Closed loop control of laser soldering through optical and thermal imaging

Laser soldering provides an attractive means of selective soldering in electronic assemblies since it can provide very focussed heating and very rapid joint formation. However, selecting the appropriate laser pulse intensity and duration for a particular solder joint is not straightforward since it depends on many design, material and process parameters. Typically, laser parameters are set through trial and error but this can be time consuming and cannot take account of process variations. One approach to broadening the process window is to use feedback from the joint formation process to control the laser energy. In this paper we discuss the use of optical and thermal imaging as a means of providing. In the case of optical imaging, video images are processed to determine the extent to which the solder balls in the solder paste have been transformed into a pool of liquid solder. A number of image processing techniques are presented and compared in terms of their accuracy and computational load. In the case of thermal imaging, the use of temperature measurement is considered. The two approaches will be outlined, results presented for each and their relative merits compared. It is shown that the use of imaging to control laser pulse duration can automatically compensate for significant variations in track width and laser power.