Up to now, lasers have been well established in the field of materials processing for cutting, welding, and surface treatments. Recently, lasers in the medium power range have been of increasing interest in the production of electronic components. Higher integration density of electronic circuits demands improved mounting technology. Due to the reduced contact area of modern surface mounted packages, more sophisticated soldering systems are required to ensure product quality. Standard reflow soldering techniques may damage thermally sensitive devices, and mechanical tensions in the solder joints will occur, due to different thermal expansion coefficients. These problems can be avoided using a laser, as the amount of heat induced into the component is very small. Another advantage is the step-wise heat input resulting in a minimal overall thermal loading of the device and the possibility to control individually the heatflow for each solderjoint. In some applications lasers are the only reasonable tool, e.g., repairing printed circuit boards (PCB) produced in surface mounting technology or soldering of three dimensional PCBs. To improve quality and productivity of laser soldering tools, the time required for melting and wetting has to be minimized in the same manner as defective solder joints should be detected online. There are some commercial laser soldering systems available, using different types of process control, e.g., pyrometrical temperature measurement, detection of the reflected laser beam energy, or evaluating the sound emission while melting the solder with a pulsed laser. To obtain certain time-temperature curves, an analogous regulation of the beam power is required. Therefore a pyrometer offers the best approach to get optimal thermal input, even if the measurement is difficult due to complex geometry and unknown emissivity of the surface. This paper outlines the behavior of the solder paste under irradiation of different wavelengths and the possibility of controlling the solder process via the above mentioned setup.
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