Fiber Laser Welding of High Integrity Implantable Medical Devices

Transcription

1 Fiber Laser Welding of High Integrity Implantable Medical Devices Authors: Sergey Safarevich, Serdar Unal, Silke Pflueger, Tony Hoult, David Braman LaserStar Technologies Corporation, One Industrial Court, Riverside, RI 02915, USA w w w. l a s e r s t a r. n e t

2 FIBER LASER WELDING OF HIGH INTEGRITY IMPLANTABLE MEDICAL DEVICES Paper (Paper Number) Sergey Safarevich 1, Serdar Unal 1, Silke Pflueger 2, Tony Hoult 2, David Braman 3 1 St. Jude Medical, Valley View Court, Sylmar, CA, 91342, USA 2 SPI Lasers LLC, 1700 Wyatt Drive, Suite 1, Santa Clara, CA, 95054, USA 3 Crafford LaserStar Technologies Corporation, 1 Industrial Court, Riverside, RI 02915, USA Abstract Medical implantable devices like pacemakers, ICDs and leads are progressively becoming more compact and lighter. Some of the components used for hermetic and structural welding are still large and thick enough for conventional Nd:YAG pulsed lasers. However, some of the welded components, particularly used for electrical connections, have become so small and thin that the conventional Nd:YAG laser performs like a big and rough tool. The greatest challenge for welding is that the miniature components have limited heat sink ability, particularly if the components contain heat sensitive elements, such as electronic chips, lower melting point plastics, and organic materials. Properly selected laser beam characteristics, such as diameter of the beam and energy distribution within the focused spot in welding area, may significantly improve welding process increasing yield during production. The results of this study have identified three fundamental criteria relating to the laser beam welding of the miniature components. First is a comparable diameter of the beam in welding area. Second is the consistency of energy output. Third is the minimization of the component heat input. The presentation discusses the analysis made on the application of the FiberStar portable laser workstation, which incorporates the SPI High Power 1090 nm 100W air-cooled fiber laser engine, to production components such as super thin Pt, Cu-Ni, Cu and MP35N conductors, cables, electrodes and compact electronic subassemblies. Introduction Unique properties of Nd:YAG pulsed-laser beam has found wide array of applications in medical devices such as pacemakers, ICD and leads. For example, average device may contain more than dozen laser weld joints, which provide hermeticity, mechanical strength for the device body and reliable electrical connection for the electrical circuits. Each type of weld joint (hermetic, structural and for electrical connections) has specific requirements [1], which reflect functional characteristics of the joint. These devices are designed to be implanted in humans for many years improving and some times saving people s lives. Some of the weld joints are relatively big and massive for the conventional laser welding application. These joints easy withstand much heat generated during welding and accept output energy variation of approximately +/- 10% and more. However, all the implantable devices have a tendency to be designed smaller and smaller. Due to this trend, some of the welded components are now so small, even the precise laser beam becomes a relatively big and rough tool. To weld miniature components successfully three fundamental requirements to the laser beam must be met. They are a small diameter of the beam in focus, which has to be comparable to the component size, consistent pulse-to-pulse energy output and minimal heat input into the components. Hermetic Welding It is important for a hermetic joint, that weld has deep penetration, which provides reliable hermetic sealing between welded components. Components for hermetic welding usually built from relatively thick ( ) Titanium. Figure 1 shows typical hermetic welds. To provide pure atmosphere inside the device, hermetic welding occurs in controlled atmosphere glove boxes with integrated lasers and CNC motion, which provide reliable and repeatable processes. Laser parameters allow sealing the joints with weld penetration deeper than at least 30% of the component thickness. This can be achieved using approximately 2.0 Joules consequent laser pulses with ~80% overlap. The beam diameter in focus should be adjusted to produce relatively wide weld (approximately ). The wide weld minimizes welding rejects caused by possible gap and mismatch between the components. The hermetic welding of the implantable devices is successful area for conventional laser application.

3 Seam Weld Coil to Electrode Weld Feedthrough Weld Crimp Slug to Ring Weld Case Seam Weld Brace to Case Weld Feedthrough to Case Weld Figure 2 Typical structural joints welded using conventional laser. Component thickness is around Width of the welds is around Figure 1 Hermetic welds (using conventional laser) of Titanium components with thickness of Weld widths are around Structural Welding Mechanical strength is the major requirement for structural joints. Usually the welded joint consists of large and strong components. Some of them could withstand even more than 50 lbs of tensile force. These joints have to have large fusion areas between the components, which can be provided by the conventional lasers. Fig 2 exhibits typical structural joints. Many of structural joints are used as elements of electrical circuits. In this case, large fusion areas provide excessive electrical continuity. Coil and crimp slug structural welds are large and strong (Figure 2). Electrical connection in the joints is assured, and the joints have overwhelming strength. Electrical Connection Joints For an electrical joint, weld must provide reliable electrical continuity for the device circuit with minimum electrical resistance. Components and fusion area do not need to be big and strong, as it required for structural welding. Contrary to hermetic and structural, joints, electrical connections may contain very small and thin components close to range (see Figure 3) or even less. Some of new generation of implantable leads equipped with miniature electronic components (chips, diodes, etc.), which are encapsulated inside the leads closer to the heart. The leads may have tens of welded electrical joints located in limited space surrounded and connected to the electronic elements. This sets very special and strict requirements to the laser beam size, minimization of heat input and energy output pulse-topulse stability.

4 High-Reflectivity FBG ( >99% ) GTWave Gain Medium Output Coupler FBG (~5% Reflectivity) High Brightness 915nm/977nm Diode Pumps / Modules Tap couplers + Monitor Diodes Output Fiber / Beam Delivery Optics Figure 4 Block diagram of fiber laser Quality of the beam Figure 3 Conductor ribbon thickness and width needs to be welded inside the ring electrode. Wall thickness of the ring electrode is Fiber Lasers for Welding The resonator of the fiber laser is single mode optical fiber, as is the delivery fiber to the work piece. A single mode fiber only allows a TEM00 mode to exist and be transmitted, resulting in a beam with a diffraction limited beam quality of M 2 =1.1 to 1.5. In the beam delivery optic, the laser is coupled out of the fiber and collimated while preserving the beam quality. Fig. 5 shows a beam scan of the fiber laser. Fiber lasers were introduced commercially in the 1990s mainly for printing applications. They have now become a production standard for many printing and medical device applications due to their size and reliability. Where standard Nd:YAG lasers are lamp pumped crystals with external resonators that may be fiber coupled, fiber lasers are truly generated in a fiber. Figure 4 shows the schematic of a fiber laser. The diode pump lasers are directly fiber coupled, bundled, and then spliced to the main laser fiber. The mirrors are fiber optical elements, so called Fiber Bragg Gratings, which are also spliced to the laser fiber. A spliced-in power pickup in the form of a tap coupler allows monitoring and controlling the forward laser power and monitors for any back reflections to prevent damage to the laser. The laser beam is brought to the work piece via an external fiber and beam delivery optic. At the beam delivery optic, the laser beam exits the fiber and is collimated with a lens. Figure 5 Beam shape measurements of 100W fiber laser Pulse-to-pulse Stability Pulse-to-pulse stability is a good measure of how a laser performs when modulated. Fig. 6 shows a measurement of a pulse train of 5J pulses, 100W peak, 50ms pulse length. The measurement shows good stability, with 99% of pulses within +/-0.5% of the average energy. Fig. 7 shows a similar measurement, with pulse energy plotted vs. time. Pulse Energy Distribution for 10Hz, 50msec pulses at 100W No. Pulses Avg 4.857J Min 4.832J 20 Max 4.882J 10 Sigma 0.007J Pulse Energy (J)

5 Figure 6 Pulse-to-pulse stability for modulated CWM fiber laser 0.11 Pulse Energy (J) :59:59 16:00:03 16:00:07 16:00:12 16:00:16 16:00:20 time(h) Figure 7 Short term power stability over 20 second at 100Hz, 2 ms pulse length, 50 W peak power and 10 W average power [2] This energy stability is achieved twofold: Fiber lasers have a closed loop feedback for power which keeps the output power constant by controlling the pump diode power. This system has a response time in the microsecond region, keeping the power within less than 0.5% when run cw or modulated. For spot welding applications, the laser is modulated with pulses in the millisecond region. The length of the pulse is controlled with an external pulse generator, which can keep pulse lengths extremely stable, guaranteeing a stable pulse. Portable Fiber Lasers Systems for Welding The advent of compact and air cooled fiber laser engines made it possible to incorporate this technology into a complete portable welding system. The FiberStar Laser System (Fig. 8) is a next generation complete laser workstation incorporating the latest technology and advances to ensure precise control of the laser pulse wattage, pulse energy, pulse width, and beam diameter using either a keypad or joysticks.. This accurate control of the laser energy is important to the reliability of the weld for miniature components as discussed. Figure 8 FiberStar portable welding system The laser pulse energy and pulse widths are controlled with repetition rates to 20 Hz and pulse widths to 250 ms, also allowing pulse shaping. The control and high energy stability is maintained by constantly monitoring and adjusting the laser engine laser energy. The cw mode allows for use with other applications. The system has incorporated the feature to limit or prohibit the operator from making adjustments or other changes to weld parameters. Operator safety is maintained through redundant monitoring by the control systems and engineering design. The system incorporates a dual window large triopening welding chamber with the capability of adding deep trays. This design also allows for easy access of hands thru ergonomically designed hand openings. Automation stages are easily incorporated. The precise alignment of the microscope cross hair to the laser beam is accurately and easily accomplished using a refined alignment device. This precise alignment is needed when working with the extremely small components being discussed and the small laser spot size. Fiber Laser Welding Applications The experience of using the FiberStar laser demonstrates that the laser can be used not only for welding extra small electrical components, but for some structural joints as well. In the pictures below, the examples of the components (electrical joints) welded using the FiberStar are shown. Figure 9 exhibits examples of welded miniature electrical joints demonstrating significant advantage of fiber laser an ability to concentrate welding heat in the extremely small area. That is why the dimension of the fusion zone is as small as the size of the miniature conductors (see Figure 9 left and middle).

6 Figure 9 Application of FiberStar laser. Top left: conductor is a solid wire with diameter of welded to a ribbon. Top right: conductor is a cable with diameter of welded to a ribbon. Bottom: small conductor ribbon with thickness of welded to the ring electrode In Figure 10, the MP35N cables welded to the thin (0.002 ) Platinum conductors. The welds placed with close proximity to the electronic component. In Figure 11, the wire diameter single filar micro-coil (MP35N) welded to the thick Platinum conductor. The weld placed with close proximity to the electronic components. In both cases, no damage of electronic components found. Figure 12 exhibits the fiber laser welding of cables. Cables are used widely in implantable leads as electrical conductors. Usually they consist of more than seven strings bundled together. The cable could be welded to other components or to another cable as well. Next fiber laser application is splicing of Pt/Ir wires. The laser is capable to weld the wires using a single pulse for each joint demonstrating excellent consistency of the weld characteristics (see Figure 13). Figure 10 Application of FiberStar laser. MP35N cable welded to Platinum attached to diode weld conductor. The welds are within a few mils of the electronic component

7 Figure 11 Application of FiberStar laser wire diameter single filar micro-coil (MP35N) welded to the thick Platinum conductor Figure 13. Application of FiberStar laser. Splicing of Pt/Ir wires with diameter of The laser provides excellent consistency of the weld quality and appearance The fiber laser could be useful for structural welding as well. See Figure 14 where two relatively big and thick components circumferentially welded together. In this application fiber laser provides excessive mechanical strength of the joint. Figure 12 Application of FiberStar laser. Top: cable termination. Middle: Cables joining. Bottom: Cable to ring electrode electrical connection weld

8 received his Masters and Bachelor s in Electrical Engineering from the University of RI. He has a number of patents in dimensional measurement instruments, resistive welding and machinery automation. Figure 14. Application of FiberStar laser. Set screw housing structural weld References [1] Safarevich, S., (2006) Laser Welding of Implantable Devices, in Proceedings ALAC 2006 Medical Devices, Vol. 10. [2] K. Kleine, W. Fox, K. Watkins (2004) Micro Welding with Pulsed Single Mode Fiber Lasers, in Proceedings of 23rd International Congress of Lasers and Electro-Optics Dr. Tony Hoult is trained as a Materials Engineer. He has now been working in the laser industry for many years as a Laser Materials Processing specialist in both industry and in academia. He is currently Applications Manager for SPI Lasers, a major industrial fiber laser supplier. Using these novel laser sources, he has improved a number of existing leading edge laser applications and has identified entirely new processing regimes in others. Dr. Silke Pflueger has 20 years of experience in industrial lasers and their applications and is currently the Director Sales North America for SPI Lasers. Before joining SPI Lasers in 2004, she held Engineering and Marketing positions in SDL / JDSU working with high power laser diodes and fiber lasers. Ms. Pflueger received her Ph.D. in Mechanical Engineering from the Technical University in Aachen, Germany, where she worked at the Fraunhofer Institute for Laser Technology. Meet the Author(s) Dr. Sergey Safarevich is the Senior Principal Engineer of St. Jude Medical in Sylmar, CA. He is responsible for welding (development and manufacturing) of pacemakers, ICD and leads. He received his Ph. D. in laser welding in 1986 and M.S. in welding engineering in 1974 from St. Petersburg Technical University, Russia. He has 20 patents and 19 international publications related to the laser robotic systems, FMS and precision welding technology including laser and micro plasma. FiberStar Workstation Manufacturer s Information - Contact: Crafford-LaserStar Technologies Corporation One Industrial Court, Riverside, RI USA T: F: sales@laserstar.net Mr. Serdar Unal is a Process Engineer III at St. Jude Medical in Sylmar, CA. He is responsible for developing and supporting laser welding processes for Leads manufacturing. He received his Masters and Bachelor s in Mechanical Engineering from University of Missouri-Rolla in 2000 and 1998, respectively. He is currently attending distance learning program at California Polytechnic State University, San Luis Obispo to receive his Master s degree in Biomedical Engineering. Mr. David P. Braman is the Vice President of Engineering at Crafford-LaserStar Technologies Corporation in Riverside, RI. He is responsible for all laser product development and engineering. He