INVESTIGATION OF A LASER SOLDERING PROCESS FOR THE INTERCONNECTION OF THIN-FILM SENSORS WITH SPUTTERED MULTILAYER METALLIZATIONS

Miniaturized thin film sensor elements can be simply carried by their lead-in wires. Laser soldering seems to be a favorable technique for attaching relatively thick wires to thin film metallizations. Work described in this paper concerns soldering experiments carried out with a Nd:YAG laser. We have investigated two types of metallizations (NiCr-Au and Al-NiCr-Au) deposited on glass, alumina, and silicon substrates by rf-magnetron sputtering. Special test patterns with integrated thin film thermocouples were developed to analyse temperature dependent dynamic effects of the laser soldering process. The thermocouples formed by NiCr and W tracks were placed below the solderable metallization and electrically insulated by silicon nitride and silicon oxide layers. We show temperature versus time curves recorded during soldering processes. A further part of this paper is focused on the dependence of solder joint quality on soldering parameters, type of metallization, and substrate material. Solder joint quality has been evaluated by means of visual inspection of solder formation and mechanical strength testing. We have established the lower limits of the range of parameter settings (laser power and pulse duration) leading to an acceptable bonding quality. Results of strength tests are correlated to metallization type and substrate material.