Experimental and numerical investigations regarding laser drop on demand jetting of Cu alloys

Active noise reduction, structural health monitoring and energy harvesting are possible applications of active structure components. An integration of piezoceramic modules into casted Al-structures can be defined as a long term goal to achieve high functional integration and thus to address lightweight construction. Since the liquidus temperature of standard electronic solder is not sufficient to withstand the thermal loads during Al die casting processes, a suitable wire bonding process is an enabling technology to generate active Al structures. In the scope of this work, a laser based drop on demand joining process is introduced. The process consists of four main steps. First a spherical CuSn12 braze preform of 600 µm diameter with a liquidus temperature of 1233 K is inserted into a ceramic capillary and being molten by a laser pulse. The droplet is subsequently ejected by gas overpressure and impinges on the joining area, consisting of the electrode structure of the piezo element and the Cu wire, where it forms a firm joint of the Cu wire and the electrode structure. In order to evaluate time–temperature profiles of the capillary during melting and detachment of the braze preform a simulation model was set up, indicating heating and cooling rates of 22,100 K/s. Further, the impact of the capillary geometry on the velocity fields of a passing medium was evaluated. By changing the capillary geometry, the gas flow velocity could be reduced by about 10% according to the simulation model, which resulted in a reduction of droplet height deviation of 7.5% and a reduction of the droplet diameter deviation of 32.2% in the experiment.