How a Laser Welds:
When the laser energy is absorbed by the material, heat is conducted into the material, creating a weld pool in a very localized area. With many metals no filler material is needed, but a tight fit-up of the parts is essential.
Figures l-a through l-h (located at end of document) show top views and cross-sections of welds in various hermetic weld sealed packages.
Typical weld penetrations can be adjusted within a range from 0.01" to 0.06", depending on the materials and configuration of the package joint. The heat input to the part is kept to a minimum because of the laser's ability to deliver short duration pulse of high energy in a small (< 0.020”) weld nugget. As a result, even high reflection materials such as aluminum, copper, and gold pulsed nd:YAG laser can produce consistent quality weld joints.
Laser welding is typically performed in an inert atmosphere (nitrogen – argon), with less than 100 ppm of moisture and oxygen. A slow-flow cover-gas also should be supplied through the beam-delivery nozzle, or through a side jet, to keep debris from being deposited on the focusing lens. This keeps the weld free of contaminates. Additionally, the inert gas with 10% helium can be contained within the welded package, so that the helium can be used as a tracer gas for leak detection of the hermetic seal.
Joint Considerations:
There are three types of laser weld joints - butt, lap, and fillet - each with its associated joint configuration. Figures 2a through 2e below show the geometry of each of these joint types.
A butt weld design is typically used for larger housings. Tight fit-up tolerances between the lid and base are important. Usually no special tooling is required to hold the lid in position. Since this laser welding process does not usually require any filler material, the fit-up of the lid to the package is critical in order to ensure a hermetic seal. If the gap is too large, the materials may not flow together to make the weld joint.
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