A glass-to-metal seal is a way to protect electrical components by sealing them within glass and metal. There are two types of glass-to-metal seals. Matched seals use varieties of glass and metal that have similar thermal expansion properties, while compression seals use materials that respond to heat differently. The glass-to-metal seal technique is often used in the aerospace and military sectors, where electronics need to perform under harsh environmental conditions.
The purpose of a glass-to-metal seal is to create an airtight, or hermetic, seal around the electronic components. This is accomplished by placing the electronics inside a glass component that is surrounded by a metal part. The glass is formed using sintering. Sintering is a process of creating a solid mass out of powdered or crushed elements by heating the material to just below its melting point and then applying pressure to shape the material. This results in a pure, uniform final product.
A matched glass-to-metal seal is made out of materials that are chosen for their similar thermal expansion coefficients. The thermal expansion coefficient of a material is a value that can be plugged into a formula to find out how much a material will expand for each degree of temperature increase. Using matched materials means that both the glass and the metal will expand and contract at the same rate with heat and cold. This ensures that the glass-to-metal seal will not break when used under the most extreme conditions.
Matched seals are often made using kovar (KV) housings and pins. Kovar is an alloy made of iron, nickel and cobalt. It is ideal because it has a thermal expansion coefficient that is similar to glass over a wide range of temperatures. The glass part of the glass-to-metal matched seal is made using borosilicate glass, a glass made of silica and boron oxide. Matched seals can be created in almost any shape.
Compression seals use a housing material that has a thermal expansion coefficient that is higher than that of the glass. The housing contracts around the glass as it solidifies. There are three types of compression seals: steel and stainless steel compression seals, iron compression seals, and number 42 alloy compression seals. Number 42 alloy is an alloy made of nickel and small quantities of other materials. The glass in a compression seal is usually a barium alkali, and the pins are made of either nickel or iron.