Abstract: The screen printing technology of thick film integrated circuits and the factors influencing the printing quality are introduced in detail. Taking the thick film resistors as an example, the production process and the influence of screen printing defects on thick film circuits are briefly described.
Keywords: thick film integrated circuit; thick film screen printing; thick film resistor; screen printing defect
Foreword
Screen printing, as an ancient printing method, has been widely used in China's electronics industry, ceramic applique industry, and textile printing and dyeing industry. With the advent of the electronic era in the 1960s, screen printing has been widely used in printed circuit boards, thick film integrated circuits, solar cells, resistors, capacitors, piezoelectric elements, photosensitive elements, thermal elements, liquid crystal display elements, etc. In manufacturing, especially since the 1980s, the continuous application of various new materials and technologies, continuous exchanges of technology with foreign countries, and the continuous development of new high-precision automation equipment have greatly improved the level of screen printing technology. The following only describes the process technology for the production of thick film integrated circuits using screen printing thick film technology.
1 Integrated Circuit Overview
Integrated circuits can be roughly divided into two major categories: semiconductor integrated circuits and hybrid integrated circuits, and hybrid integrated circuits can be divided into two types. One is a thin film hybrid integrated circuit, which is manufactured using a thin film technique of a vacuum spray method. The other is a thick film integrated circuit manufactured using a screen printing thick film technology.
The so-called film refers to a film thickness of about 1 μm, and the thick film refers to a film thickness of 10 to 25 μm. Both the film and the thick film have their own advantages. For example, thin film technology, whether active or passive, is directly processed into integrated circuits according to their respective technical characteristics. However, thick film technology cannot process active components directly to circuits. Therefore, these components have to be soldered. Direct processing will be realized as the research progresses in the future. In addition, in terms of cost, the thick film is much lower than the film. The conductor and thick film resistors and capacitors are formed by screen printing. Compared with the resistors and capacitors fabricated by the thin film forming technology, the thick film technology is easy and reliable. It is good and requires less investment in production equipment.
2 thick film integrated circuit screen printing process
2.1 Ceramic plates
The use of 90% to 96% alumina ceramic substrate is a material mainly composed of alumina (Al2O3), which has good conductivity, mechanical strength and high temperature resistance. When making thick film, attention should be paid to the material, size, roughness, warpage, surface defects and contamination of the ceramic plate, and ultrasonic cleaning is performed in the clean room.
2.2 Slurry
There are three kinds of conductive paste, resistor paste and insulating paste. The paste is generally composed of precious metal and low-melting glass. When making the slurry, pay attention to the material, viscosity and expansion coefficient of the slurry.
The paste used for printing thick film circuits includes gold, silver, platinum, and palladium. The above-mentioned metal powder is dispersed in an organic resin adhesive to form a paste, and then printed on a ceramic substrate by a screen printing plate. After high-temperature firing, the organic resin binder is burned off, and almost all of the rest is pure noble metal, which adheres to the substrate due to the vitreous effect. This film can be used as the bottom metal sheet for thick film lines, thick film resistors, thick film capacitors, and semiconductor integrated circuits.
(1) The electrical resistance of silver as the conductive material is very low, so silver-palladium, silver-palladium-platinum mixtures are sometimes used as conductive materials.
(2) In order to form a resistive film on a substrate, the resistive materials used are mainly metal powders such as silver, gold, palladium, and ruthenium.
(3) Conductors, electrodes, etc. of small capacitors are manufactured by the overlay printing method. The electrode material consists of platinum-gold, palladium-gold, and silver.
2.3 Screen Printing Plate Production
(1) Frame: The screen mesh frame for printing thick-film circuits mostly uses hard aluminum and aluminum alloy. The frame specifications are generally 100 mm × 150 mm and 150 mm × 200 mm. The frame shape is usually rectangular.
(2) Wire Mesh: Printed thick film circuits use stainless steel mesh or nylon mesh. General circuit printing 200 ~ 300 mesh screen; multi-layer wiring or when the accuracy is higher, you can use more than 300 mesh screen.
(3) Photoresist: Due to the thick ink film (slurry film) required for the thick-film circuit screen printing, it is required that the photo-resist be applied to a 20 to 30 μm thick photo-resist, but no edge defects will appear after the development.
2.4 thick film silk screen
The screen printing image of the integrated circuit is miniature and requires high printing precision. Therefore, the printing machine, the printing plate, the substrate (substrate), and the ink all need high precision, and the printing site must also be kept at a constant temperature to remove the dust.
The scraper material is generally polyurethane rubber or fluorinated rubber and has a hardness of A70° to A80°. In addition, when selecting the shape and hardness of the squeegee, consideration should be given to the factors that need to be printed on the circuit board. In general, the blade edge is 90° or 60° (see Figure 1) and the blade angle is 70° to 75° (see Figure 2).
There are two types of semi-automatic and fully-automatic screen printing machines for printing thick-film ICs. The semi-automatic screen printing machine only uses the substrate to supply it manually, and other processes are completed automatically. For example, the WY-155 and WY-203 precision screen printing machines developed by the 45th Institute of Electronics and Information Industry of the Ministry of Information Industry have the printing accuracy. Less than 0.01 mm, squeegee pressure and printing speed can be adjusted, vacuum table x, y, θ three-dimensional precision adjustment, the whole PLC control, etc., all indicators meet or exceed the technical level of similar foreign equipment, is the domestic thick film integrated circuit The preferred equipment of the manufacturer.
