Process spread reduction of laser microspot welding of thin copper parts using real-time control

Abstract

In present-day industry, particularly in the area of microelectronics packaging and assembly, there is a strong demand for highly reliable, miniature joints of thin copper parts. Laser welding could be the perfect solution for making such joints if this process were not highly sensitive to various parameters, such as the reflectivity of the copper workpiece, the gap between the product parts to be welded, and laser-power density. The robustness of the process is further limited because two important product properties (reflectivity and heat conductivity) change strongly during welding. An investigation has been performed to increase the robustness by means of real-time feedback control, based on several parameters that are monitored simultaneously during the process. It is shown how this drastically decreases the influence of the above-mentioned variations with "heat conduction" welds. The control algorithm was based on an approximate model of the (non-linear) welding process. In addition, it is shown how adaptive feedforward control is required to cope with the limited response time of the system. Finally, some remarks are made on experiences iterative learning control. This investigation was part of the European co-operation project SLAPS, performed within the framework of the IMS/Brite-Euram III program. The support from the European Commission and the IMS regional offices is gratefully acknowledged.