Pulsed Nd:YAG Lasers Push the Welding Envelope
Dr. Geoff J. Shannon
The growth of industrial welding applications continues to push the welding envelope of lasers in terms of faster speeds, deeper penetration, more-weldable materials and smaller spot sizes. The welding applications emerging from the medical, photonics and electronics industries, in particular, typically involve more exotic materials and very fine precision welds -- demands that are providing Nd:YAG laser manufacturers and integrators with a unique set of challenges.
Pumped Nd:YAG laser welding applications span diverse industries from medicine to semiconductor manufacturing. Examples are joining 0.004-in.-diameter nitinol guide wires (top) and seam welding Kovar electronic packages (bottom).
Medical applications.The welding applications being developed for the medical industry revolve around joining very small, thin-walled and mechanically delicate components. These may be catheters, guide wires or hypo tubes, and joining them requires a highly stable low-power laser source. Typically, a weld may use a spot size below 100 µm with less than 0.1 J. When creating welds this small, heat sinking the weld and keeping the material from balling around the joint are critical (balling occurs when the joint material melts and forms a ball under surface tension, pulling away from rather than wetting the other interface). The other half of the equation is tooling, which, because of the small size and fragility of the parts, is crucial in achieving reliable welds.
Photonic applications.Many optical devices, such as pump and source lasers for fiber communications networks, are manufactured using laser welding. The key requirements for the welds are long-term stability and low and consistent postweld shifts, caused by movement of the optical components during welding from solidification contraction forces and thermal distortion of parts.
The laser welding system must provide long-term process stability and low and consistent postweld shifts when welding photonic components. In this application, a gold-coated Kovar ferrule is joined to a Kovar weld clip attached to the welding base.
In some of the worst-case scenarios, the components are aligned to 200 to 300 nm prior to welding, while the welding occurs between interfaces with a 10- to 20-µm gap. The amount of shift is minimized by careful selection of beam-delivery optics, weld schedule, symmetrical welding (using multiple welds at once) and part design. When the shift has become reliable and predictable, alignment of components prior to welding is offset so that welding realigns them. This usually achieves alignment accuracy approaching 1 to 2 µm. Thereafter, the parts are optimally realigned using microbending techniques or even low-power pulses from the laser.
Electronic applications.A common welding application in the area of electronics is the hermetic seam sealing of packages. This process usually requires the use of a glove box to guarantee that the internal volume is free from particulate contaminants that would affect the lifetime of the device. Seam sealing requires an optimization of speed vs. penetration, while ensuring that the package is not thermally loaded during the welding process. Therefore, high pulse repetition rates and short pulse durations minimize the average power required to create the seam.
In some instances, engineers can use a shaped pulse combined with a ramped leading edge to achieve high speed and to reduce weld spatter. With improved brightness, higher-power lasers now offer fiber delivery through smaller cores, enabling smaller spot sizes and faster welds.
Pulsed lamp-pumped Nd:YAG welding lasers are still the preferred choice in most medical, photonic and electronic applications. In seam welding, they have occasional competition from CW Nd:YAG and emerging fiber and disk laser technology, although the applications seldom overlap for reasons of materials, cost and speed. For spot welding, there is no real laser alternative to the lamp-pumped Nd:YAG, because diode-pumped alternatives still have issues related to peak power generation.
Future applications in microwelding may see the use of Nd:YVO4 lasers, which have efficient frequency conversion leading to the possibility of a green welding laser. This would offer advantages over the Nd:YAG laser in absorption and minimum spot size.
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