Low temperature co-fired ceramic (LTCC) substrate materials are a branch of ceramic packaging substrates that meet the technical requirements for multi-chip assembly or single-chip packaging of low frequency, digital, RF, and microwave devices with their excellent electrical, mechanical, thermal, and process characteristics.
LTCC substrate has the advantages of high frequency characteristics, thermal stability, passive component integration, etc.:.
(1) have excellent high frequency, high Q characteristics and high-speed transmission characteristics.
(2) has good temperature characteristics, can adapt to the characteristics of high current and high temperature resistance requirements.
(3) easy to achieve multi-function and improve assembly density, high reliability, high temperature, high humidity, shock vibration, can be applied to harsh environments.
(4) In the frequency range of 2.4MHz~80GHz, the signal loss caused by LTCC technology is much lower than that of multilayer line technology.
(5) Due to the introduction of mass production equipment and process, the module size is reduced by 20%~40%, and the cost can be greatly reduced.
Therefore, LTCC technology is considered to be the most promising technology for future integrated components and substrate materials for high-frequency applications.
At present, there are three major types of low-temperature co-fired ceramic materials: microcrystalline glass systems, glass + ceramic composite systems, and amorphous glass systems.
1. Microcrystalline Glass System
Microcrystalline glass is a composite of a large number of tiny crystals and a small amount of residual glass phase made from a certain composition of glass by controlled crystallization. It has the characteristics of easy to adjust the formula, simple process, and better performance. Such as low dielectric loss, suitable for the production of working frequency in 20 ~ 30GHZ devices, cordierite (2MgO-2Al2O3-5SiO2), calcium silica (CaO, SiO2), and lithium pyroxene (Li2O-Al2O3-4SiO2) is most widely used. The microcrystalline glass according to the composition of the base glass can generally be divided into silicate system, aluminosilicate system, borosilicate system, borate system, phosphate system, and other five categories. Microcrystalline glass uses silicate type of glass-ceramic materials, adding one or more oxides, such as P2O5, Li2O, B2O3, ZrO2, ZnO, TiO2, SnO2, sintering temperature in 850 ~ 1050 ℃, small dielectric constant and thermal expansion coefficient.
2.Glass + ceramic composite system
This is currently the most commonly used LTCC material. Adding low melting point glass phase in the ceramic, the glass softens during sintering and the viscosity decreases, which can reduce the sintering temperature. Glass is mainly a variety of crystallized glass, ceramic filling phase is mainly Al2O3, SiO2, cordierite, mullite ceramic, etc.. The sintering temperature is around 900℃, the process is simple and flexible, easy to control and adjust the sintering characteristics and physical properties of the composite material, the dielectric constant and its temperature coefficient are small, the resistivity is high and the chemical stability is good.
3.Amorphous glass system
The oxides that form the glass are mixed thoroughly, calcined between 800~950℃, then ball-milled and sieved, and sintered into dense ceramic substrates according to the ceramic process molding. This system is a simple process, the composition is easy to control, but the comprehensive performance of the ceramic substrate is less desirable, such as lower mechanical strength, dielectric loss is large, and is rarely used.
LTCC production process
There are certainly many types of LTCC ceramics, but the preparation of its ceramic material is generally divided into two methods, namely, the high-temperature melting method and chemical preparation method. The high-temperature melting method is to mix various oxides in a predetermined proportion, the liquid phase reaction in a high-temperature melting furnace (generally higher than 1400 ℃), after water quenching, and finally ball milling or ultrasonic crushing, to obtain glass-ceramic powder; chemical preparation method is to dissolve different proportions of oxides and reactants into a specific solution, after the reaction to produce precipitation, the precipitate for the glass-ceramic powder, this method to produce the powder The activity is higher.
The LTCC process involves the preparation and formulation of a ceramic slurry, which is cast into raw ceramic strips of up to several millimeters. The raw ceramic strip is then cut into small individual pieces and the desired through-holes are punched by mechanical or laser methods. In the next step, metallic conductors (Cu, Ag, Au, etc.) are filled with holes in the raw porcelain tape using techniques such as screen printing and micro-hole grouting, and conductive patterns are created. Finally, the single layer of raw porcelain tape is stacked together according to the process requirements, combined together by uniaxial and isostatic pressure lamination, low temperature (900~1000℃) sintering molding, and finally made into a high density integrated circuit, also can be built-in passive components, in its surface, mount IC and active components, made of passive/active hybrid integrated functional modules.
There are certainly many types of LTCC ceramics, but the preparation of its ceramic material is generally divided into two methods, namely, high-temperature melting method and chemical preparation method. High-temperature melting method is to mix various oxides in a predetermined proportion, the liquid phase reaction in a high-temperature melting furnace (generally higher than 1400 ℃), after water quenching, and finally ball milling or ultrasonic crushing, to obtain glass ceramic powder; chemical preparation method is to dissolve different proportions of oxides and reactants into a specific solution, after the reaction to produce precipitation, the precipitate for the glass ceramic powder, this method to produce the powder The activity is higher.
Due to the excellent performance of LTCC, it has been successfully used in the manufacture of integrated circuit packages, multi-chip modules (MCM), microelectromechanical systems (MEMS), various chip inductors, chip capacitors, chip transformers, and chip antennas. Application areas include communication, automotive electronics, medical electronics, aerospace, and military electronics.
Features and Advantages of LTCC
Modern substrates have the following characteristics:
(1) High resistivity to ensure insulation between signal lines;
(2) Low dielectric constant to improve the signal transmission rate;
(3) Low dielectric loss;
(4) low sintering temperature, can be co-fired with Cu.Ag and other high conductivity metals;
(5) low sintering shrinkage to ensure the accuracy of the circuit design;
(6) Suitable coefficient of thermal expansion to ensure compatibility with silicon and other chips;
(7) High thermal conductivity to prevent overheating of the substrate;
(8) comprehensive physical, chemical “and mechanical properties.