Basic principles of glass sealing

    Glass and other materials are two materials with fundamentally different properties. There are many basic requirements if they are to be tightly combined. For a selected other material, not any kind of glass can be used for sealing, it must meet certain conditions. Investigating its sealing mechanism will involve two most basic conditions.

    (1) The expansion coefficient of the two should be very close

    When choosing glass to be sealed with other materials, the expansion coefficients of the two materials must be matched, which is the key to the success or failure of the sealing. Specifically, glass and other materials should be within the temperature range from room temperature to below the upper limit of the annealing temperature of the glass, and their expansion curves should be as consistent as possible, so that it is easy to produce a stress-free sealing body. If the expansion rate and contraction rate of the two are not the same, no matter glass or other materials can generate stress in the sealing body, when the stress value exceeds the strength limit of the glass, the sealing part will crack, resulting in air leakage and failure of the component . Even if there is no cracking in a short period of time, after a long period of time, microcracks will gradually occur due to the inability of the vitreous body to withstand the force. This is what people often call chronic air leakage. Especially when electronic devices are subjected to vibrations and collisions, micro-cracks will spread and expand rapidly, resulting in damage to the sealing parts.

    Generally speaking, the presence of compressive stress in the sealing body is not dangerous, because this compressive stress is much smaller than the compressive strength of glass. However, the tensile strength of glass is only 1/10 of the compressive strength, so the selected glass and other materials must have less than the tensile strength of the glass due to the different thermal expansion caused by the tensile stress.

    Of course, it is impossible to make the expansion coefficient curves of two materials with fundamentally different properties completely consistent. If the difference between the expansion coefficients of glass and other materials does not exceed ±10% over the entire temperature range, the stress can be controlled within a safe range. Inside, the glass will not burst.

    (2) The glass must be able to wet the surface of other materials (that is, the glass can be fully spread on the surface of other materials)

    Another key to the success or failure of sealing is that the selected glass can be well wetted with other materials during sealing. The wettability reflects the binding ability between two substances. For example, a drop of water can roll around on the lotus leaf, it looks spherical and cannot be spread out. It can be said that it is not wet at all. A measure of the quality of wettability is expressed in terms of wetting angle. The geometrical method of the wetting angle takes the boundary line between the droplet and the substrate as one side of the wetting angle, and the tangent line is made at the place where the edge of the droplet connects with the substrate to form the other side of the wetting angle. The included angle is called the wetting angle. From a thermodynamic point of view, the fluidity of a liquid on a solid surface is determined by the surface tension of the solid and the liquid and the tension between the solid and liquid.

(Huiputech)