Laser soldering for lead-free assembly

Selective soldering is a process where some joints of a printed circuit assembly are prepared separately from the many reflowed ones. The most popular selective soldering processes are derivative wave-soldering processes. They are usually used to solder the pins of larger through-hole components (e.g. connectors) on a board, which is fully populated and reflowed using surface mount components. In the paper the laser soldering technology is discussed in detail, which - as the most promising alternative selective soldering technique - might replace the less energy-efficient derivative wave-soldering processes. Application oriented test boards were designed and applied for the investigation of the laser soldering process and qualification of the joints. Among the good results, the most interesting is that high quality solder joints could be prepared even when the temperature limit of the board was much lower than the melting point of the applied solder paste.