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Soldering with Diode Lasers
December 31, 1969 |Estimated reading time: 12 minutes
Soldering
with
Diode Lasers
Two trends have lifted laser soldering into new prominence: the replacement of wave soldering with reflow soldering, and the emergence of diode lasers with increased power output and high service life.
By Armin Rahn, Ph.D.
asers (light amplification by stimulated emission of radiation) are used in many branches of industry. Cutting, hardening and heating are common applications. For years, eximer, Nd:YAG and ion lasers have dominated the field. Several of these have been tried in soldering with debatable success. Then, in the ,90s, diode lasers achieved outputs that permitted their use in soldering. These lasers are available to approximately 2,000 W, where the usual range for soft soldering lies between 40 and 60 W (Figure 1). As lasers go, the quality of the light produced by diode lasers does not match that of more traditional ones. When soldering, however, this is an advantage, as the nonfocal area is used to avoid too fast an energy transfer.
Advantages of diode lasers lie in their cost - they are much cheaper than traditional lasers - and their life expectancy. Current diode lasers are guaranteed up to about 5,000 hours; their actual working life, however, is at least 10,000 hours - or up to 50 million solder joints. The fact that they are easily modulated and pulsed favors their application in soldering.
Maintenance is extremely low and limits itself to the cooling unit and the occasional cleaning of the outer optical system.
Tasks
Today`s usual laser soldering application is limited to special functions. Most prominent are those soldering situations where neither wave nor normal reflow soldering may be applied, among them the soldering of components with thermally sensitive bodies or substrate material that cannot be exposed to higher temperatures. Molded interconnect devices (MID) where the 3-D printed circuit board (PCB) offers an additional challenge are an extreme example.
Lasers are suitable particularly for joining flat cables or high-temperature applications where a spring (e.g., for a battery) has to be "riveted" with solder. Smart Cards offer the challenge of joining the antenna to the chip, a job best accomplished with a laser (Figure 2).
In all these cases, laser precision is required to minimize thermal exposure to the surrounding area. Obviously, the laser beam needs line-of-sight access to the joint. Only in rare cases may the joint actually be heated through the substrate material.
Laser soldering has also gained a standard place in selective soldering - i.e., soldering of the remaining through-hole components after reflow. It either supplements or replaces mini-wave soldering.
Options
Depending on the object, the laser may be used in two different ways: as a point source of energy, or (for multiple joints) to produce lines with the laser light. As a point source, laser soldering becomes a strictly sequential process - joint after joint is soldered. The time needed to heat the base metals and to melt the solder amounts to between 500 and 1,000 milliseconds (ms). As in the case of many selective soldering applications, the few remaining solder joints will be completed in the allotted cycle time. Adding the time needed to move from one joint to the next will give an idea as to how many joints can be completed within 10 to 20 seconds.
The amount of energy required to melt a joint with slightly low paste deposition is 2.4 Ws; i.e., with a power setting of 8 W, the laser needs 300 ms to complete the soldering task. Assuming that another 300 ms are needed for beam positioning, the total time per joint is slightly more than 0.5 seconds.
If, however, multi-leaded devices need to be joined, a point-by-point approach requires too much time and may actually lead to induced stress. Here, the laser line - or several lines from several lasers - may be the correct solution (Figure 3). The line covers all of the leads on one or more sides of the component, and all are soldered simultaneously. To safeguard the laminate, soldering times must be extended substantially. Typical exposure times for laser bars range from about 4 to 20 seconds.
Solder Deposition
Laser soldering is a typical reflow operation. Hence, the solder must be deposited independent of the heat source. There are several options; most obvious is the use of paste that is applied either in a stencil printing operation or by dispenser. When paste is used, the laser power must be controlled exactly. Otherwise, the impact of the laser leads to paste splattering or explosions. The sudden heat can cause the solvent to sublimate quickly, and the rapid expansion ejects solder particles from the paste.
Preforms have been applied with success. Their main advantage is the use of a precise amount of solder and flux. Preforms are available in a large variety of shapes and sizes, as well as in custom configurations. However, placing the preforms is usually an exacerbating task. Furthermore, they have to be held in place, which is accomplished by sticky flux coatings or by placing the leads through doughnuts.
Because a general reflow process usually precedes selective laser soldering, some companies have opted to supply solder by printing paste on the required surfaces prior to reflow. The solder amount required to form the joint is thus present on the pads. The component, though, has to be held in place mechanically during soldering, and additional flux may be required to ensure proper lead wetting.
The same situation is encountered if deposition technology is used. Here, the different methods supply a flat solder deposit. The first reflow pass will automatically destroy the flatness and, thus, the surface reference. However, the solder amount remains and may be acceptable for selective laser soldering. Again, the question of how the component is positioned and held in place will have to be solved.
A common form of solder supply is the feeding of solder wire. The accuracy of the wire-feeding mechanism determines process repeatability. Contrary to the usual practice in hand-soldering operations, the solder does not need to be melted on the pad or component lead. Once the required heat has been determined, the process ensures proper reflow by providing the necessary energy.
Fluxing
If paste is applied, flux is present and is applied simultaneously. Preforms usually contain flux as well, so the question of flux deposition limits itself to two cases: wire feed and already existing deposits.
Because of geometric location, fluxing can cause a problem. Several application methods have been tried: spray, dip, wave, brush, stamp and roller. Whether any of these is successful depends on the user-imposed boundary conditions. Deposition localization is usually better with direct application methods than with any of the spray methods. However, as direct application methods have to contact the board, the free space around the joints is critical. Direct methods commonly deposit more flux than sprays, making it important to note the balance between available activity and rest contamination. Laser heating is very much restricted to the joint - any flux outside the joint will not reach the usual temperature from normal reflow environments. The residue question, thus, is a different one.
Spray application can be rather precise and splatter can be limited, but never totally eliminated. Proper preheating can help limit flux spread prior to the soldering operation.
Preheat
Whether preheat is required at all when laser soldering is debatable. As the thermal load is very localized, general preheating would only "age" the other joints on the board. Localized preheating is often seen as a way of eliminating stress in the area. It has helped with filling holes when through-hole joints have to be formed in thicker boards.
Preheating may use convection, but infrared (IR) is more commonly applied. There are various preheating sequences, each with its own philosophy. The traditional place of preheating is post-flux application. Although flux does not actually increase activity substantially during preheat, solvents are evaporated and the spreading of the flux is somewhat limited.
Instead of fluxing and then preheating, flux applied after preheating has proven to work and to shorten cycle times in some cases. With the limited amount of flux supplied in spray soldering, solvents evaporate readily on the heated board. No studies have been done thus far on possible contamination around such application areas.
As in many repair stations, preheat also can be used during the soldering process. Here, the preheater is placed under the board and activated just prior to laser beam application. The two-action heat transfer is particularly helpful when hole filling is required.
Heat Transfer
The laser must have line of sight to the joint. Only in rare cases is it possible to melt solder on top of the PCB when applying the laser from below and through the board. Precise positioning of the laser beam is a must. If the target is missed, the power of the beam can damage the substrate; therefore, locating the board relative to the beam is a major prerequisite for successful execution. Vision systems can be helpful but will slow down the process. Locating the board by using local holes or other references is the preferred method. The next question is the amount of energy required to make each joint. The laser can be operated so that each joint receives the exact amount of power and time needed to make an optimal joint. However, determining this set of parameters still requires a fair measure of experimental effort, as no scientific technique has yet been established for it (e.g., using a thermal imprint from a pyrometer still lacks the reliability of a proper method and a scientific scale that determines "optimal," "good" or "deficient").
Joint reliability is one aspect of quality. Is it better to begin with a high-energy input and then reduce the amount, or would a reversed order yield "better" joints? Are there some tricks still to learn? A series of appropriate tests on the reliability of such joints is needed.
Defect rates during processes are the other quality aspect. Because of a high-energy exposure, solder balls are just an example of the problems that may be encountered. Others may relate to wetting or joint appearance, and could indicate internal problems such as entrapped flux and solvent residues.
As formidable as this approach may look, the amount of information available today is growing and there are already individuals with a rather good "feel" of how to set the power rating and the heat-transfer characteristics in each case. Some fine-tuning may still be required, but the overall approach is headed in the right direction.
Once set, the process repeats itself with great precision and the resulting joints faithfully resemble each other.
Incorporating the Selective Process
Most processes requiring selective soldering are mass production processes. To minimize cost and handling, as well as to reduce defect rates, such processes typically are fully automated. Any selective soldering process has to match in-line capability as well as cycle times. Laser soldering offers a variety of options, whether in-line or stand-alone, single point or simultaneous.
As a stand-alone, laser soldering is commonly combined with some other functions such as component placement, fluxing, preheat and final test. Indexing tables have a long tradition in stand-alone soldering applications and are useful in this case, as well (Figure 4).
Rather than supplying an entire piece of equipment, some companies have opted to install the laser just above the conveyor. Figure 5 shows an application where twin-bar lasers are used to solder two rows of contacts at the same time. The stationary twin lasers are triggered whenever the boards are in the proper location below them. The distance from the board is chosen to ensure proper beam length, density and clearance for high components.
Quality
Whereas the usual soldering processes are "democratic" in nature (i.e., treat every joint in the same way, regardless of its thermal requirement), the laser soldering process can tailor the energy input. If it is accepted that the amount of intermetallic compound in the joint and wetting quality relates to joint reliability - the more diffusion material is present, the more likely the joint will have a lower life expectancy - the laser offers a way to create very reliable joints. Although there is relatively little that the soldering process can contribute to this aspect of quality, optimizing thermal exposure will help produce a good joint. When other soldering methods target a diffusion layer of less than 3 µm in thickness to achieve a reliable joint, laser soldering can stay well below that goal. The difficulty is finding the proper setting for the laser: too much energy and the joint will show a large amount of intermetallic; too little, and wetting may be inhibited because of low energy. With lasers, there is a narrow line between these two conditions. Adding time constraints to this picture does not make things easier.
Nevertheless, tests repeatedly have shown that excellent joints can be produced with laser reflow. Damage can also occur when the laser hits the target area properly. As the cladding adheres to the laminate by some gluing action, extremes in heating or heating rate may cause blistering or other adhesion problems. These problems may be induced because of chemical reactions or based on pure physical extension differences.
Safety
Although the power rating of diode lasers as applied in selective soldering is low compared to other lasers and applications, the beam concentration warrants special safety precautions. Full units usually incorporate all necessary safeguards, such as warning labels, classification and required shielding with appropriate locks. If the laser is mounted on existing equipment, care must be taken to meet all legal requirements and restrictions, and, most importantly, to advise and train personnel on the use and potential dangers of the equipment. SMT
ACKNOWLEDGEMENT
Photographic material compliments of SEHO GmbH.
WORKS CONSULTED
1 S. Bader, et al., Entstehung und Aufbau isothermer erstarrter Schichtverbindungen, DVS 129.
2 T. Fröhlich, et al., Simultanes Laserstrahllöten, Productronic, October 1997.
3 S. Hierl and M. Geiger, Simultaneous Laser Soldering for SMDs on 3-D MIDs, Proceedings of the Technical Program SMTIA, 1999.
4 S. Pongratz and G.W. Ehrenstein, Soldering on Plastics, Proceedings, International Congress Molded Interconnect Devices, September 1998, Erlangen, Germany.
5 A. Rahn, Selective Soldering - Part 1: Laser Soldering, SMTAnews, June 1999.
6 M. Walter and W. Hirth, Selektives Löten mit dem Laser, Productronic, November 1997.
ARMIN RAHN, Ph.D., may be contacted at RahnTec Consultants, 3 Ridgewood Rd., St. Cathereine, Canada L2R3S2; (819) 382-9906.
Figure 1. A typical laser unit.
Figure 2. Lasers are often used to join the antenna to the chip on a Smart Card.
Figure 3. To join multi-loaded devices, a laser bar can be created that solders all sides of the component simultaneously.
Figure 4. Indexing tables are often used when combining laser soldering with other functions, such as placement and test.
Figure 5. This in-line laser configuration consists of twin-bar lasers soldering two contact rows simultaneously.