What are Hermetically Sealed Packages?

A hermetic seal is an air tight seal that is used to prevent contaminants such as solids, liquids, or gases from entering a package. Hermetic seals used in a wide range of applications including semiconductor electronics, thermostats, optical devices, MEMS, and switches. Electronic parts can be hermeticly sealed to ensure water vapor, dust particles or other particles do not enter package to ensure proper operation of the component. Many Semiconductor devices and components require hermetic seals to be used in High Reliability (Hi-Rel) applications.

Some RF and Microwave Parts that are often available with Hermetic Seals are:

Coaxial Connectors: Hermetically sealed RF connectors are used in a wide range of applications where entry of liquid, air or gas has to be avoided.

RF Semiconductors: Many Hi-Rel Applications like Space, Military and Critical communications require semiconductors ICs to be Hermitically Sealed.

Hermetically Sealed Relays: The device is sealed into a metal cover via welding to ensure zero intrusion of any form of materials. This is done to make the relay free from the impact of ambient conditions such as harmful gas, humidity, dust, etc.

Hermetically Sealed MEMS: To secure the operations of MEMS devices, it is advised to design a hermetically sealed package which protects the device from wet environments. There are multiple ways in which MEMS devices can be hermetically-sealed. In the one show below, a lid (metal) is placed on a ceramic package which is hermetically sealed via seam welding.

How do you Test the Hermeticity of a Seal?

The most common method used to determine the hermeticity of a seal is to measure the rate at which helium escapes from a package that has been pressurized with helium. As stated by MIL-STD-883C, a hermetic package is one in which the leak rate of helium after pressurization is below a rate specified with reference to the package size (refer to Table 1). The sealed package is placed in a chamber which is pressurized with helium (helium bomb) to a level depending on the package volume.

For example, a package with a volume of less than 0.05 cm³ is subjected to a pressure of 75 PSIG for at least 2 hours, but less than 2.2 hours (bomb condition exposure time). A helium detector is used to measure the leak rate of helium for no longer than 1 h (maximum dwell) after the circuit is removed from the bomb. The package is considered to be hermetic if the leak rate is less than 5×10^-8 atm cm³ s⁻¹ He (reject limit).