Hermetic Glass-to-Metal Sealing is a remarkable fusion of metal and glass, engineered to create vacuum-tight electrical connectors, packaging, feedthroughs, or optical windows/lenses. It acts as a secure barrier against moisture, gas intrusion, and leakage, offering extreme reliability. Unlike organic materials, glass seals do not age, making them the ideal choice for protecting sensitive electronic components in the long term.
A fundamental challenge in creating glass-to-metal seals lies in the differing coefficients of thermal expansion (CTE) of glass and metal. When exposed to heat, metal and glass expand at different rates, creating mechanical stresses during the sealing process. If the metal expands faster than the glass, it can cause the glass to crack, while slower expansion can lead to gaps in the seal.
When water vapor reacts with metal at high temperatures, three types of metal oxides, FeO, Fe3O4, and Fe2O3, are formed. Among these, Fe2O3 has a loose structure and exhibits lower bonding strength with glass. The degree of oxidation directly affects the thickness of Fe2O3, which in turn directly impacts the sealing effectiveness of the bond. The control of dew point in the furnace is essentially controlling the thickness of the oxidation layer. By controlling the thickness of the oxidation layer, the amount of Fe2O3 can be controlled, thus affecting the sealing effectiveness. The chemical bond between glass and Fe3O4 is similar, resulting in a tight bond.
Different glasses used in glass-to-metal seals have varying properties, including flow behavior, density, and impurities. These properties can impact the strength and viscosity of the glass, making it essential to select the right glass for a specific application. Irregularities in the glass's surface can also affect its strength and create stress points, potentially leading to seal failure.
The choice of metal is equally critical in glass-to-metal seals. Surface flaws in the metal, such as subsurface cracks or surface imperfections, can jeopardize the seal's integrity. Some defects may not be immediately visible but can develop into cracks when exposed to heat during the sealing process. Even minor surface conditions like die lines on the metal's surface can lead to cracks and eventual leaks in the seal.
The key to achieving consistent sealing performance lies in the intake structure during the high-temperature ramp. Any non-uniformity in the intake can result in inconsistent oxidation on the surface of the metal casing, leading to variations in the sealing effectiveness of the products. To address this, several critical points should be considered for atmosphere control. First, it is crucial to maintain a stable and consistent intake atmosphere throughout the process, as any fluctuations can affect the oxidation process and subsequent sealing performance. Additionally, achieving a homogeneous mixture of humidified nitrogen and dry nitrogen is essential to ensure consistent moisture levels, which directly impact the oxidation process and the quality of the seal. Lastly, the method of introducing the intake into the furnace chamber should be carefully designed to ensure uniform distribution, as uneven distribution can lead to localized variations in the atmosphere, ultimately affecting the oxidation and sealing process.
Preventing gaps that cause leaks in glass-to-metal seals involves modifying the surface of the metal. Techniques like centerless grinding can alter the surface finish of the metal, reducing the likelihood of gaps that could compromise the seal's integrity. By changing the surface features to radial patterns, the metal can provide better adhesion for the glass, minimizing the risk of longitudinal leak points.
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