This paper deals with basic investigations in order to control the laser spot micro welding process when packaging electronic components onto three dimensional molded interconnect devices (3-D MID) or flexible printed circuit boards. A wide range of experiments has been carried out for both successful and fail welds. Typical failures appearing during welding are either damage of the circuit board due to overpower or loss of connection between the welded components due to gap formation between the leads of the component and the circuit board. The optical radiation emitted from the process was firstly measured off-axially and co-axially with a spectrometer. To aid the spectrometric analysis, an optical sensor based on a silicon photo diode and an appropriate optical filter was applied for detecting the emitted radiation. The signal was acquired, analyzed, and saved using a dedicated software program. Changes in the detected radiation due to different weld conditions were evaluated. Moreover, the weld quality was investigated by Scanning Electron Microscope (SEM) measurements and cross-sectional analysis. A correlation has been found between the signal course and the weld quality. Primarily, there are three relevant signal phases (high peak, flat stage, and small peak) appearing during the weld. Any changes in the characteristic signal during these process phases can be used to predict the quality of the welds.
Nd:YAG solid-state lasers have been integrated in many seam welding applications. They provide a good ability of integration into existing manufacturing sequences and allow its easy automation. Appropriate process monitoring systems are needed to decrease necessary user intervention, to ensure a high machine availability and to realize a zero defect production. In the electronics industry, laser spot welding techniques using pulsed Nd:YAG-lasers have been established in mass production applications, for example in manufacturing of electron gun components for TV monitor tubes over the last 25 years. They require different strategies and methods for process monitoring systems. Apart from these integrated laser spot welding applications, there is a current demand for new technologies to join micro components onto 3-dimensional (3-D) circuit substrates and to connect electrical plugs. In recent years, laser spot joining techniques have emerged as a viable option for packaging electrical and mechanical microparts, such as surface mounted devices (SMDs) and casings. Under most conditions, laser spot welding provides more durability as well as thermal and mechanical stability compared to traditional packaging techniques, such as simultaneous soldering. Additionally, under less ideal conditions, the packaging quality can be inconsistent, resulting in the need for optimization and monitoring of the weld parameters under different conditions. In order to achieve a stable process during packaging of electrical components despite their weak absorption of laser radiation and different surface qualities, a process monitoring system should be needed.
Simultaneous to the rapid evolution of laser welding, with increased improvements concerning reliability and effectiveness, the need for quality control has grown. The development of system which monitor process quality online, and detect faulty welding results immediately after the defect occurs, has proven to be useful. Such process monitoring systems are usually based on the evaluation of the radiation emitted from the welding spot. This procedure offers a simple way to rate the welding quality, and to detect typical errors. However, it is not possible to recognize the nature of the fault with these single-sensor- setups. To obtain more specific information about the process, additional sensoring is necessary. Within this paper, an approach with IR signal acquisition is described, and correlations are presented. It is shown that the supplementary information coming from a 2D thermo-image allows the identification of typical welding defects. Subsequently, a method for an optimized and practical evaluation strategy using a 1D, line-shaped IR-acquisition is introduced.
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