What is a thick film circuit (thick film integrated circuit)

What is a thick film circuit (thick film integrated circuit)

A hybrid integrated circuit is a type of microelectronic functional component that is made by using thick film processes such as screen printing and sintering to make a passive network on the same substrate, assembling discrete semiconductor device chips or monolithic integrated circuits or micro components on it, and then packaging it.

1. Features and applications

Compared with thin-film hybrid integrated circuits, thick-film hybrid integrated circuits are characterized by more flexible design, simple process, low cost, and are particularly suitable for small-batch production of multiple varieties. In terms of electrical performance, it can withstand higher voltages, greater power, and larger currents. The operating frequency of thick-film microwave integrated circuits can reach more than 4 GHz. It is suitable for various circuits, especially analog circuits for consumer and industrial electronic products. Substrates with thick-film networks have been widely used as micro-printed circuit boards.

2. Main process

According to the circuit diagram, several functional component diagrams are first divided, and then the planar layout method is used to convert it into a planar circuit layout diagram on the substrate, and then the thick film network template for screen printing is made by photoengraving. The most commonly used substrate for thick film hybrid integrated circuits is alumina ceramics with a content of 96% and 85%; when particularly good thermal conductivity is required, beryllium oxide ceramics are used. The minimum thickness of the substrate is 0.25 mm, and the most economical size is 35×35~50×50 mm. The main processes for making thick film networks on substrates are printing, sintering and resistance adjustment. The commonly used printing method is screen printing.

The process of screen printing is to first fix the screen on the frame of the printer, and then stick the template on the screen; or apply photosensitive glue on the screen, make the template directly on it, and then put the substrate under the screen, pour the thick film slurry on the screen, use a scraper to press the slurry into the mesh, and print it on the substrate to form the required thick film pattern. Commonly used screens are stainless steel and nylon, and sometimes polytetrafluoroethylene.

During the sintering process, the organic binder is completely decomposed and volatilized, and the solid powder melts, decomposes and combines to form a dense and strong thick film. The quality and performance of the thick film are closely related to the sintering process and the ambient atmosphere. The heating rate should be slow to ensure that the organic matter is completely removed before the glass flows; the sintering time and peak temperature depend on the slurry used and the film structure. In order to prevent the thick film from cracking, the cooling rate should also be controlled. The commonly used sintering furnace is the tunnel kiln.

In order to achieve the best performance of thick film network, the resistor needs to be adjusted after firing. Commonly used adjustment methods include sandblasting, laser and voltage pulse adjustment.

3. Thick film materials

Thick film refers to a film layer with a thickness of several microns to tens of microns formed on a substrate using printing and sintering technology. The material used to make this film layer is called thick film material.

Thick film materials are a type of coating or slurry, which is formed by uniform suspension of one or more solid particles (0.2 to 10 microns) in a carrier. In order to facilitate printing and forming, the slurry must have appropriate viscosity and thixotropy (the property that viscosity changes with external force). Solid particles are components of thick films and determine the properties and uses of the film. The carrier decomposes and escapes during the sintering process. The carrier contains at least three components, a resin or polymer binder, a solvent, and a surfactant. The binder provides the basic rheological properties of the slurry; the solvent dilutes the resin and then evaporates to dry the printed pattern; the activator allows the solid particles to be wetted by the carrier and properly dispersed in the carrier.

According to the properties and uses of thick films, there are five types of slurries used: conductor, resistor, dielectric, insulating and encapsulation slurries.

Conductive paste is used to manufacture thick film conductors, forming interconnects, multi-layer wiring, microstrip lines, welding areas, thick film resistor terminals, thick film capacitor plates and low resistance resistors in thick film circuits. The welding area is used to weld or paste discrete components, devices and external leads, and is sometimes used to weld metal covers to achieve the encapsulation of the entire substrate. Thick film conductors have different uses, and no paste can meet the requirements of all these uses, so a variety of conductor pastes are used. The common requirements for conductor pastes are high conductivity, strong adhesion, anti-aging, low cost and easy welding. The metal components in commonly used conductor pastes are gold or gold-platinum, palladium-gold, palladium-silver, platinum-silver and palladium-copper-silver.

In the thick film conductor slurry, in addition to metal powder or metal organic compound with suitable particle size, there are glass powder or metal oxide with suitable particle size and shape, and organic carrier for suspended solid particles. Glass can firmly bond the metal powder to the substrate to form a thick film conductor. Alkali-free glass is commonly used, such as borosilicate lead glass.

Thick film resistors are the earliest developed and most advanced thick film components in thick film integrated circuits. They can be used to manufacture various resistors. The main requirements for thick film resistors are high resistivity, small resistance temperature coefficient, and good stability.

Like conductor paste, resistor paste also has three components: conductor, glass and carrier. However, its conductor is usually not a metal element, but a compound of a metal element, or a complex of a metal element and its oxide. Commonly used pastes include platinum-based, ruthenium-based and palladium-based resistor pastes.

Thick film dielectrics are used to manufacture micro thick film capacitors. The basic requirements for them are large dielectric constant, small loss tangent, large insulation resistance, high withstand voltage, and stability and reliability.

The dielectric slurry is made by evenly suspending low-melting glass and ceramic powder in an organic carrier. Commonly used ceramics are titanate ceramics of barium, strontium, and calcium. By changing the relative content of glass and ceramic or the composition of ceramic, dielectric thick films with various properties can be obtained to meet the needs of manufacturing various thick film capacitors.

Thick film insulation is used as the insulation layer of multi-layer wiring and crossover lines. The requirements for it are high insulation resistance, small dielectric constant, and linear expansion coefficient that can match other film layers. Commonly used solid powders in insulation slurries are alkali-free glass and ceramic powders.