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www.us-tech.com
December, 2020
Hermetic Sealing Protects Welds in Harsh Environments
By Mark Sunico, Product Engineer, Amada Weld Tech
resistance seam welding or opposed electrode pro- jection resistance welding. It is a key manufactur- ing process utilized in assembling microelectronic packages for communication, aerospace and med- ical device manufacturing. Microelectronic devices are commonly used in
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industrial and commercial communications, trans- portation, and the military and aerospace indus- tries and include optical sensors, pressure sensors, communications devices, thermal and laser imag- ing, and power amplifiers. By sealing these electronic packages, external
contaminants — such as moisture — are kept out, preventing degradation of the electronic compo- nents inside and extending lifetime usefulness. Implantable medical devices, such as pacemak-
ers and defibrillators also require careful hermetic sealing to protect both the device and the patient.
Packages and Welding Methods There are two primary types of packages:
metallic tub and ceramic. The preferred material for a metallic tub base package is Kovar™, which has a similar coefficient of thermal expansion (CTE) as glass. The use of this material prevents the metal-
to-glass seals of the feedthrough connectors of the package from leaking, due to material expansion from heat generated during the welding process. Ceramic packages are made of a ceramic sub- strate with a brazed metal seal ring. Kovar is also
Metal tub package.
process utilizes opposing electrodes to join a head- er (containing the electronic device) to a cap de- signed with a ring or annular projection, by run- ning current across the electrodes through both the cap and the header. The generated heat is directed through the projection in order to weld the parts together. A
ermetic sealing is the encapsulation of elec- tronic components into an airtight metal or ceramic housing, using either parallel gap
used in ceramic packages. The Kovar is brazed onto the ceramic base as a seal ring to which the lid is welded. Parallel gap seam welding is one way to exe-
cute a hermetic seal. A seam welder with rolling wheel electrodes is connected to a power supply, which is responsible for delivering electric current across the electrodes, through the lid and through the package. The seam welder delivers multiple overlapping weld spots, thus creating a continuous weld.
Another method used to execute hermetic seals is opposed electrode projection welding. This
successful weld should have at least a 50 to 90 per- cent projection collapse. Linear displacement measuring device sensors, also known as LVDTs, can be added to the weld head to measure this col- lapse. A fillet formation is typically seen at the perimeter of the cap indicating a successful weld. As with parallel gap seam welding, the part
design of the metal packages in opposed electrode projection seam welding is very important. The projection can be either on the cap or the header, but there must be a constraining feature between the two so that the parts self-align. The preferred material, again, is Kovar. For best results, the pro- jection should be located in the middle of the flange, so that as the projection collapses, the dis- placed material is evenly distributed across the width of the flange.
Testing and Troubleshooting Weld strength destructive testing can be per-
formed in hermetic sealing applications to ensure that welds are secure. In destructive testing, see- ing at least a 25 percent weld joint still intact after significant attempts to mechanically separate the lids or caps from the base is a good indicator that a strong weld was achieved. Other methods of testing hermetic reliability
include helium fine leak and gross leak bubble testing, optical fine leak detection, internal gas analysis, particle impact noise detection, and tem- perature cycling. Hermetic seam sealing technology can be crit- ical to success in a number of demanding applica-
Continued on next page
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