Hermetic sealing is the encapsulation of electronic components into an airtight metal or ceramic housing using parallel gap resistance seam welding or opposed electrode projection resistance seam welding. It is a key manufacturing process utilized in assembling micro-electronic packages for communication, aerospace and medical device manufacturing.
Microelectronic devices are commonly used in industrial commercial communications, transportation, military and aerospace industries and include optical sensors, pressure sensors, communications devices, thermal and laser imaging, and power amplifiers. By sealing these electronic packages, external contaminants – like moisture – are kept out, preventing degradation of the electronic components inside and extending lifetime usefulness.
Implantable medical devices, like pacemakers and defibrillators, also require careful hermetic sealing to protect both the device and the patient.
There are two primary types of packages: metallic tub and ceramic (see Figure 1). The preferred material for metallic tub packages 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 substrate with a brazed metal seal ring. Kovar is also used in ceramic packages; the Kovar is brazed onto the ceramic base as a seal ring to which the lid is welded.
Executing the seal
Parallel gap seam welding is one way to execute 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 the package. The seam welder delivers multiple overlapping weld spots, thus creating a continuous weld (see Figures 2 and 3).
Another method used to execute hermetic seals is opposed electrode projection seam welding. This process utilizes opposing electrodes to join a header (containing the electronic device) to a cap designed with a ring or annular projection by running current across the electrodes through the cap and the header. The generated heat is directed through the projection in order to weld the parts together (see Figure 4).
A successful weld should have at least a 50 to 90 percent projection collapse; linear displacement measuring device sensors can be added to the weld head to measure this collapse. Additionally, a fillet formation typically is 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 important. The projection can be 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 projection should be located in the middle of the flange, so that as the projection collapses, the displaced material is evenly distributed across the width of the flange.
Weld strength destructive testing can be performed in hermetic sealing applications to ensure that welds are secure. In destructive testing, seeing 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 temperature cycling.
Hermetic sealing technology can be critical to success in a number of demanding applications such as industrial 5G commercial communications, aerospace and military electronic devices. Ensuring an excellent seal through welding is extremely important, and only trusted weld device manufacturers that account for all the demands explained here can ensure highly accurate and reliable welds.
Manufacturers such as Amada Weld Tech are incorporating these standards into current controlled environment welding technology and are also developing technology for the future that will enable even greater accuracy and weld success. The future of hermetic sealing is likely to be robot-assisted seam sealing pick and place with smart vision systems, which have the potential to eliminate current margins of error and lead to even greater success in hermetic sealing of sensitive electronic devices.