Low-melting glasses for the potting of thick-film microcircuits

Abstract

Low-melting glasses have attained broad use in thick-film microelectronics, including the prevalent lead-containing borate compositions. Since lead glasses have a number of valuable properties, they are used successfully in manufacture of microcircuits as the lowmelting constituents of capacitance and resistance elements, as glassy materials for interlayer insulation, and as protective coatings and glass solders. We studied the SiO2-B203-AI203-PbO system with a goal of developing low-melting glasses for potting thick-film microcircuits. We studied the effect of composition on glass-forming, crystallization, and physicochemical properties and also the glass compatibility with the substrates used in microelectronics of ST-50-1 crystal, steatite, and high-alumina ceramic. The data of Fig. 1 attest that the section SiO2-B203-1OAI203-PbO has a vast region of noncrystallizing glasses with a different combination of softening temperature and thermal expansion coefficient (LCTE). The absolute values of the latter are within the limits 315575~ and 50-90.I0-7~ -I, respectively, and depend basically on the PbO content of the glass. High-lead glasses of reduced wetting temperature have a relatively high LCTE. The glasses studied are typified by a comparably good water stability, a deciding effect on which is the B203 content. Accordingly, on increase of molar B203 content from i0 to 50~, the loss of glass in water increases from 0.02 to 2.75~. The most chemically stable are compositions of elevated SiO2 content. The glass dielectrics in thick-film microcircuits should combine a low melting temperature with a high chemical stability and a LTCE corresponding to that of the substrate material in order to ensure the spreading and adhesion of the glass film to the substrate while not initiating a chemical reaction with microcircuit materials. Figure 2 gives data on a study of the compatibility of glasses in the SiOz-BzO3-!0Al203-PbO system with substrates used in microelectronics of ST-50-1 crystal (LCTE -52.10-7~ 22KhS ceramic (LCTE -65.10-7~ -I, and steatite (LCTE -70.I0-7~ The coatings were obtained by applying a thin layer of glass powder to the substrate and annealing at 400-700~ As apparent, the glasses studied with a LCTE below the substrate LCTE form monolithic coatings without cracks, having good spreadability and strong adhesion of the glass film to crystal, steatite, and 22KhS ceramic. Coatings based on glasses with a LCTE above that of the substrate have cracks. There is an opinion that materials used in thick-film microelectronics should have a LCTE comparable to that of a substrate. It is known that for silver-palladium resistance compositions the LCTE of the glass binder should be 3-7~ below that of the substrate. Since glass is subject to substantial compressive stresses and sight tensile forces, according to our data, the glass composition for monolithic glass coatings without cracks must provide that the glass is in a state of compression; the LCTE of the glass should thus not exceed the LCTE of the substrate. We achieved during production of glasses for microelectronics, the resulting rules concerning the effect of glass composition on its properties and compatibility with the substrates used in electronics, and in microelectronics. Using modifying additives to optimize the oxide properties (RO and ROz), low-melting glasses of the types 71, 82, and 83 were developed in the SiO2-B203-AI203-PbO system for the potting of thick-film capacitance and resistance elements. The basic properties of the glasses are shown in Table 1 (patents 1081135, 1227605, 1313817). We used pure and analytical grade raw materials for synthesis of the glasses: sand, boric acid, alumina, and oxides of diand tetra-valent metals. The glasses were founded in a gas furnace in quartz crucibles at a temperature of 1300 • 50~ With this regime, the