Laser beam soldering

Laser beam soldering provides very precise heat source for precision work, but at high cost. 

FIGURE 5.12 Schematic of simultaneous laser beam soldering of a two-terminal component... 

Consequently, like other selective soldering processes, laser beam soldering is used when... 

This method requires a laser beam that is smaller in diameter than the length or width of the component lead or bonding pad. The beam is stepped to each lead-pad combination and delivered with sufficient energy to result in solder reflow. The beam can be CW, pulsed, or... 

During the soldering cycle, the glass can accumulate spattered flux and flux by-products that may change the delivered laser beam intensity, adding variability to the process. 

Laser is a recent innovation in rework and repair. The fundamentals of laser for initial soldering or for repair have already been discussed in Chap. 47. In the commercial incarnation of this technique, a laser beam is quickly scanned around component leads or, package surface, in the case of area-array devices such as EGAs and chip-scale packages (CSPs). It is most effective for plastic packaged components rather than the thermally massive CBGAs, CCGAs, and so on. In this technique, the component body is heated. Otherwise, there is little difference between this and alternative rework or repair techniques. 

The crimping process applied pressure to the top part of the battery containing the current break assembly, the positive temperature coefficient (PTC) and cap-up. After sealing the current break, the PTC, and the cap-up either soldered shut by a laser beam or into a heat-sealed soft pouch, the crimped ceU is pressed to maintain a constant height. The cell is ready for inspection by X-rays to check for any defect and verify the internal assembly of all these elements. After washing with water to remove electrolyte and any impurity at the surface, the ceU is dried to eliminate moisture. FinaUy, the battery in imprinted with the manufacturing factory, line number and date. 

The laser beam is focused directly on the surfaces to be joined and the solder depot. The localized energy input rapidly heats the partners and the solder, with ramps from 500 K/s to well in excess of 1000 K/s possible, depending on the masses to be heated and the power of the laser. 

Imperfections in the lens or beam shape and other factors prevent practical achievement of these minimal spot sizes. Generally, the attainable focused beam diameter on the factory floor is about two to three times larger than the ideally calculated spot size. Note that for an Nd YAG laser, the spot size is at least 10 times smaller than that of the CO2 laser, providing a fine, high-energy-density spot for soldering. 

One of the attractions of working with a laser is the fact that its beam can be split for multiple use within one station or even shared between two or more stations. The split can be accomplished by the use of bifurcated fibers or beam splitting mirrors. The beam of a single laser cavity can be duplexed to solder two sides of a surface-mount component simultaneously. It is entirely possible to share a common beam between two or more laser soldering stations either simultaneously or in a time-shared manner. 

---