Laser Welding - Design Guideline for Hermetic Sealing
Precision components or sensors exposed to corrosive conditions, are required to have extreme performance characteristics, and need to be sealed hermetically into special packages. Epoxy gluing, resistance welding, soldering, electron-beam welding, and laser welding are several of the techniques available for sealing these packages. The disadvantage in the use of epoxies is that they do not create a hermetic seal as moisture migrates through the epoxy, destroying its hermetic seal.
Resistance seam welding is a reliable process that has been around for many years, but it has several disadvantages. For one, the process requires that the materials to be welded have high electrical resistance, and therefore cannot be used to weld materials such as aluminum or copper. Second, the electrodes must be “burned in" on several packages before production welding begins. Furthermore, resistance welding can be used only for a lap-weld joint.
Problems develop when solder-sealing is attempted on large packages. The process requires the whole assembly to be heated, which can compromise performance of some electronic components, and the use of low-melting-temperature alloys may result in the materials not being fully wetted.
Electron-beam welding has many of the same advantages as laser welding, but E-beam systems require vacuum chambers, X-ray shielding, and a fill of inert gas after welding, requiring a sealing operation by conventional means thereafter.
Reliability, minimal heat distortion, high processing speeds, a non-contact process, and the flexibility of CNC programming are just a few of the advantages that have resulted in the increased use of laser welding. However there are several considerations in package design that need to be addressed to enable the designer to maximize the unique functions, features and benefits of laser welding.
First, a short review of laser welding systems and the welding process. In its simplest form, a laser-welding system consists of a laser, beam delivery, and workstation. Lasers with wave lengths ranging between 808 nm to 1064 nm (Direct Diode and nd:YAG laser) are best suited for hermetic seam welding. Pulsing capabilities of nd:YAG lasers deliver high energy (joules/cm2) with short pulse duration to the workpiece which generate minimal heat input beyond the immediate weld joint area. These short wave length laser can also weld reflective materials, e.g.; aluminum, gold, silver, copper, beryllium copper, etc. The laser beam can be delivered to the workstation by standard hard optics or through a fiber optic delivery (FOBD) weld head. When standard optics are used, the laser is typically positioned on top of the workstation and a mirror angled at 45° directs the beam downward through the focusing lens to the work piece. A FOBD uses a flexible, optical cable to deliver the beam to the workstation allowing the laser to be remotely located away from the processing area. Time-sharing or energy-sharing fiber systems permit the output of one laser system to be used at several workstations. A closed-circuit television coaxial viewing system is an effective processing tool, as the camera is configured for a coaxial view directly down the beam-delivery path. Cross-hairs are electronically generated on the TV screen and centered where the focused laser beam strikes the work piece. The focused spot then can be automatically positioned to the weld joint by moving the part and/or moving the FOBD weld head. The CNC controller can also be programmed to include laser process parameters, e.g., shutter controls, laser power and pulse parameters, shield-gas delivery, etc.. Programs for each type of part to be welded can be stored in the CNC controller requiring little to no adjustments in the set-up.
These features facilitate the design and development of integrated automated welding systems. An enclosure such as a manual glove-box system or an automated enclose workstation that is sealed from the outside atmosphere while its interior is purged and filled with a purified, inert-gas mixture. Parts are loaded through a two-way isolated transfer chamber, where they are positioned for welding.
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