimated reading time: 8 minutes
Due to increasing pressure to improve product quality and shorten cycle times in many electronics manufacturing processes, it is becoming more advantageous to integrate selective soldering techniques such as miniwave, laser or hot gas systems. Selective soldering can meet many process requirements, offering companies an opportunity to minimize product size, reduce costs and improve quality.
By Heike Schlessmann
Looking at process development in the electronics industry over the past few years, a trend towards the use of reflow soldering technology is evident. While the wavesoldering process remains a part of electronics production, it clearly has lost ground. Unfortunately, not all components are available as SMDs, or are much more expensive than their respective leaded components.
For a long time, manually soldering remaining conventional components into place was considered acceptable. Due to increasing pressure to improve product quality, shorten cycle times, and reduce non-controllable variables, however, demand is for new, more appropriate production processes.
In principle, conventional components also can be processed by reflow. This method means all soldering joints can be processed at the same time, keeping production costs to a minimum. However, reflow technology reaches its limits when temperature-sensitive leaded or surface mount components are soldered into place.
Additionally, many factors must be considered when reflow soldering conventional components, such as pin length or the size of soldering joints relative to the size of the pin. In addition, the amount of soldering paste, which has been removed while placing the leaded components, may never be precisely defined, thus becoming an extremely critical parameter.
Consequently, in a majority of applications it makes sense to integrate a selective soldering system after reflow soldering. This will make it possible to economically solder the remaining components and avoid problems with lead-free soldering. The most frequently used processes are miniwave soldering and laser soldering. However, hot gas soldering also is gaining ground.
Selective Miniwave Soldering
This method is the most frequently used, and several systems are on the market. Three basic variations are available: systems with robot handling, as well as axis and pass-through systems.
Selective soldering systems with robot handling are featured with highest precision and flexibility with regard to the handling and soldering process.
Modular-designed systems may be completely tailored to meet specific production requirements. They even may be upgraded to suit subsequent production developments at a later date.
The robot's action radius may cover different fluxer systems, preheaters and solder baths. Thus, different soldering processes may be performed with the same machine. Simultaneous processes shorten the cycle times enormously. Processing multiple panels in combination with multiple nozzles leads to the highest throughputs. The same goes, of course, for the fluxers.
In the preheating area, which, depending on application may vary in length, there is a choice between software-controlled quartz radiators or a convection module. Miniwave systems with robot handling also feature flexible fluxing and soldering applications.
In addition to spray fluxers with one spray nozzle or a micro drop-fluxer, which excludes any contamination of surrounding areas, a simultaneous spray fluxer also may be integrated in these machines. Additionally, especially for high throughputs a separate XY fluxing module may be installed in front of the soldering system.
The soldering process with the well-established miniwaves, in the form of single and/or multiple solder waves, may be flexibly upgraded with additional solder baths for dip soldering. With the dip process, even geometrically difficult points may be reached and elegantly soldered. Dip soldering processes are used frequently, especially in the automotive industry. In this business sector, high throughputs and short cycle times range higher than flexibility in soldering process. Multi-nozzle tools for dip soldering processes ensure high soldering quality and shortest cycle times with both lead-containing and lead-free solder alloys (Figure 1).
Figure 1. Selective soldering system with two solder pots. Miniwaves are for drag soldering processes, and multi-nozzle tools for dip soldering processes shorten cycle times.
Depending on requirements, different grippers such as claws, tension or vacuum units may be used. Mask and direct gripper systems also are available. Even adjustable grippers to suit a particular assembly may be integrated in the machines. Additionally, the printed circuit board (PCB) may be turned 180°, permitting processing of the top and bottom side.
Selective miniwave soldering systems with robot handling feature highest precision and soldering quality. Decisive for high soldering quality is, among others, the handling process.
With its constant precision of 0.05 mm and directional function for non-planar PCBs, the gripper guidance unit offers maximum precision. This precision repeats itself in each subsequent working station as follows:
The fluxer targets the flux to the exact point on the solder joint to be wetted. The micro-drop fluxer makes any contamination of the surrounding area impossible. The state-of-the-art nozzle technology affects the highest degree of precision in the soldering area. Additionally, this technology permits small distances between the solder joints. It also is possible to solder points that are difficult to get at.
Another reason for a defined soldering profile and the resulting high soldering quality is an oxide-free wave, affected by a locally operated nitrogen gas unit. This reduces maintenance to a minimum.
An active solder wave height measurement guarantees the maximum in process reliability. Modern systems with robot handling are equipped with a touchless wave height and solder level sensor. The wave height, as well as the solder level, is permanently measured without any influence on the cycle time.
Selective miniwave machines with axis systems are featured with nearly the same precision than systems with robot handling; however, they usually do not offer the same flexibility. In any case, the user should pay special attention to a modular construction of the system to make sure that there is a possibility to reconfigure the soldering system at a later date in case production requirements should change.
Figure 2. Pass-through system with product-specific fluxing and soldering nozzles.
Pass-through selective miniwave systems are operated with product-specific spray fluxers and solder nozzles (Figure 2). They are not as flexible as the robot systems with regard to product variations, but allow large volume production with short cycle times. Depending on the size of the PCB to be soldered, these systems feed one or more assemblies into the fluxing, preheating or soldering modules. All joints will be processed in parallel and at the same time.
Selective Laser Soldering
This is the most frequently used process on PCB layouts where some solder joints may not be soldered risk-free even with a miniwave process, such as with difficult-to-access solder joints or with surface mount components on the surface of the PCB. This process also is advantageous when processing temperature-sensitive substrates such as MIDs, as these may not be reflow soldered because of the temperatures governing the process.
Laser soldering machines apply the required solder volume via an integrated solder wire supply, an integrated solder paste dispenser or an already available reflow solder depot with the necessary additional fluxing.
New opportunities offer laser soldering systems of the latest generation with a 2-D scan head. This unit makes it possible to redirect the laser beam within a maximum operating area of 80 × 80 mm (3.12 × 3.12"). In addition to positioning the laser beam on the solder joints, different laser beams may be produced, such as linear, circular or triangular arrangements in different dimensions (Figure 3). Thus, parts of casings may be economically soldered. Furthermore, the scan head makes it possible to cycle two IC sides, meaning it may heat them simultaneously.
Figure 3. Laser soldering system. New scan heads allow different laser beams to be produced.
The laser capacity of selective laser soldering systems is supplied without oscillations, drifts or wear, and it is totally reproducible. Additionally, the laser energy supply may be precisely and linearly tailored. Laser systems apply all energy directly to the solder joint, without any effect on its direct environment.
Round Table Selective Soldering
These systems usually are laid out as miniwave systems, but it also is possible to equip them, for example, with a laser soldering station. Depending upon the application, up to six workstations may be realized, such as an loading/unloading station, PCB turning station, fluxing, preheating, soldering and another PCB turning station.
At the fluxing station, a spray fluxer with product-specific mask or a micro drop fluxer may be implemented. Variations also are possible at the preheating station. Depending upon the application, either an IR heating station or a convection module are possibilities.
The solder nozzles also are product-specific. The solder pot with the nozzles, however, is not stationary but moves upwards for soldering and downwards after the process. To guarantee the highest process reliability, the solder station may be equipped with a solder wave height measurement and automatic solder wire supply. High soldering quality is achieved due to oxide-free waves, affected by a locally operated nitrogen gas unit.
Roundtable selective soldering systems are known by their flexible concept and high reliability. They either may be used as standalone systems or integrated into an existing production line.
Selective Hot Gas Soldering
These systems are the quick, low-cost alternative to laser systems. They are suited for processing single surface mount components that cannot be soldered with a reflow system due to their thermal sensitivity.
Depending on the application, pretinned or not pretinned components may be processed with selective hot gas soldering systems. For not pretinned components, the machine optionally can be equipped with a micro-drop jet fluxer and a dispense unit for solder paste. The soldering process itself is performed with a hot air nozzle. All workstations are fixed at a three-axis robot, which allows operation in the x-, y- and z directions.
Conclusion
There are enough different techniques and machine technologies for selective soldering processes to meet the most disparate requirements. Nonetheless, the user always should pay special attention to throughput requirements, process flexibility as well as the lead-free soldering process. This new technology especially will require a new way of thinking. Since lead-free solder alloys usually show a lower specific gravity compared to conventional SnPb solders, the flow properties of the solder will change. This may require new nozzle geometries to minimize solder faults, such as bridging.
Automatic selective soldering will grow in importance. This process helps ensure high quality standards in electronic production processes. Such an advanced machine technology offers the electronic designer more possibilities to minimize product size, reduce costs and improve product quality.
Heike Schlessmann may be contacted at SEHO USA Inc., 325 L Hill Carter Parkway, Ashland, VA 23005; E-mail: sehousa@aol.com.