The term soldering denotes thermal processes for joining parts. In contrast to welding, they work not by melting the parts to be joined (exceeding their solidus temperature), but rather only melting a solder (soldering agent, filler metal), which has a lower melting point. When the solder solidifies after removing the heat source, it forms a firm connection with the parts to be joined. Some amount of diffusion of material can support the strength of the formed solder joint (diffusion soldering). A flux is often used to facilitate the soldering process, e.g. by inhibiting detrimental effects of oxidation or dirt and by wetting (reducing the surface tension).
Frequently, soldering produces an electrical connection, e.g. in an electronic circuit, and not only a mechanical connection.
Related to soldering is desoldering, i.e., dissolving solder joints. For example, defect electronic chips need to be desoldered from circuit boards for repairs.
An essential advantage of soldering in comparison to welding is that the process can often work at a substantially lower temperature, reducing the risk of damage and decreasing the processing time. Also, it facilitates the joining of dissimilar materials, e.g. having substantially different melting points. Coatings can often be preserved – for example, zinc coatings on steel, which often cause problems in welding processes, while they can even support high-quality soldering. On the other hand, the requirement of a solder, which must be properly applied during the process, can introduce various additional complications, e.g. related to toxicity.
Laser Soldering Methods
Laser soldering utilizes the very controlled heating by absorption of laser light, provided in the form of a moderately focused laser beam. The laser beam diameter is often well below 1 mm, e.g. for fine soldering applications in micro-electronics, but can also be a few millimeters for some soldering processes on larger parts.
Special laser processing heads are used for laser soldering. The main functions of such a laser soldering head are the following:
- The laser beam is focused to the right position on the workpiece. An integrated laser scanner may provide increased flexibility of positioning without moving the head as a whole.
- Often, the solder is also applied through the processing head, or with some facility attached to it. It can come in various forms, depending on the process requirements – for example as a wire, as small droplets or irregularly shaped solid fragments, or as a powder. In some cases, the solder is deposited in a separate process before starting the soldering.
- Some means for process monitoring (e.g. a camera) are also often integrated into the head.
The applied solder materials are usually some (often eutectic) metal alloys such as tin/lead, tin-zinc, lead/silver or tin/silver. (Unfortunately, some common solders contain poisonous materials such as lead or cadmium; one more and more tries to replace such substances.)
For the soldering of ceramics, glass materials are used.
According to the different melting temperatures (and thus process temperatures), one distinguishes soft soldering, hard soldering and high-temperature soldering:
- Soft soldering (below 400 °C) can be advantageous e.g. in electronics for avoiding the damage to components, but is obviously not applicable to parts which need to withstand high temperatures. One often requires some kind of flux material, which may be applied together with the solder or separately. The required laser power is normally between 30 W and 100 W.
- Hard soldering (e.g. at 500–600 °C), often done with multi-kilowatt laser powers applied to spots with a few millimeters' diameter, can produce mechanically more stable connections and often works without a flux. It is often done with silver-based solders, then also called silver soldering.
- High-temperature soldering processes are applied to ceramics, for example.
Processes with working temperatures above 450 °C are also called brazing.
A process gas like nitrogen is often applied for soldering metals at higher temperatures. It can effectively prevent oxidization of the heated surfaces, supporting or replacing the function of a flux. When using solders in the form of powders, the process gas may also be used to transport the solder to the soldering spot.
Discrete Point vs. Line Soldering
Some laser soldering processes work on discrete points, for example for connecting wires on circuit boards, while line soldering methods (with the solder joint being a straight or curved line) are also common for the mechanical joining of various kinds of parts.
Process Control and Automation
In industrial manufacturing, laser soldering processes are often automated and fully integrated into a larger manufacturing environment. In other cases, hand soldering is applied, similar to traditional methods with a soldering iron (contact tip soldering).
Accurate process control is essential for high-quality results. One often first heats the location, then applies the solder and continues the heating for a short while. The laser beam may then be switched off abruptly, or one gradually reduces the laser power to give more time for solidification.
The reached peak temperature, a critical process parameter, depends on the applied laser intensity and heating time, but also on heat conduction in the given setting (which may vary, e.g. due to the solder flow). A disadvantage of laser soldering compared with tip soldering can be that there is no guaranteed maximum temperature (set by the temperature of the soldering tip). One may solve that problem with proper process monitoring, for example utilizing the heat radiation.
Applications of Laser Soldering
Laser soldering processes are utilized in many areas of industrial fabrication. In the following, some typical examples are described.
One of the early applications of lasers in manufacturing was laser soldering of mainsprings in mechanical watches. Such a mainspring, a spiral ribbon of fine spring steel, is the essential part of the mechanical oscillator which determines the accuracy (time drift) of the watch. It needs to be attached to some base material in a careful and reproducible manner. Traditional soldering methods are not very suitable for this process, not producing sufficiently fine and reproducible results.
It has been recognized long ago that the melting of the tiny amount of solder can be accurately done with a single pulse of light, delivered as a laser beam. This was originally generated with a small lamp-pumped YAG laser; nowadays, diode-pumped lasers or direct diode lasers are used. The early development of lasers which are suitable for fine soldering has initiated the formation of a much larger and very versatile laser industry in the Black Forest in the southwest of Germany, for example.
Similar fine soldering processes are used in other areas, e.g. involving sensors.
In the context of fabricating automotive bodies, e.g. doors, it is often necessary to join metal parts. Although methods of laser welding are widely used for such purposes, soldering processes are also very important because in various cases they work better than welding. In particular, they make it easier to join parts consisting of different types of metals, having substantially different melting points. Also, soldering processes are less affected by protection coatings, for example of zinc. Other advantages are faster processing speeds and a better look of joints after varnishing.
Direct diode laser technology is increasingly used for flux-less hard soldering in car manufacturing. Multiple kilowatts of laser power at wavelength around 0.8 μm to 1 μm can be generated with diode stacks, particularly when also using spectral beam combining, and delivered through multimode fibers. The moderate amount of beam focusing can be done even with non-perfect laser beam quality.
Laser soldering is also essential in micro-electronics, where it is used mostly for making electrical contacts. For example, one uses the solder bumping technique, where a single small solder ball is applied through a capillary, together with the process gas (e.g. nitrogen) and the laser beam. A single laser pulse can melt the solder to produce a solid connection.
Such techniques are particularly needed where many closely spaced contacts need to be made, e.g. for microprocessors with many pins.
Credit cards contain microelectronic chips which need to be equipped with contact pads. These are soldered to the chips with automated laser soldering machinery.
For the fabrication or the repair of jewelry, fine soldering processes are needed. Usually, one uses silver-based solders, which require relatively high process temperatures – more than is possible with a soldering iron. The high temperature together with the need to process very fine structures with good-looking results favor the use of laser soldering in this area.
Soldering of Ceramics
Ceramics can be used for encapsulating temperature sensors and other types of sensors, which need to work at high temperatures and/or need to be well protected against chemicals. After inserting the actual sensor device into a ceramic capillary, at least one end must be closed in a stable, tight and durable manner. This can be done, for example, by soldering another ceramic piece to the end of the capillary.
Some glass materials are suitable as solders for such purposes. They melt at a temperature (e.g. 1300 °C) which the ceramic can easily withstand, but which is still well above the operation temperature of the sensor. Upon solidification, the glass melt can form a stable connection with the ceramic.
The moderate heat conductivity of ceramics, as compared to metals, is beneficial for the soldering process. At the same time, ceramics exhibit reasonably efficient absorption of laser radiation. For such reasons, quite high quality of the joints can be achieved.
Some medical applications have similar requirements, although the high temperature resistance may not be relevant here.
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