Plating Considerations:
Many hermetic packages employ plating to improve corrosion resistance, solderability, or for better absorption of laser energy. Nickel and gold are the most common plating choices. Nickel can be used alone, and it is almost always used as an under-plating for gold or tin. These metals are plated on top of the nickel, because they do not plate well directly to other base metals. Nickel can be plated by an electrolytic or electroless process. Ferrous alloys such as Kovar and stainless steel must be plated with electrolytic nickel. Electroless nickel plating contains phosphorous, which produces inter-metallics within the weld that cause porosity and cracking. Gold-plated Kovar packages weld well, as long as there is not an under-plating of electroless nickel. Gold-plated mild steel produces weld voids due to inter-metallic segregation in the weld. Aluminum and copper can benefit from nickel plating, because nickel absorbs the nd:YAG laser-energy much better than the base metals. This technique is not often used with aluminum but is an excellent aid in the welding of copper and its alloys. Tin and zinc have low melting and vaporization points, which result in porous welds. Tin and zinc plating should be absent from the weld joint.
Even when plating beneficial to the laser welding process are chosen there are other plating additives, such as organic brighteners, that can cause problems. Organic brighteners are gaseous when metals are liquid and, as discussed before, they create voids in the weld. Matte finishes with no organic brighteners work best for improving laser welding performance. If a plating is used that are not compatible with laser welding, they must not be present in the weld joint. Masking the weld area before plating, or machine removal of the plating in the weld zone is a common practice. In all cases, it is important to specify the correct plating, and perform weld tests before finalizing the weld joint design.
The Welding Process:
The hermetic weld sealing process begins by placing the parts in a vacuum bake-out oven to remove moisture. The parts are typically baked for 24 hours at 125 °C. Since this is a long process, more than one oven may be needed to fully utilize the laser's welding capabilities. When the baking cycle has been completed the chamber is filled with an inert atmosphere. The parts can now be transferred to the glove-box or automated enclosure for further assembly and for laser welding.
The component parts need to mounted into a fixture designed to hold them securely for the accurate positioning during the programmed weld cycle. The fixture can be a tray matrix designed to position a number of component parts. Complex parts may need to be tack-welded before seam welding can be performed. The welding cycle can last from a few seconds to a few minutes, depending on the length of the weld and the number of parts being welded. After welding the parts are removed from the enclosure (glove-box). If rework is required, the weld joint can be machine out and the lid removed, and a new lid can be welded in place. In some cases an over size lid may be required.
Conclusion:
The laser welding process allows designers to expand upon the types of joints, material, and plating options for their products. These options, along with the consistent quality, speed, reliability, flexibility, reduced energy consumption, and other advantages, are the reasons the use of laser welding is becoming the leading technology for hermetic sealing.
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