ASTM F-15 / KOVAR MATERIAL

Glass-to-metal seals (GTMS) and ceramic-to-metal seals (CerTMS®) are vacuum tight glass and ceramic assemblies with metals, which are utilised to feed electrical and electronic conductors through hermetically sealed package walls.

The term “Hermetically Sealed” comes from Greek / Egyptian mythology, with Hermes the Olympian God magically sealing treasure chests to keep them impervious to entry. Today, hermetic packages protect sensitive electronic devices from challenging conditions such as the earth’s atmosphere, space and marine environments.

Glass-to-metal sealing comprises of the process of bonding glass & metal to create a hermetic seal, also allowing isolated electrical current through conductive wires external to the metal package to the assembly inside, without compromising the internal environment . The sensitive electronic components inside the package remain sheltered from the harsh environment outside, receiving electricity via the isolated wires only. Or in the case of optical devices optical quality glass is also used to transmit visible and nonvisible wavelengths of light to optical sensors inside the metal package.

There are a vast number of products where metal and glass or ceramic are joined together. Applications include x-ray tubes, lighting tubes, microwave tubes, transistors, vacuum tubes, diodes and power tubes and photography flash bulbs. This technique has also been used in integrated circuits, such as flat pack and dual-in-line packages and optoelectronic component cases and bases.

A common glass-to-metal seal (GTMS) comprises of an external metal part into which a pre-formed sintered glass component element is sealed. The sintered glass element encloses one or more metal leads which are sealed into it. Due to the varying expansion coefficients of various glasses and metals, mechanical stresses during the melting process are unavoidable. However through adopting stringent design procedures and use of alloys such as F-15 alloy / Kovar it’s possible to produce glass to metal seals in which these stresses do not create a weakening of the assembly.

There are two types of glass-to-metal seals (GTMS) matched seals and compression seals. Kovar / F-15 is only used for matched seals. For compression seals the thermal expansion of the housing is much larger than glass so stainless steel is commonly used for this type of seal.
The seal’s function is to hermetically isolate environments from each other. One environment is the electrical conductor or pin and the other is the housing itself. Between these two is the glass. These three components are chosen based on their similarity of Coefficients of Thermal Expansion (CTE) at different temperatures between their transformation points. During the process a chemical bond is formed between an oxide and the glass on the conductor and the housing. This bond increases the strength of the the matched seal. Borosilicate glass and Kovar metal components are the most common materials to be used for matched seal engineering processes.

For example consider a traditional electrical lighting tube which creates immense heat that transfers to the glass shell. The glass can become extremely hot very quickly so using other metals which don’t possess the expansion properties of F15 / Kovar to connect the glass tubing to the metal structures would be dangerous. The reason is that traditional metals have thermal expansion and contraction properties much different to hard glass and therefore each component part, the glass and the metal, would expand and contract at different rates. This would therefore lead to increased pressure on the glass tube which would invariably not be able to maintain its hermetic seal, or worse still crack with the additional load.

There are several ways to produce glass-to-metal seals, but fundamentally they involve melting the glass and bonding it to the metal components using a high temperature furnace with a controlled atmosphere.

F15 alloy / Kovar is also frequently used in the telecommunications, military and defense, aerospace and electronic industries.

Thanks to its thermal expansion coefficient, fatigue strength and formability, F-15 Alloy / Kovar is often used as electronic packaging material and also in hybrid circuit packages.

The main properties to be considered when selecting materials for leads and leadframes are electrical conductivity, solderability, corrosion resistance, coefficient of thermal conductivity, fatigue properties, yield strength, and coefficient of thermal expansion.

The lead material must be electrically conductive with as low a resistance as possible, and be resistant to corrosion (corrosion increases the electrical resistance causing electrical failure and eventually mechanical fracture).

Leads have to be soldered to the board hence good solderability is required. Silver undercoats and gold overcoats are often used to improve F-15 / Kovar’s solderability level and to prevent surface oxidation.

Leads can also serve as a medium for dissipation of heat during device operation, so high thermal conductivity is required for the lead material.

Yield strength and ductility is required for formability (which is the ability to accommodate sharp bends without the material weakening). Fatigue properties are the ability of the lead to cope with cyclic stresses induced through vibration or varying temperatures.

Kovar is commonly used in lead and leadframe fabrication in ceramic chip carriers.