Overview of common ceramic substrates

What is a ceramic substrate?

The ceramic substrate is based on the electronic ceramic, and the film circuit element is attached to the cutting element to form a sheet material supporting the base!

The advent of ceramic substrate products has opened up new developments in the heat dissipation application industry. Due to the heat dissipation characteristics of ceramic substrates, ceramic substrates have the advantages of high heat dissipation, low thermal resistance, long life, and withstand voltage. With the improvement of production technology and equipment, products The price has been accelerated and rationalized, which has expanded the application areas of the LED industry, such as indicator lights for home appliances, car lights, street lights and outdoor large billboards. The successful development of ceramic substrates has provided better services for indoor lighting and outdoor lighting products, making the LED industry a broader market in the future.

  1. Properties of ceramic substrates

Physical properties:

High enough mechanical strength, in addition to components, can also be used as a support member.

Good processability, high dimensional accuracy, and easy multi-layering.

Smooth surface, no warpage, bending, micro cracks, etc.

Electrical properties

Insulation resistance and insulation breakdown voltage are high.

The dielectric constant is low and the dielectric loss is small.

It is stable under high temperature and high humidity conditions to ensure reliability.

Thermal properties

High thermal conductivity

The coefficient of thermal expansion matches the relevant material, especially the coefficient of thermal expansion of Si.

Excellent heat resistance.

Other properties

It has good chemical stability, easy metallization, and strong adhesion to circuit patterns.

It is non-hygroscopic, oil-resistant, chemical-resistant, and has a small amount ofα radiation.

Rich in raw materials, mature technology and easy to manufacture.

  1. Method for preparing ceramic substrate

Typical molding methods before ceramic firing include the following four types: powder press forming (molding forming, isostatic pressing), extrusion molding, tape casting, and the like. Among them, the tape casting method is widely used in the manufacture of substrates for LSI packages and hybrid integrated circuits in recent years because it is easy to realize multilayering and high production efficiency.

  1. Various ceramic substrates

Alumina (Al2O3) substrate

  Raw material: The typical manufacturing method of Al2O3 raw material is the Buyer method. In this method, the raw material is bauxite (aluminite/boehmite and corresponding compounds);

Production method: Al2O3 ceramics are generally formed by green sheet lamination method, and the adhesive is generally polyvinyl alcohol polybutyraldehyde (PVB). The firing temperature varies depending on the added sintering aid, and is usually 1550 to 1600 °C. The metallization method of the Al2O3 substrate is mainly based on the thick film method and the co-firing method, and the slurry from the use to the process technology are relatively mature, and can meet the requirements of various applications at present;

Applications: hybrid integrated circuit substrates, LSI package substrates, and multilayer circuit substrates.

Aluminum nitride (AlN) substrate

  Raw materials: AlN is a non-naturally occurring but an artificial mineral that was first synthesized by Genther et al in 1862. The representative preparation method of AlN powder is a reduction nitridation method and a direct nitridation method. The former is formed by using Al2O3 as a raw material, reduced by high-purity carbon, and then reacted with nitrogen, and the latter is directly reacted with N2 to directly nitrite. ;

Manufacturing method: Various methods for manufacturing Al2O3 substrate can be used for the manufacture of AlN substrate, and the most used one is the green sheet lamination method, that is, the AlN raw material powder, the organic binder, the solvent and the surfactant are mixed to form a ceramic slurry. Material, cast, laminated, hot pressed, degreased, fired;

Characteristics of AlN substrate: The thermal conductivity of AlN is more than 10 times that of Al2O3, and the CTE is matched with the silicon wafer. The AlN material has higher insulation resistance and dielectric withstand voltage and lower dielectric constant than Al2O3. Features are very rare for package substrate applications;

Applications: for VHF (Ultra High Frequency) band power amplifier modules, high power devices and laser diode substrates;

Silicon carbide (SiC) substrate

Raw material: SiC is not produced naturally but is made of artificially produced minerals. It is mixed with silica, coke and a small amount of salt in powder form. It is heated in a graphite furnace to a temperature above 2000 °C to form α-SiC, which is then precipitated by sublimation. Obtaining a dark green block polycrystalline aggregate;

Manufacturing method: SiC has very good chemical stability and thermal stability, and it is difficult to achieve densification by ordinary method. Therefore, it is necessary to add a sintering aid and fire it by a special method, usually by vacuum hot pressing;

Characteristics of SiC substrate: Its most characteristic property is that its thermal diffusivity is particularly large compared with other materials, even larger than copper, and its thermal expansion coefficient is closer to Si. Of course, it also has some shortcomings, relatively speaking, its dielectric constant is high, and the insulation withstand voltage is worse;

Application: For SiC substrates, low-voltage circuits and VLSI high-heat-dissipation substrates, such as high-speed, high-integration logic LSI with heat dissipation package, for ultra-large computers and optical communication. Substrate application of laser diodes, etc.

Beryllium oxide ceramic (BeO) substrate Its thermal conductivity is more than 10 times that of Al2O3, it is suitable for high-power circuits, and its dielectric constant is low, which can be used in high-frequency circuits. The BeO substrate is basically produced by a dry pressing method, and a BeO substrate can be produced by a green sheet method by adding a trace amount of MgO or Al2O3 thereto.

  1. Low temperature co-fired ceramic multilayer substrate (LTCC)

The substrate discussed above has a relatively high sintering temperature of 1500 to 1900 ° C. Therefore, if the simultaneous firing method is used, the conductor material can only select the refractory metals Mo and W, etc., which is bound to cause a series of difficult problems to be solved: For example, the co-firing needs to be carried out in a reducing atmosphere, increasing the process difficulty, and the sintering temperature is too high, and a special sintering furnace is required; since the Mo and W resistivity is high, the wiring resistance is large, the signal transmission is likely to cause distortion, increase loss, and wiring. The miniaturization is limited; the dielectric constant of the dielectric material is too large, which increases the signal transmission delay time, and is particularly unsuitable for UHF circuits.

In order to solve the above problems, a low temperature co-fired ceramic substrate (LTCC) in which glass and ceramic are co-fired has been developed. Since the firing temperature is about 900 ° C, various resistivities can be used. The low material enables fine wiring, in which the precious metal paste can be fired in the atmosphere.

Preparation of low-temperature co-fired multi-layer ceramic substrate: This technique requires first adding inorganic alumina powder and about 30% to 50% of glass material to the organic binder to make it evenly mixed into a slurry, and then using a doctor blade. The slurry is scraped into a sheet shape, and then the sheet-like slurry is formed into a thin piece of green embryo through a drying process, and then the through hole is drilled according to the design of each layer, and the LTCC internal circuit is used as a signal transmission of each layer. Screen printing technology, respectively, fill holes and printed circuits on the raw embryos, and the inner and outer electrodes can use silver, copper, gold and other metals respectively. Finally, the layers are laminated and placed in a sintering furnace at 850-900 °C. In the middle of sintering, it can be completed. As shown in FIG. 2, a schematic diagram of preparation of a low temperature co-fired multilayer ceramic substrate is shown.

Application of low temperature co-fired multilayer ceramic substrate: LTCC is suitable for three-dimensional wiring multilayer ceramic substrate for high-density electronic packaging because of its low conductor resistivity, small dielectric constant of dielectric, high thermal conductivity, and silicon chip The matched low thermal expansion coefficient and easy multi-layering are especially suitable for RF, microwave, and millimeter wave devices. Applications include multi-layer substrates for supercomputers, multi-layer substrate ECU components for next-generation automobiles, interface modules for optical communication and HEMT modules, and high-frequency components VCO and TCXO.

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