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.
In order to improve the reliability of micro-spot welding of metal parts in production such as e.g. in electron guns for TV picture tubes, real-time information about the evolution of the welding process should be available to allow on-line modification of the laser parameters. Such information can be derived from a set of sensors that are mounted on a laser-scanning head. Different sensors are used to monitor the optical fiber output power to determine the evolution of temperature during the spot welding process, to measure plasma emission and back-reflected laser light. A vision channel and a CCD camera are used to control the position of the laser spot on the parts to be processed. The compact scanning head is composed of a tip/tilt laser scanner, a collimating lens and a focusing lens. The scanner is fast steering, with a bandwidth of 700Hz, and can tilt by +/- 3.5 degree(s) with a repeatability better than 50(mu) rad. The settling time for maximum deflection is less that 10ms. The scanning lens is a newly developed focusing lens designed to replace commercial cumbersome scanning lenses such as F-(theta) lenses, which have large volume, weight and price. This lens is based on the well-known Cooke triplet design and guarantees a constant shape of the spot all over the scan surface and is specially well suited for high power beam delivery. The scan field achieved by the system is limited to 25mm x 25mm. The laser used for this application is a pulsed Nd:YAG laser delivered by an optical fiber to the optical head. However, the system can be adapted to different types of lasers. Laser micro-spot welding on copper substrate has been performed in the frame of the Brite-Euram project MAIL. Smaller tolerances (a factor of 2 less) on the spot diameters were obtained in the case of a sensor controlled operation compared to the case where sensor control is not used.