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Analysis of key factors affecting thermal conductivity of low temperature co-fired ceramics (LTCC)* materials
LTCC is a new type of microelectronic packaging technology based on glass-ceramics or glass/ceramic two-phase composites. LTCC technology with its excellent electrical, thermal, mechanical and interconnection characteristics, has become a new generation of passive device miniaturization, integration, multi-functional and system-level packaging of the preferred way, is widely used in a variety of microelectronics device fields, such as high-precision chip components, passive integrated functional devices, passive integrated substrate and micro-electronic functional modules and other packaging products. In recent years, the emergence of emerging technologies such as 5G/6G mobile communication, virtual reality, and artificial intelligence has brought new opportunities for the development of LTCC materials.
The development of system-level integrated packaging in the direction of high density, multi-function, high power and intelligence has become an inevitable trend, and with the continuous increase of integration, the heat dissipation power density rises sharply, resulting in an increasingly prominent heat dissipation problem of microelectronic devices, which seriously threatens the reliability and service life of microelectronic devices. This puts a very strict requirement on the thermal performance of LTCC substrate materials. In general, the thermal conductivity of solid materials is closely related to the mean free path of phonons. However, the atomic arrangement of glass materials has the characteristics of short-range order and long-range disorder, resulting in the mean free path of the phonons of the glass matrix is small, which makes the thermal conductivity of LTCC materials based on glass-ceramics or glass/ceramic two-phase composite materials is low, and it is difficult to meet the application requirements of large-scale and ultra-large scale integrated circuits.
LTCC technology is an advanced passive integration and hybrid circuit packaging technology. Compared with other integrated circuit packaging technologies, LTCC technology shows great advantages and can significantly improve the flexibility of multi-layer microcircuit chip packaging design. LTCC technology features:
(1) The adjustability of dielectric properties. LTCC material has low dielectric loss and can adjust the dielectric constant by adjusting the raw material composition and its ratio, which can be adapted to the substrate with different needs and improve the activity of circuit design.
(2) Lower sintering temperature. LTCC technology can be sintered at low temperatures, reducing energy consumption, thereby avoiding high temperature damage to electronic components and materials.
(3) High density wire wiring capability. The LTCC process optimizes the line width spacing to be smaller, allowing for a tighter wiring structure that provides higher conductivity and better signal transmission performance.
(4) Integration of passive components can be achieved. The number of components such as inductors and filters is reduced by embedding to improve package density and reliability.
(5) Good process compatibility. It can be combined with a variety of materials and is compatible with layer wiring technology to obtain high-performance hybrid multi-chip components for more efficient and flexible design and production.
Factors affecting the thermal conductivity of LTCC materials
The heat conduction carrier of LTCC material is mainly phonons, and the thermal conductivity is determined by the mean free path of phonons. Its thermal conductivity is related to its composition, impurity content, defect and porosity, particle size and structural density, heat treatment procedures and other factors. In order to improve the thermal conductivity of the material, it is often used to improve the purity and density of the material, reduce the generation of pores and defects, reasonable heat treatment procedures and the construction of thermal conductivity channels in the crystal to improve the performance of the material.
(1) Material composition
Although LTCC material has better thermal conductivity than ordinary PCB substrate, its thermal conductivity is only 2.0~2.5W/(m·K) due to its glass ratio. In order to meet the needs of more efficient and energy-saving, improve the thermal conductivity of LTCC material. The most common method used is component doping. Some materials have abnormal grain growth in the process of preparation, and the addition of filler can effectively change the crystallization rate, delay the ion migration rate and improve the thermal conductivity. At the same time, the addition of some special materials can also build a high thermal conductivity network inside the material to improve the thermal conductivity of the material.
(2) Stomata
During the preparation of LTCC materials, the properties of LTCC materials need to be improved by sintering. In this process, pores will be generated inside the embryo due to impurities and more glass phases. The size of the aperture directly affects the internal heat transfer efficiency of the material, the larger the aperture, the faster the heat transfer; The uniformity of pore distribution affects the uniformity of heat transfer inside the material. The way in which the pores are connected directly determines the transmission channel of heat energy inside the material.
(3) Internal defects
The internal defects and microstructure of materials will affect the propagation of phonons and change the thermal conductivity. Defects create centers that cause phonon scattering, reducing the mean free path and thermal conductivity of phonons. Therefore, the defects can be reduced by increasing the sintering time and adding sintering additives when preparing LTCC materials. The thermal conductivity of LTCC material largely depends on the material added, but during the sintering process, these materials will react physically and chemically with the original material, causing its internal structure to change and resulting in defects. Therefore, when selecting raw materials, attention should be paid to the effect of added materials on the structural changes of LTCC materials.
(4) Heat treatment process
Heat treatment is the most important step in the process of preparing LTCC materials. A series of physical and chemical changes occur during this process, affecting the microstructure of the material. The sintering temperature, heating rate and holding time of heat treatment all have a certain effect on the internal structure of the material, which also leads to the different thermal conductivity of the material treated under different conditions. The properties of LTCC materials are mainly affected by factors such as the type of crystalline phase precipitated, crystallinity and grain size. Reasonable heat treatment temperature and time can effectively promote the crystallization of glass, increase the crystallinity, make the grain size larger, and then improve the thermal conductivity.
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