Alumina ceramic packaging substrates

Modern microelectronics technology is developing very rapidly, and electronic systems and equipment are developing in the direction of large-scale integration, miniaturization, high efficiency, and high reliability.

The increase in the integration of electronic systems will result in an increase in power density and an increase in heat generated by the overall operation of electronic components and systems. Heat dissipation has become a key technology that affects device performance and reliability. For electronic devices, usually every 10°C increase in temperature reduces the effective life of the device by 30% to 50%.

Good heat dissipation of electronic devices depends on optimized heat dissipation structure design, packaging material selection (thermal interface material and heat dissipation substrate), and packaging manufacturing process. Among them, the selection of the substrate material is a key factor, which directly affects the cost, performance and reliability of the device, and puts forward higher requirements on the performance of the semiconductor packaging substrate.

Generally speaking, electronic packaging substrates should have the following properties:

(1) Good thermal conductivity. Thermal conductivity is one of the main performance indicators of electronic packaging substrate materials. If the packaging substrate fails to dissipate heat in time, it will affect the life and operating conditions of electronic equipment.

(2) Linear expansion coefficient matching. If the linear expansion coefficients of the two differ greatly, the rapid thermal cycle of the electronic device during operation is likely to introduce thermal stress and cause failure.

The packaging substrate mainly utilizes the high thermal conductivity of the material itself to export heat from the chip (heat source) to achieve heat exchange with the external environment. For power semiconductor devices, the package substrate must meet the following requirements:

(1) High thermal conductivity. At present, power semiconductor devices are packaged with thermoelectric separation, and most of the heat generated by the device is transmitted through the package substrate. A substrate with good heat conduction can protect the chip from thermal damage.
(2) Match the thermal expansion coefficient of the chip material. The power device chip itself can withstand higher temperatures, and changes in current, environment, and operating conditions will change its temperature. Since the chip is directly mounted on the package substrate, the matching of the thermal expansion coefficient of the two will reduce the thermal stress of the chip and improve the reliability of the device.
(3) Good heat resistance, meet the high temperature use requirements of power devices, and have good thermal stability.
(4) Good insulation, meeting the requirements for electrical interconnection and insulation of components.
(5) Low dielectric constant and dielectric loss.

In addition, the electronic packaging substrate should also have high mechanical properties, stable chemical properties, easy processing, suitable for large-scale production, reasonable price, and suitable for large-scale applications.

At present, the commonly used substrate materials for electronic packaging are mainly: polymer substrates; metal substrates; composite substrates; ceramic substrates.

Therefore, polymer substrates (such as PCB) usually have low thermal conductivity and poor reliability, and are not suitable for higher requirements; metal substrates (such as MCPCB) have high thermal conductivity, but generally do not match the thermal expansion coefficient , And the price is high, the use is greatly restricted; and the ceramic material itself has the properties of high thermal conductivity, good heat resistance, high insulation, high strength, and thermal matching with the chip material, which is very suitable as a power device packaging substrate. It has been widely used in semiconductor lighting, laser and optical communications, aerospace, automotive electronics, deep-sea drilling and other fields.

Compared with polymer substrates and metal substrates, ceramic substrates have the following advantages:
(1) Good insulation performance and high reliability;
(2) The dielectric coefficient is small, and the high frequency performance is good;
(3) Small thermal expansion coefficient and high thermal conductivity;
(4) The air tightness is good, the chemical performance is stable, and it has a strong protective effect on the electronic system.

Therefore, the ceramic substrate is suitable for the packaging of products with high reliability, high frequency, high temperature resistance and strong air tightness in aviation, aerospace and military engineering. Ultra-small chip electronic components are widely used in the fields of mobile communications, computers, household appliances and automotive electronics, and their carrier materials often use electronic packaging ceramic substrates.

At present, several commonly used ceramic substrate materials for electronic packaging are alumina (Al2O3), aluminum nitride (AlN), silicon nitride (Si3N4), silicon carbide (SiC), boron nitride (BN), beryllium oxide (BeO) ).

Alumina ceramic substrate

At present, alumina ceramic is the most mature ceramic packaging material, and is widely used because of its good thermal shock resistance and electrical insulation, and mature manufacturing and processing technology.

Alumina ceramics generally refer to various ceramics with alumina as the main raw material, α-Al2O3 as the main crystal phase, and alumina content above 75%. It has a rich source of raw materials, low price, high mechanical strength and hardness, and insulation Good performance, thermal shock resistance, good chemical resistance, high dimensional accuracy, and good adhesion to metals. It is a ceramic substrate material with good comprehensive performance.

Alumina ceramic substrates are widely used in the electronics industry, accounting for 90% of the total ceramic substrates, and have become an indispensable material in the electronics industry.

Compared with polymer substrates and metal substrates, the advantages of alumina ceramic substrates are:
(1) Low dielectric constant, good high frequency performance;
(2) Good insulation and high reliability;
(3) High strength and good thermal stability;
(4) Low thermal expansion coefficient and high thermal conductivity;
(5) Good air tightness and stable chemical performance;
(6) Good moisture resistance and not easy to produce microcracking.

The electrical insulation performance, thermal conductivity and impact resistance will increase with the increase of alumina content, but at the same time will lead to the increase of the sintering temperature of the ceramic substrate and the increase of production cost.

In order to reduce the sintering temperature of the alumina ceramic substrate, while ensuring the mechanical and electrical properties of the alumina ceramic substrate, a certain amount of sintering aid is often added to promote sintering.

Disadvantages of alumina ceramic packaging materials:
High cost, suitable for the packaging of advanced microelectronic devices, such as high-reliability, high-frequency, high-temperature resistance, and air-tight component packaging for aerospace and military engineering;

The thermal conductivity of alumina ceramic substrate is higher than that of silicon single crystal, so alumina ceramic substrate is limited in the use of high frequency, high power, and large scale integrated circuits.