Reliable laser micro-welding of copper

Abstract

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. At the beginning of the pulse the high intensity 1 micron radiation is partly converted to 532 nm radiation which preheats the surface to a level where the absorption for 1 micron radiation is high enough. From this point on the pulse power is reduced and welding is finished by the 1 micron radiation only.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.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. At the beginning of the pulse the high intensity 1 micron radiation is partly converted to 532 nm radiation which preheats the surface to a level where the absorption for 1 micron radiation is high enough. From this point on the pulse power is reduced and welding is finished by the 1 micro...