Based on the development of electronic chips with higher and higher comprehensive performance and smaller overall size at the current stage, the heat flux presented during the working process of electronic chips has also increased significantly.
For electronic devices, the effective life of the device will be reduced by 30% to 50% for every 10°C increase in temperature. Therefore, selecting suitable packaging materials and processes and improving the heat dissipation capability of devices have become the technical bottlenecks in the development of power devices.
Taking high-power LED packaging as an example. Generally, 70%~80% of the input power is converted into heat (only about 20%~30% is converted into light energy). Besides, the LED chip size is small, and the power density of the device is large (greater than 100W/cm2 ). If the heat from the chip cannot be exported and dissipated in time, a large amount of heat will accumulate inside the LED. Then the junction temperature of the chip will gradually increase. Thermal stress is generated inside the LED device causing a series of reliability problems.
How does packaging substrate take effect?
Packaging substrate mainly uses the high thermal conductivity of the material itself to conduct heat away from the chip (heat source) to achieve heat exchange with the external environment. At present, commonly used packaging substrates can be mainly divided into polymer substrates, metal substrates and ceramic substrates.
For power device packaging, in addition to basic wiring functions, the packaging substrate is also required to have high thermal conductivity, heat resistance, insulation, strength and thermal matching performance.
Therefore, the use of polymer substrates and metal substrates is greatly limited. Ceramic materials themselves have high thermal conductivity, good heat resistance, high insulation, high strength, and thermal matching with chip materials, and are very suitable as power device packaging substrates. At present, it has been widely used in semiconductor lighting, laser and optical communication, aerospace, automotive electronics, deep-sea drilling and other fields.
At present, commonly used ceramic substrate materials for electronic packaging include alumina, silicon carbide, aluminum nitride, silicon nitride, and beryllium oxide. Among these several materials, who is the best choice for chip heat dissipation?
Different ceramic material comparison
Alumina ceramics are the most common ceramic substrate materials. As early as 1929, German Siemens successfully developed Al2O3 ceramics, published research results in 1932, and began industrial production in 1933. Because of its low price, good stability, good insulation and mechanical properties, and skilled technology, it is currently the most widely used ceramic substrate material.
However, the thermal conductivity of Al2O3 ceramics is low (20W/(m K)). And the thermal expansion coefficient does not match Si, which limits its application in high-power electronic products to a certain extent. Its application range is limited to the packaging field where the voltage that the circuit can withstand is low and the integration level of the circuit is not too high.
BeO ceramics are currently more commonly used high thermal conductivity ceramic substrate materials. They have good overall performance and can meet high electronic packaging requirements.
However, their thermal conductivity varies greatly with temperature fluctuations, and their thermal conductivity decreases significantly as the temperature rises. In addition, BeO powder is highly toxic, and it will cause acute pneumonia after a large amount of inhalation to the human body, and chronic beryllium lung disease after long-term inhalation, which greatly limits its application. It is understood that the production of BeO is no longer allowed in Japan. And BeO-related electronic products are also subject to certain restrictions in Europe.
SiC single crystal has high thermal conductivity. The thermal conductivity of pure SiC single crystal at room temperature is as high as 490W/(m·K), but due to the difference in grain orientation, the thermal conductivity of polycrystalline SiC ceramics is only 67W/(m·K)).
In addition, SiC has a low degree of insulation, a large dielectric loss, and poor high-frequency characteristics. Therefore, SiC, as a circuit substrate material, has been studied less for many years.
What are the benefits of aluminum nitride?
From this point of view, the above three ceramic materials are not good choices as substrate materials, especially in terms of high-power devices. Let’s take a look at aluminum nitride and silicon nitride.
In contrast, the performance of aluminum nitride ceramics is excellent, especially the characteristics of high thermal conductivity. Its theoretical thermal conductivity can reach 320W/(m K), and its commercial product thermal conductivity is generally 180W/(m ·K)~260W/(m·K), which makes it suitable for high power, high leads and large size chip packaging substrate materials.
As early as the early 1980s, some developed countries in the world began to engage in the research and development of AlN substrates. Among them, Japan was the earliest and the technology was the most mature. In 1983, it developed a thermal conductivity of 95W/(m K ) transparent AlN ceramics and 260W/(m·K) AlN ceramic substrates, and has been popularized and applied since 1984.
In addition, aluminum nitride ceramics also have high mechanical strength and chemical stability, and can maintain normal working conditions in harsh environments. It is precisely because aluminum nitride ceramics have many excellent properties that aluminum nitride ceramics will stand out among many ceramic substrate materials and become a representative product of a new generation of advanced ceramic packaging materials.
What are the benefits of silicon nitride?
In terms of silicon nitride, before 1995, the thermal conductivity of Si3N4 at room temperature was 20~70/W (m K), which was far lower than that of AlN and SiC. Therefore, the thermal conductivity of Si3N4 has not attracted people’s attention.
In 1995, the view of the thermal conductivity of silicon nitride in public changed. A scientist named Haggerty calculated through classical solid transport theory that the main reason for the low thermal conductivity of Si3N4 materials is related to defects and impurities in the crystal lattice.
And it is predicted that its theoretical value can reach up to 320W/(m·K). After that, researchers have done a lot of research on improving the thermal conductivity of Si3N4 materials. Through process optimization, the thermal conductivity of silicon nitride ceramics has been continuously improved, and it has exceeded 177W/(m·K).
In addition, the biggest advantage of Si3N4 ceramics is the low thermal expansion coefficient. Among ceramic materials, except SiO2 (quartz), the thermal expansion coefficient of Si3N4 is almost the lowest, which is 3.2×10−6/℃, which is about 1/3 of Al2O3.
Overall, the biggest advantage of aluminum nitride ceramic substrates is that it has high thermal conductivity and has a thermal expansion coefficient that matches semiconductor materials such as Si, SiC and GaAs. So it is indeed a tool in solving the heat dissipation of high-power devices.
The main feature of silicon nitride ceramics is comprehensiveness. Among the existing ceramic materials that can be used as substrate materials, Si3N4 ceramics have high bending strength (greater than 800MPa) and good wear resistance, and are known as the best comprehensive mechanical properties. Ceramic materials are stronger than other materials in heat dissipation environments with high strength requirements.
Therefore, aluminum nitride ceramics and silicon nitride ceramics, these two materials have become the most important substrate materials in recent years and in the future.
Ceramic materials themselves have high thermal conductivity, good heat resistance, high insulation, high strength, and thermal matching with chip materials, and are very suitable as power device packaging substrates.
High thermal conductivity, high mechanical strength and chemical stability.
Low thermal expansion coefficient, high thermal conductivity and high bending strength capability.