Compression seals employ a coefficient of thermal expansion mismatch between glass, housing and pins. The hermetic seal is established because the thermal compression of the outer housing acts on the glass as the parts cool down after heat treatment. Housing materials with a high CTE, such as stainless steels or titanium are well suited for compression seals. Depending on design, a compression seal can withstand a very high level of pressure (up to 3000 bars possible). This type of glass-to-metal seal is, among other uses, perfectly suited for high-pressure sensors.
In a matched glass-to-metal seal, the coefficient of thermal expansion of glass, housing and pins are matched as closely as possible. Matched seals have the advantage that only small levels of residual stress are present and the feedthrough can withstand very high temperatures. The seal between glass and metal is mainly established chemically through an oxide layer. Typically, Kovar is used as a housing material for matched seals. This type of glass-to-metal seal is often used for hermetic electronics packages.
Glass-ceramics feedthroughs are matched seals which can used for high performance applications. The glass-ceramic material has amorphous and crystalline phases and is produced through controlled crystallization during the sealing process. Glass-ceramics seals have many advantages over conventional seals: They can be matched to many types of metals, can sustain temperatures up to 700 °C and are even more chemically robust. Using glass-ceramics seals, we can manufacture feedthroughs with copper conductors and stainless-steel housings.
|Compression Seal||Matched Seal|
|Sealing principle||Thermal compression between glass and housing||Chemical bonding at the interfaces|
|Typical housing materials||Carbon steels, various stainless steels, Titanium, Inconel, Hastelloy, etc.||Kovar, alloy 42, Molybdenum, etc.|
|Typical pin materials||NiFe47 / alloy 52, Inconel, copper-core alloy 52, Kovar, Molybdenum, etc.||Kovar, alloy 42, Molybdenum, etc.|
|Typical glass materials||Barium oxide, Soda-lime, Aluminoborate, Borosilicate, etc.||Borosilicate|
|Special properties||High bust pressure capability, very robust, chemically very inert||Very high temperature capability, stress-free design|
|Applications||Sensor-housings, hermetic connectors, sight glasses, power feedthroughs||Electronics packages|
Example material combinations for compression seals:
Stainless steel housing – Barium alkali glass – Nife47 / alloy 52 pins: For small-signal sensor feedthroughs
Carbon steel housing – Barium alkali glass – Carbon steel pin: For high-power terminal plates
Carbon steel / stainless steel housing – Soda-lime glass: For sight glasses
Example material combination for matched seal:
Kovar housing – Borosilicate glass – Kovar pins: For RF electronics packages
Brazing can be combined with the glass-to-metal sealing process. This is often used to attach an additional component that does not need to be electrically insulated (e.g. adding a filling tube to pressure sensor housing).
Common brazing materials that can be used alongside glass sealing:
Ag-Cu-Pd: Various compositions for brazing temperatures of 800 to 1000 °C.
Au-Ni: For brazing at around 950 °C.
Surface treatment options
Various surface treatment options exist, which are typically performed after the sealing process. They serve the purpose of either corrosion protection or to enable the electrical interconnection. For small-signal feedthroughs, the plating is often only performed selectively on the pin to save precious metal.
Common surface treatment options include:
Nickel + soft gold: For wire-bondable and solderable pins
Nickel + hard gold: For connector pins (also solderable)
Palladium / Nickel: For connectors as an alternative to hard gold
Nickel: Corrosion protection
Zinc + thick-film passivation: Corrosion protection
Specifying your custom design
We are here to assist you with finding the right design for your hermetic application. Please do not hesitate to contact us with your requirements or general design idea. Here are some tips on specifying your requirements:
The Dietze Group helps its customers with custom interconnection solutions from concept to series production. Our products are designed in close collaboration with our customers and engineered to meet the application’s specific needs.