News Item - Measuring and maintaining weld quality

The quality of laser cutting and marking is in the main easy to assess. Visual inspection using relatively low magnification will in general easily reveal process defects. By contrast, for the inspection of laser welding, there is no comparable optical method for determining weld volume or defects, except to identify obvious gross defects. Most of the metallic implantable medical devices used within the industry are welded with lamp-pumped Nd:YAG lasers, using a combination of single-shot spot welding and seam welding. The pulsed laser operation enables rapid local heating and cooling, avoiding collateral thermal damage. The devices produced using this process are often wires, tubes, or assemblies combining both, with outside diameters as little as 200-300 μm and made from Stainless Steels, Nitinol or combinations of materials. Validating weld quality by destructive testing can of course only be performed on a few assemblies, therefore laser welded medical device conformity requires, amongst other things, extremely tight monitoring and control of the laser output parameters. In the past it has been difficult to qualify lamp-pumped laser welding in the manufacture of medical devices because of fluctuations in the lamp output, and therefore the laser output. These variations include shot to shot fluctuations in laser output power, which increase as the lamp ages, and the variations in output from lamp to lamp, which are significant, even when lamps are new. Historically the problem was addressed by frequent user intervention, which the medical device industry prefers not to have to deal with because of the validation regulations. To overcome these obstacles, some form of control of the laser output is required.

In control with Rofin’s (RCU) Closed loop control of electrical power and laser output pulse energy, ensures consistent lamp pump output, consistent laser energy output, and eliminates the interference of back reflections from the work piece. The Rofin Control Unit (RCU) provides such control to pulsed Nd:YAG laser welding resonators and it's use allows a pulse-to-pulse stability of <±1% to be achieved over the full range of laser powers. In addition, WYSIWYG (what you see is what you get) with regard to pulse shaping and ramping. These features provide the stable and predictable welding process required for medical device production.

Not only is it essential to be able to control electrical power and laser output pulse energy, to provide a stable and consistent weld, process traceability also forms a crucial part of medical device validation procedures. The traceability required for laser welded implantable parts is far reaching because of FDA (US Food and Drug Administration) regulations. According to the FDA, process validation is defined as "establishing documented evidence which provides a high degree of assurance that a specific process will consistently produce a product meeting its pre-determined specifications and quality characteristics." With this in mind, laser controllers should be capable of monitoring and recording the key laser parameters for each individual part which is processed and controlling the access levels to the system to avoid undocumented intervention.

Back to Current News