Basic investigations for controlling the laser spot welding process when packaging 3-dimensional molded interconnect devices

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.