A leading manufacturer of solar cells is looking for a new fully automated system for the metallisation of the rear of the silicon wafers with sizes up to 156mm x 156mm. The potential printing processes are silk-screen or pad printing. Due to their high sensitivity to breakages, the wafers present special demands for the printing process.
Tamponcolor as a systems provider developed a fully automated printing system with multiple servo-electric TC 200 E pad printing machines. Using a robot system, three wafers are placed next to one another in a workpiece holder on a rotary indexing table and conveyed into the first printing station, in which all three wafers are printed simultaneously with contact points ("silver dots"). The contact points are then dried at four drying stations using specially designed nozzles.
Three print stations follow, in which the wafers are printed individually using aluminium paste. Due to the relatively even pressure during printing, the breakage rate of the wafers in the pad printing process is greatly reduced in comparison to the silk-screen printing process. The programmable servo drive permits optimum paste take-up and transfer time with a thick paste coating applied at a high cycle rate. Using an image processor, the surface of the print image is then checked, as well as its position on the wafer. Poor quality parts are rejected and good quality parts are transferred to the drying oven for further processing on a conveyor belt. The system works in 24h three-shift operation and prints approx. 2500 parts/h.
In contrast to the silk-screen printing process, pad printing offers a much lower breakage rate of the delicate wafers at a high cycle rate. Other advantages include high availability of the system thanks to shorter cleaning times and reduced costs for consumables thanks to longer service life durations of the pads and clichés.