By Mark Rodighiero
Implantable medical device showing typical contours.
AT A GLANCE Nd:YAG laser system
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.
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 plot showing rapid changes in acceleration in X and Y axes of focus head postioner while executing a contour seam weld.
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 LightInteraction 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.
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.
Autogenously welded gold-plated copper leads onto semiconductor connection pads using 532 nm pulsed Nd:YAG laser welder.
ONLINEFor 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 firstname.lastname@example.org . en3lidgeu