KOVAR alloy (4J29) is the most commonly used metal casing material in the electronic packaging industry. Due to its linear expansion coefficient being the closest to that of molybdenum group glass and the fact that it can generate the minimum sealing force during the sealing (melting sealing) process with molybdenum group glass, it can achieve good air tightness. To ensure that the metal tube shell can achieve airtight sealing, the annealing process undoubtedly plays a crucial role in connecting the previous and subsequent sealing procedures. It not only eliminates the internal stress generated by the metal parts during the previous manufacturing process but also prepares the material structure for the implementation of the metal parts in the subsequent sealing process.
2 Purpose of Annealing

The main purpose of annealing the Kovar shell before sealing is:
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(1) Eliminate mechanical processing stress. When Kovar undergoes plastic deformation during cold working, approximately 10% to 15% of the applied energy is converted into internal energy [1], which is commonly referred to as internal stress. This causes the Kovar material structure to be in an unstable state. At room temperature, this state can be maintained for a considerable period of time without significant changes. Once it is heated, a series of changes in its structure and properties will occur, causing the material structure to tend towards a stable state. Such changes in structure and properties, especially in the microstructure, are reflected on the sealing surface as tensile stress on molybdenum group glass during high-temperature sealing, which may lead to the formation of tiny cracks in the glass and cause air leakage.

(2) Eliminate work hardening. During the cold working manufacturing process of Kovar parts, due to defects such as grain elongation and grain breakage in the internal material structure, the crystal defects increase significantly and the dislocation density rises. The smaller the distance between dislocations, the greater the interference between them, and the greater the distortion in the surrounding lattice. Each dislocation line has a stress field. Dislocations interact with each other through their respective stress fields, increasing the hardness and elasticity of Kovar while reducing its plasticity, which is known as work hardening. If work hardening is not eliminated, during high-temperature sealing, the stress field of the interaction between dislocations will be disrupted and lose balance due to the recovery or recrystallization of the crystal, which also has a certain stress impact on the sealing of metal and glass.

(3) Make the material structure uniform, refined, stable and consistent;

(4) Decarburization of metal surfaces. Carbon, as a reducing substance, has a strong reducing property. At high temperatures, it will absorb oxygen to form CO2, thus creating bubbles at the sealing surface between metal and glass or within the glass. This not only reduces the sealing strength between metal and glass but also seriously affects the performance of the glass. This is absolutely not allowed in the sealing process.

(5) Remove the internally adsorbed gas.

In addition to the above five functions, high-temperature heat treatment carried out in an appropriate atmosphere also has the effect of further cleaning metal parts.