The reliability of copper welds is still a problem today concerning the high demands of spot or contact welding for the
electronic or medical industry. Due to the low absorptivity at wavelengths of 1 micron and the very high thermal
conductivity of copper, even small surface contaminations lead to drastic variations in weld quality. The wavelength of
532 nm (frequency-doubled Nd:YAG laser) is much better absorbed by copper at room temperature. Combining the two
wavelengths and using the drastic increase in absorption with increasing temperature leads to an efficient spot welding
solution. By the use of intelligent pulse forming with the thermal pulses of a Nd:YAG laser the spot weld reliability is
improved significantly. This paper discusses a solution where Nd:YAG laser pulses composed of 85 - 90% of 1 micron
and 10 - 15% of 532 nm radiation are used for spot welding of 80 - 300 micron thick copper ribbons. A weld spot
diameter variation below 6% combined with 100% full penetration welding is achieved. The process efficiency is
improved by more than a factor of two compared to conventional spot welding with 1 micron radiation.
High speed contour weldings with feed rates in the range of meter per second are performed by single pulses of Nd:YAG lasers with real time pulse control.The pulse length may be up to 100ms leading to a possible contour length of more than 100mm. The properties of the weld pool was investigated with direct beam and a beam delivered via fiber on the metallic workpiece. Micrographic pictures show, that with fiber the profile (depth and shape) of the melted zone is quite constant along the contour, while for the direct beam the profile is strongly influenced by the change of thermal lensing of the laser rod during the pulse. Corrections are possible by applying temporal pulse forming. The effect of this highly dynamic welding strategy on the joining of similar and dissimilar metals, which show quality problems in conventional seam welding, is investigated. The results demonstrate an improvement in the metallurgy (e.g. cracks), mechanical and visual properties in most joining tasks.The advantages and limits of this micro-welding strategy will be discussed.
Conference Committee Involvement (1)
Laser-Based Micro-Packaging II
27 January 2005 | San Jose, California, United States
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