C6c. laser soldering - is a reflow method that uses laser energy to heat each solder joint, one at a time, melting the solder and making the desired electrical connection. The laser is a beam of coherent light, closely focused and directed at each joint for a precise amount of time. (See 3O and 7C5 for applications of more powerful laser energy.) A computer controls the movement of the workpiece or laser beam from joint to joint and controls the dwelltime of the beam at each joint. Several different laser systems can be used. Nd:YAG lasers have the advantage of higher thermal efficiencies because less energy is reflected. CO 2 lasers operate at a higher wavelength and have a less concentrated spot of light.  

The advantage of laser soldering is that the heat is highly concentrated; each individual circuit board connection can be heated without affecting the connected components (except that ceramic components may require more care). The rapid heating and cooling of the joint metal reduces undesirable intermetallics, and provides desirable ductility and fatigue resistance. However, reflectance of the joint material reduces the heating effectiveness of the laser beam, although solderingflux can be used to reduce or prevent this effect. Additionally, laser equipment is costly and output rates with the oneby- one heating may be slower than other reflow methods, although two or three joints per second has been reported to be feasible. 6 Spattering may occur when solder paste is heated rapidly by laser. The laser beam must be enclosed for safety reasons.  

There are two basic laser soldering systems: blind laser soldering and intelligent laser soldering.  

Blind laser soldering is open-loop laser soldering; there is no feed back. (See section 3U and Fig. 3U-1.) The laser heating time, the diameter and power of the beam, and other aspects of the machine’s settings, are programmed beforehand for each joint. Differences in joint mass, contamination, or reflectance from board to board, unless programmed beforehand, do not change the heating cycle. This approach is useful when conditions are very predictable and constant, as is the case in many mass-production situations, but is not so suitable for most printed circuit board reflow soldering.  

Intelligent laser soldering is a closed-loop process. The equipment incorporates an infrared detector, mounted to be concentric with the laser beam. The detector senses when the joint metal changes from solid to liquid and the computer control then reduces the laser power for a few milliseconds of dwell and then shuts off the power, allowing the solder to cool and solidify. The combination of detector and control obviates the need for joint inspection that would be necessary with blind laser soldering. Reflow soldering of fine pitch printed circuit boards or those with tape automated bonding of components is a major application.  

C7. hot gas soldering - is primarily a manual rework procedure with hand-held heat guns. Air is the most common gas used, but nitrogen or nitrogen-hydrogen mixtures can be employed when it is important to limit oxidation. Heating of the gas is usually by electrical resistance. The method is used to repair defective solder joints or replace defective components on printed circuit boards. The process can be used in other applications such as the soldering of small electronic devices, where only a small area needs to be heated. In printed circuit board operations, care must be taken not to overheat adjacent electronic components. Appropriate nozzles can limit the area heated and baffles can be used to protect critical components nearby. Gas flow rate and temperature are also controlled to avoid overheating in the vicinity of the work.    

C8. soldering iron soldering - is also primarily a rework procedure. Production soldering uses more-automatic methods. Soldering irons are very common, however, for repairs, touch-up, and limited quantity or prototype work. Most irons are heated by electrical resistance. The tips or “irons” (usually copper with an iron or nickel coating) are large enough to serve as heat reservoirs and the current is always “on”. Soldering irons transfer heat to the joint by conduction when the iron is brought into contact with the joint surface. Soldering guns are soldering irons with small tips that are part of a secondary transformer coil. The tips heat very rapidly as the trigger is pulled and do not heat otherwise. Another advantage of these guns is that the small tips that can be inserted easily into narrow spaces.  

C9. using lead-free solders - As public and governmental awareness has grown of the potential safety hazards of lead-containing materials, there has been a movement toward the elimination of lead from soldering alloys. Prior to this movement, the most common solders for electronic products contained 37 or 40% lead. These lead solders are relatively inexpensive, reliable, and easily recycled from discarded circuit boards. However, the industry is now switching to solders with typical compositions containing chiefly tin, alloyed with 3 to 4% silver and about 1/2% copper. One commonly-used alloy is SAC305 with 96.5% tin, 3% silver and 0.5% copper, popularized by Japanese companies. These solders require peak temperatures for wave or reflow soldering of 455 to 500 ° F (235 to 260 ° C) compared with 406 to 455 ° F (208 to 235 ° C) for tinlead solders. The methods employed in making and assembling circuit boards with these lead-free solders are basically the same as with lead-bearing solders but require tighter process controls. The lead-free solders are more costly, primarily because of their silver content. Problems can arise with these alloys, especially in applications with severe thermal cycling. Problem areas are surface finish, solder joint integrity, thermal damage to boards, components, and connectors, and in testing, cleaning and rework. 1 Solutions to the problems involve changes to less temperature-sensitive materials and components, minor changes in tooling or methods, and more careful monitoring of process conditions.