Technical requirements for metal substrates of Thick film heating elements:
The substrate material has great influence on the performance of thick film elements. Therefore, there are certain technical requirements for substrates used for thick film hybrid circuits.
1-substrates shall have good insulation performance;
2-Substrate generally should be able to withstand 850℃ firing process;
3-The substrate should have a certain mechanical strength;
4-Substrate material shall be easily machined to a flat surface;
5-The chemical and physical properties of the substrate shall be suitable for the conductor and resistance slurry;
6-The substrate shall have high thermal conductivity;
The coefficient of linear expansion of substrates shall be matched with the thick film size used.
Metal materials have high mechanical strength, good thermal conductivity and easy to process. But whether it can be used in thick film circuit substrates, the key lies in whether the metal surface can be fired medium layer (glass glaze), the main technical requirement of metal substrates is to design the composition of glass medium material, to meet the "burn up" and insulation performance requirements. At the same time, the resistance and conductor slurry should be suitable for the chemical and physical properties of the substrate.
As a dielectric material, the thermal expansion characteristic of glass must be suitable for the selected metal material within the range of room temperature to firing temperature, that is, the expansion coefficient of glass is close to that of metal and the expansion coefficient of both is matched.
The insulation of the metal substrate is guaranteed by the dielectric layer. The insulation of the dielectric layer can be improved from two aspects: the component of the 7:0 glass does not contain (or contains a very small amount of) active ions such as alkali metals, because the active ions are easy to diffuse in the residual glass phase, resulting in the phenomenon of ionic conductivity. (2) the appropriate addition of large atomic coefficient of metal elements. These elements do not participate in the formation of crystalline phase , but can isolate the crystalline phase from the residual glass phase, thus blocking the movement of the active ions in the glass network.
From the point of view of use, glass components should not contain lead, cadmium and other toxic elements. In order to improve the endurance impact and thermal impact of thick film elements, cobalt and other elements that enhance the wettability of metal can be added to glass to enhance the bonding strength of medium layer and metal surface.