Research On Laser Micro-welding Based On Scanning Galvanometers

Laser micro-welding technology based on galvanometer scanning, within the welding zone dimension of less than one millimeter, is an advanced laser processing technology, which under the control of the computer, the laser micro-welding is implemented through reflecting laser beam by scanning galvanometers. It is a novel welding technology, which developed along with the development of laser technology.Focus on the existing problems and shortcomings in laser micro-welding technology, starting with the characteristic of laser micro-welding, this paper studies more deeply laser micro-welding technology based on galvanometer scanning. The major contents are as follows:Laser micro-welding mechanism and the physical phenomena that laser beams with different power density irradiate the metal surface are studied in this paper. Especially, the circumstances are more analyzed that laser beam, which can be used in laser micro-welding and has the power density of 104~107W/cm2, irradiates the metal surface. Then the characteristic of polarized laser beam and its applications in laser micro-welding are analyzed. Through the numerical simulation to the process of laser micro-welding, obtaining the distribution of the temperature field and coming to the conclusion that the laser micro-welding is a flash heating and cooling process. The results also demonstrate that the heating speed is higher than the cooling speed, which because the temperature are raised passively due to laser energy input and the temperature decreases initiatively due to the heat conduction. The metal molted pool is also simulated. In addition, its formation, characteristic, fluid state and application in the laser micro-welding are studied too. The research shows that the liquid metals flow state in laser micro-welding molted pool is very complex, which the surface tension is relatively stronger, Marangon effect is significant and the Reynolds number is low.Through the deep study of laser micro-welding technology based on galvanometer scanning, the development trend of laser micro-welding is proposed, namely, precision, accuracy, reliability and rapidity. Then the precise, accurate, reliable and rapid methods and ways of laser micro-welding are studied. (1) The precision and accuracy of laser micro-welding can be realized by transforming laser beam, equipping with CCD real-time monitoring system, and selecting laser, f-theta lens, scanning galvanometers and so on. (2) The reliability assurance in leaser micro-welding can be realized through adjusting process parameters. The process parameters include laser power, welding speed, laser spot in the focal plane, defocusing amount, laser pulse frequency, pulse width, pulse waveform, etc. Moreover, the relation in the laser seam welding among laser pulse frequency f, laser spot d and scanning speed v is obtained through the research, that is f ? d?(1 .2~1.4)?v. (3)Finally, how to improve the laser welding speed is studied. It is considered by analysis that laser, solder (or the matrix material for welding) and the system structure of laser micro-welding had influence on the welding speed. Optimization the scanning path can increase the welding speed through genetic algorithm to the cycle-matrix of distance. It can improve laser micro-welding system utilization rate through using multiple-channel and time-sharing scheme in multi-spot welding. Then, the total laser welding speed is improved.The system structure of laser micro-welding based on galvanometer scanning is studied. Two kinds of laser micro-welding systems based on galvanometer scanning, which equipped with Nd:YAG laser with an energy negative feedback control system and fiber laser, are obtained through the research.Laser micro-welding technology based on galvanometer scanning is validated through three typical examples, namely laser soldering bump of lead-free, welding between enameled wire and IC aluminum electrode, and welding for thin copper wire and IC aluminum electrode. (1)Laser lead-free soldering bump is a representative application for laser micro-welding in electronic packaging field. It is considered that the scanning mode of decussate or cycle isn't the best for laser soldering bump. Comparatively speaking, the linear scanning mode has certain superiority in soldering bump. Soldering bump of lead-free with diameter of 300μm, which has a good bond with the matrix material of CCL(Copper Clad Laminate), can be obtained in linear scanning mode. We can solder 25 bumps per second in this way. (2)The laser micro-welding can be implemented between enameled wire with diameter of 0.10mm and the IC aluminum electrode directly without removing insullac. This method simplifies the process of laser welding for enameled wire and creates the conditions for automatic batch welding. (3)The laser micro-welding can be implemented between thin copper wire with diameter of 0.25mm and the IC aluminum electrode. The thin copper wires are irradiated directly by laser beam and melted. The molten liquid metals Cu flow cocurrently downward and coat IC aluminum electrode, Then the welding joint would be formed. It overcomes the welding difficulty due to the characteristic difference of dissimilar meter Cu-Al, and avoids several disadvantages in laser welding for Al and Al-alloy like porosity, hot cracking, welding joint softening and defect etc. This method has universal significance, and can serve for welding other diisimilar metals.