Quartz-crystal resonators, monolithic filters and oscillators are electronic components which place extreme demands on hermetic sealing due to the environments they are used in. Hence, glass-to-metal seals are the safest choice for such applications.
Quartz crystals with a highly precise and stable resonant frequency are sensitive to environmental influences such as moisture, temperature and impact stress, which can impair their performance.
In order to protect the quartz crystals from such influences, they must be hermetically encapsulated in special enclosures that have been specifically designed for this purpose. Hermetic encapsulation means that the lead wires are embedded vacuum-tight in glass using the glass-to-metal sealing process. HC and UM types generally can have either a rectangular or a round geometry.