The viscous flow of glass has effects to compensate and planarize surface irregularities, convenient for bonding wafers with a high roughness due to plasma etching or deposition.
The glass has to flow and wet the soldered surfaces well below the temperature where deformation or degradation of either of the joined materials or nearby structures (e.g., metallization layers on chips or ceramic substrates) occurs.
This bonding procedure also allows the realization of metallic feedthroughs to contact active structures in the hermetically sealed cavity.
Glass frit as a dielectric material does not need additional passivation for preventing leakage currents at process temperatures up to 125 °C (257 °F).
[4] Glass frit bonding is used to encapsulate surface micro-machined sensors, i.e. gyroscopes and accelerometers.
Other applications are the sealing of absolute pressure sensor cavities, the mounting of optical windows and the capping of thermally active devices.
[4] Screen printing, as a commonly used deposition method, provides a technique of structuring for the glass frit material.
[3] The risk of glass frit flowing into the structures can be prevented by optimization of the screen printing process.
The exact positioning of the screen print structures to the cap wafer are required to ensure an accurate bond.
[3] The conditioning process consists of: The initial step comprises drying for 5 to 7 minutes at 100 to 120 °C in order to diffuse solvents out of the interface.
[3] The cooling process leads especially at higher temperatures to thermal stress in the glass frit layer that has to be considered in the lifetime analysis of the bond frame.
Devitrifying solders undergo partial crystallization during solidifying, forming a glass-ceramic, a composite of glassy and crystalline phases.
Devitrifying solders usually create a stronger mechanical bond, but are more temperature-sensitive and the seal is more likely to be leaky; due to their polycrystalline structure they tend to be translucent or opaque.
Glass solders can be also used as sealants; a vitreous enamel coating on iron lowered its permeability to hydrogen 10 times.
They can be mixed with water or alcohol to form a paste for easy application, or with dissolved nitrocellulose or other suitable binder for adhering to the surfaces until being melted.
The solder glass can be also applied from molten state to the area of the future joint during manufacture of the part.
Due to their low viscosity in molten state, lead glasses with high PbO content (often 70–85%) are frequently used.
Several glass frit pastes are commercially available, e.g. FERRO FX-11-0366, and every single one need individual handling after deposition.
[5] The choice of the paste depends on various factors, i.e. deposition method, substrate material and process temperatures.
Those precipitations decrease the strength of the bond and are reliability risks that have to be considered for the lifetime predictions of the devices.
Outgassing of water from the glass solder during encapsulation was a cause of high failure rates of early CERDIP integrated circuits.
The expansion matching is not critical in applications where thin layers are used on small areas, e.g., fireable inks, or where the joint will be subjected to a permanent compression (e.g., by an external steel shell) offsetting the thermally introduced tensile stresses.
[18] A glass solder is used, e.g., for joining together parts of cathode ray tubes and plasma display panels.