GUADIZ LASER WELDING FACTORS

Metals with low boiling points produce a large amount of metal vapor which could initiate gas breakdown and plasma generation in the region of high beam intensity just above the metal surface. This plasma, which readily absorbs the laser energy, can block the beam passes, and bubbles tend to form at the root of the weld. If the viscosity is high, these bubbles do not escape before the molten metal solidifies.

Although the melting point of metals does not have a significant effect on laser weldability, it must be reached during the initial absorption of energy. Thus, low melting point materials are easier to weld with a laser than high melting point metals.

Chemical reactions, such as oxidation or nitriding, with atmospheric gases at high temperatures can pose problems, particularly when the oxides or other elements formed have disassociation temperatures far above the melting point of the metal. The result is brittle, porous welds. Covering the welding area with an inert gas such as argon or helium minimizes these reactions in most cases. For some materials, it may be necessary to weld within a sealed chamber to prevent outside contamination.

For welding aluminum to hermetically sealed semiconductor packages, the introduction of silicon-aluminum alloys vastly improves the weld by providing a solidification temperature significantly lower than the parent material.7

For this particular application, Simpson recommends type 4047 aluminum which has a melting point of 1,070 °F to 1,080 °F compared to the 1,200 °F melting point of the 6061 aluminum used for the housing packages. During cooling, the outside interface cools fastest. As the boundary weld passes through the brittle phase, the core of the weld bead acts like warm taffy and yields with the shrinkages, preventing the build-up of shrinkage stresses.