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Emphasis on Laser Equipment: Advances in Laser Welding Systems

Tue, 02/07/2006 - 5:39am
Requiring exceptional precision in their welding processes, medical device manufacturers are looking to laser technology experts to provide a system that enables such a high degree of accuracy. This article examines the emerging advances that specifically address the specialized needs of this industry sector.

By Mark Rodighiero

Implantable medical device showing typical contours.
AT A GLANCE
• Nd:YAG laser system
• Guidewire welding
• Precision motion control
• Welding copper and gold

The challenges of many medical device manufacturing applications are pushing the need for more sophisticated metals-joining technologies. In response to these requirements, pulsed Nd:YAG laser welding has become an important enabling technology, as single-use medical devices become smaller and smaller and reliability requirements tighten for implantable cardiac health management devices. The use of laser welding is also making practical a wider range of applications as a newly available laser light wavelength opens the door to reliable laser welding of copper and gold alloys. This article highlights the special challenges of applying pulsed Nd:YAG lasers in precision joining of guidewires and similar structures, motion and laser control techniques beneficial to hermetic laser seam welding of implantable devices, and new metals joining production methods made possible by the availability of “green light” (532 nm) pulsed Nd:YAG welding lasers.
Welding a “Hair”
Guidewires are devices that are used along with other technologies, such as catheters, to establish a pathway through a portion of the human circulatory system for drug delivery or deployment of angioplasty devices. Typically, a guidewire consists of a long (up to 3 meters) tapered central core wire, 250 to 500 microns in diameter. A short length of the wire is surrounded by a round- or flat-wound spring. The final product includes a hand-held steering tool that allows a doctor to insert the guidewire into a convenient artery in an extremity, and using a realtime x-ray machine to view his or her progress, to steer the wire through a maze of arteries to a target location such as a blocked artery supplying blood to the heart.


Spot weld on guidewire. Spot diameter is 100 microns.
A typical laser welding application in the production of guidewires is welding the spring to the core wire. This application presents challenges in both fixture design and control of the laser energy. First, there may be a considerable difference between the O.D. of the core wire and the I.D. of the surrounding spring. If the spring and the core wire are in intimate contact (no gap) during the weld, the heat generated by the laser pulse has a path to flow through both parts and an experimentally determined laser pulse profile yields a quality weld. However, if there is a gap present between the spring and the core wire, there is no place for the heat to go, and the same amount of laser peak power vaporizes the spring instead of creating a quality weld. Second, the instantaneous power profile of every delivered laser pulse must match the profile established during the qualification phase for that process. A consistently successful guidewire laser welding process requires innovation in the design of parts fixtures to ensure intimate contact between the core and the spring, and a modern laser with real-time instantaneous power feedback to guarantee that every laser weld pulse is within the process specifications regardless of the age of the laser flash lamp, temperature in the laser resonator, or even minor misalignment of the optics.

This task was challenging enough for early generations of guidewires used in heart therapy applications, but newer wires that are used to guide the deployment of stents for repair of smaller blood vessels, such as in the brain, may be as small as 100 microns in diameter. That’s on the order of the diameter of a human hair. Fortunately, advances in laser resonator design and fiber optic based beam delivery have made possible laser spot welds as small as 30 microns in diameter, enabling manufacturing engineers to scale the laser welding process as needed to build these new devices. A successful laser weld in this application requires precision aiming stability, vibration isolation between the work surface and the environment, accurate location of the weld site, and realtime optical power feedback. How all the elements of a complete laser welding workstation fit and work together determine not just the success of the initial process, but also the cost of maintaining process control through the production life of the guidewire product.
Smoothing Out the “Bumps”
Seam-welding implantable medical devices such as heart rhythm management devices requires unique motion system control solutions, in addition to precise control over the delivered laser energy. The principle task in this case is to seam-weld the two halves of the device together, achieving a reliable hermetic seal. Equipment built for this purpose usually consists of a rotary stage holding the implantable device while applying pressure to clamp the two halves together and an XYZ robot that aims the laser beam at the edge seam, maintaining orthogonality to the seam as well as tight control of the focus offset distance. Operation of the coordinated four-axis motion control system is deceptively simple: just turn the rotary stage and keep the laser beam pointed at the seam. But a number of practical concerns need to be addressed.

Polar acceleration - Fixed path speed

Polar acceleration plot showing rapid changes in acceleration in X and Y axes of focus head postioner while executing a contour seam weld.

For one, the contour of the seam to be welded is most often not a simple path. By studying the kinematics of the motion system needed to follow a typical welding contour at constant speed, one finds that there are points in the motion of some of the stages that require theoretically infinite deceleration/acceleration. These stages must instantaneously reverse direction without decreasing speed. Unless managed properly, this motion results in a large reactive force transmitted through the machine structure that is felt as a strong impulse force (“bump”) in the otherwise smooth operation of the servos. The impulse causes the holding fixture to flex or vibrate and can cause irregularities in the spot-to-spot spacing in the seam, often resulting in a hermeticity failure.

In the past, a typical solution has been to slow down the whole operation so the force impulse is low enough to not affect the performance of the weld, but this means a lower production rate and higher product cost.

A better approach is to use variable speed contour welding with “position-based firing.” The motion system is set up to minimize the magnitude of the sharp changes in acceleration by slowing down as needed at these inflection points and then speeding up along the benign segments of the contour. Using special software to achieve “position-based firing” along the contour, it becomes a simple matter to fire the laser not at a constant repetition rate, but rather in response to its actual position along the contour at any point in time. The good news is that the special software works with any contour, so there is no reprogramming or parameter “tweaking” needed to accommodate any implantable device now or in the future. In addition, a programming tool that allows the operator to import a *.DXF file of the contour and set up the entire process in a graphical environment makes this a practical process, allowing for rapid changeovers between part types and fast introduction of new part types into production.An effective laser welding production process for implantable medical devices clearly requires good coordination between the laser power supply and motion control system, and provision for setting up and managing the special challenges posed by the unique contour shapes involved.
Green Laser Light
Interaction between laser light and materials is dependent on the wavelength of the laser light and on the type of materials. For example, fundamental-mode (1064 nm) pulsed Nd:YAG lasers interact well with non-copper metals (iron, cold-rolled steel, nickel-plating, some aluminums, etc.), but fails to work consistently well with “red metals” such as copper or gold alloys.
 

Copper absorbs laser light with a wavelength of 532 nm substantially better than at a wavelenth of 1064 nm.
The availability of a reliable green light (532 nm) laser welder has opened the door for a number of key medical device applications where traditional laser welding simply could not be utilized. Some of the inherent advantages of green laser welding include:

• Precision welding of copper and gold alloys
• A true metallurgical weld as an alternative to conventional soldering
• Consistent high-reliability electrical connections with no long term resistance drift
• Non-contact process that completely eliminates risks of ESD or physical damage to the parts being joined

Because it is a true autogenous weld in which the materials are melted and joined without the requirement for a third material (solder, brazing compound, or welding wire), a laser welded connection is inherently more reliable than any soldered connection. Green laser welding can easily address critical electronics interconnect applications within active implantable medical devices, from eliminating soldering at the component and PCB level to overcoming the limitations of existing bonding methods at the semiconductor packaging level. The use of green laser welding to attach leadframe connections can provide a very effective alternative to ultrasonic bonding. In addition, because it is a non-contact process, green laser welding completely eliminates the risks of electrostatic discharge (ESD) or physical damage, which can result in expensive scrap or latent defects in the components.
Conclusion

Autogenously welded gold-plated copper leads onto semiconductor connection pads using 532 nm pulsed Nd:YAG laser welder.
The application of lasers to difficult medical device manufacturing problems is ever expanding. New advances in precision laser beam delivery are making possible smaller and smaller devices, promising the availability of newer and more effective therapies for a wider range of conditions. Combining close coordination between motion systems and lasers can improve the reliability and reduce production costs of active implantable devices. New production-grade green lasers now offer the promise of extending the reach of reliable laser welding to critical electrical and electronic connections. Far from being just another method for joining metals, laser welding can help bring the next generation of medical devices into existence.
ONLINE
For additional information on the technologies and products discussed in this article, see Medical Design Technology online at www.mdtmag.com or Miyachi Unitek Corp. at www.miyachiunitek.com.

Mark Rodighiero is in charge of the Laser and Systems Division of Miyachi Unitek Corp., 1820 S. Myrtle Ave., Monrovia, CA 91016. With an engineering degree from California State University at Los Angeles, he has 30 years of experience in a broad range of engineering disciplines and management positions, including general management of the Laser and Systems Division for the past 6 years. The Laser and Systems Division provides laser welding and processing equipment, complete turnkey laser processing workstations, process development, and 24/7 technical field support worldwide. Rodighiero can be reached at mark.rodighiero@miyachiunitek.com. en3lidgeu
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