Mark Rodighiero

Green-laser-welded connections offer "forgiving" process parameters and highly reliable connections.

Autogenous copper welds eliminate solder, provide high reliability and eliminate resistance drift over time due to chemical and metallurgical changes.

Non-contact green-laser welding eliminates mechanical damage to electrical connections in high-value products like this solar cell.

Welding pros know that lasers offer flexible configurations and a high degree of process control, making them ideal for numerous applications. Yet the inability of conventional laser technologies to handle "red" metals like copper and gold has blocked their use in many industrial applications. That's about to change with the advent of 532-nanometer (532 nm) green welding laser technology.

Why Green?
The interaction between laser light and welded materials depends on the wavelength of the light and on the base materials. For example, CO2 lasers with wavelengths near 10 microns interact well with organic materials such as plastics, wood or rubber. Also, fundamental-mode 1064 nanometer pulsed Nd:YAG lasers interact well with non-copper metals such as stainless and coldrolled steel, high nickel alloys and some aluminums. However, neither of these wavelengths work consistently well with red metals such as copper alloys and gold alloys. On the other hand, 532-nm wavelength is a natural for these metals. For example, copper absorbs 532-nm laser light four times better than 1064 nm, so the initiation of laser "threshold absorption" occurs more readily and more predictably. The result is predictable, high-quality production welds in copper-and gold-based materials.

It isn't easy being green
Q-switched marking lasers with 532-nm wavelengths have been around for years. However, until recently there have been significant technological barriers in producing green-laser pulses of sufficient amplitude and duration for productiongrade welding applications.

These limitations result from the way green-laser light is generated — as a second harmonic of 1064-nm Nd:YAG laser output. Solid State lasers use a non-linear crystal to deliver the second harmonic output. For marking, a relatively simple Q-switch technique was sufficient to produce a short pulse of high peak power.

However, a Q-switched laser won't work for welding because welding requires a much higher pulse energy and wider pulse width. A typical laser-marking pulse width might be as little as 15 nanoseconds, which is adequate for localized heating or ablation of a material to create a mark, but is insufficient to produce the time and heat profile needed to create a weld. In contrast, welding requires pulse widths on the order of 0.1 - 50 milliseconds. Miyachi Unitek's LW2AG production-grade green-laser welding system, overcomes these limitations

through a combination of techniques including improvements in the nonlinear crystal technology and innovative optical design in the laser resonator. The LW2AG uses a modified LBO (Lithium Triborate LiB3O5) crystal in a special intra-cavity optical arrangement that optimally manages power density inside the crystal and results in extremely efficient second harmonic conversion. The result is 532-nm laser-welding pulses with high beam stability and long pulse widths — ideal for green-laser welding in fullscale production environments.

New applications ahead
A laser-welded connection is inherently more reliable than any soldered connection because it is a true autogenous weld — the materials are melted and joined without using a third substance such as solder, brazing compounds or welding wires. Green lasers open the door to new industrial applications offering key benefits that include:

• Welding copper, gold alloys and silver,
• Providing a joining alternative to soldering electrical connections that eliminates long-term resistance drift,
• Eliminating the need for solder and flux that can cause problems in highreliability applications,
• Eliminating mechanical and electrical (ESD) stress to components by effecting the weld through "non-contact" means.

These features permit the use of green lasers in electronic interconnect applications where the extensive use of copper and gold alloys prevented the use of conventional laser-welding processes in the past.

Get the lead out
Green-laser welding provides an answer to worldwide initiatives calling for the elimination of lead-based materials in electronic applications. Initially driven by the European-Union (EU) rules restricting hazardous substances (RoHS), the leadfree movement has now gained impetus from a global commitment of the entire electronics industry. The EU's RoHS regulations restrict the use of certain hazardous substances in all electrical and electronic equipment shipped into the EU as of July 1, 2006. Electronics manufacturers worldwide are now focusing on meeting this timeframe for a complete transition to leadfree operation throughout their facilities. Achieving this global lead-free goal represents one of the greatest challenges that the electronics industry has ever faced. Of course, the best way to eliminate lead from electronics is to eliminate the need for soldering.

Precision and control
The precision and inherent controllability of laser welding allows it to be deployed in a wide range of production applications. Green lasers can weld lead frame connections providing an efficient alternative to ultrasonic bonding. In addition, because it is a non-contact joining process, green-laser welding completely eliminates the risks of electrostatic discharge (ESD) and physical damage that can result in expensive scrap or latent defects in components.

In addition to streamlining compliance with lead-free mandates, industries such as automotive, aerospace and medical manufacturing see other advantages to green-laser welding. Global competitive pressures are pushing the automotive sector toward the goal of providing 100,000-mile warranties on vehicles, which is difficult to achieve with existing crimpand-weld production methods. Soldered electrical connections do not have the mechanical durability and electrical stability needed to support such a warranty. On the other hand, metallurgical autogenous-welded connections produced by laser welding with 532-nm light provides the required durability because the materials are actually melted and joined together in a true weld.

Similarly, the medical-device-manufacturing industry also has to cope with challenging requirements for reliability — often with life-or-death consequences. There is no such thing as "reliable enough" electrical connections inside an implantable medical device like a pacemaker.

The never-ending push for higher reliability in these industries is providing impetus for widespread transition away from conventional soldering processes.

Manufacturing alternative energy sources
Another major application greenlaser welding is the manufacture of solar cells. Because green-laser light couples exceptionally well with both copper and even silver, it is particularly well-suited for high-volume production welding of solar-cell components. For example, a solar-cell assembly typically consists of thin copper-nickel ribbon welded to a amorphous silicon substrate with a thin plated-silver surface. The advantage of the green laser's precision and efficient interaction with base materials is the ability to quickly create consistent welds without damaging the underlying (and relatively expensive) processed silicon.

Contract manufacturers and fab shops can offer their customers an edge with green-laser welding technology. The lead-free initiatives, combined with the explosion of medical device manufacturing and other industries demanding ultra high reliability, have created a new impetus for competitive manufacturers to offer innovative technologies that deliver much more than a "me too" approach to permanent electrical connections.

The bottom line is that new production-grade green-laser welding capabilities, such as the Miyachi Unitek LW2AG, have opened a new arena of applications — and those are just scratching the surface of potential uses. For years, industry has recognized the benefits of non-contact laser-welding processes but has also accepted the limitations of not being able to laser weld copper, gold and many silver alloys. However, with green-laser welding, those basic limitations no longer apply, and it is time to rethink a broader range of application possibilities to see which ones can be "green lighted" by using this new technology.

About the Author
Mark Rodighiero is vice president of the laser and systems division of Miyachi Unitek Corp., Monrovia, Calif. In addition to advanced laser technology, Miyachi Unitek Corp. provides complete workstations for laser welding a wide range of precision industrial components.