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Silicon carbide module silver sintering technology

SiC modules face several technical challenges, including reliability and heat dissipation. For this reason, the silver sintering process is very popular, and some companies have also developed a copper sintering process.

In 2006, Infineon Easypack1 used double-sided silver sintering technology; in 2007, Semikron’s SkinTer technology also used silver powder sintering technology to connect between chips and substrates. In 2018, the silicon carbide module of Tesla Model 3 officially made silver sintering and related equipment “famous”.

In the connection between the chip and the substrate, in traditional power modules with substrate soldering, the solder connection is often the mechanical weak point on the module. Due to different thermal expansion coefficients of materials, high temperature fluctuations and excessive load cycles during operation will cause solder layer fatigue and affect module reliability.

At present, silver sintering technology has become the most widely used technology in the third-generation semiconductor packaging technology, and is adopted by silicon carbide module manufacturers in the United States and Japan. Compared with the traditional high-temperature lead-free solder, the sintered connection layer of silver sintering technology is composed of silver, which has excellent electrical and thermal conductivity. Since the melting point of silver is as high as 961°C, there will be no soldering connection with a melting point of less than 300°C. The typical fatigue effect that occurs in the layer, it has extremely high reliability, and its sintering temperature is comparable to that of traditional soldering materials.

Compared with soldered modules, silver sintering technology has an important impact on module structure, service life, and heat dissipation. The use of silver sintering technology can increase the service life of modules by 5-10 times, and the thickness of the sintered layer is 60-70% thinner than that of the soldered layer. The thermal conductivity has been increased by 3 times. Many manufacturers regard silver sintering technology as the core technology of the third-generation semiconductor packaging. Silver sintering technology has become the best choice for the connection between chips and substrates. At the same time, a double-sided silver sintering technology has been developed on this basis. The silver ribbon is sintered on the front of the chip instead of the aluminum wire, or the substrate is directly sintered on the heat sink without the bottom plate, which greatly simplifies the structure of the module package.

The chip connection uses silver sintered alloy instead of solder, the sintered connection has a higher melting point, which means that the aging rate of the connection will be much lower for a given temperature swing, the thermal cycling ability of the power module can be increased by five times, and the thermal resistance of the connection layer 95% reduction.

At present, the silver sintering technology has entered the stage of nano-silver sintering from micro-silver sintering. Compared with micro-silver sintering technology, the connection temperature and auxiliary pressure of nano-silver sintering have decreased significantly, which greatly expands the application range of the process. In silver sintering technology, in order to prevent oxidation and improve the reliability of the oxide layer, it is necessary to plate nickel on the bare copper surface of the substrate and then gold or silver. At the same time, the control of sintering temperature and pressure are also key factors affecting the quality of the module.

Silver sintering technology is cost-effective in many aspects, including high throughput, low capital cost, high yield and low labor cost. Up to now, many manufacturers have provided power modules manufactured by silver sintering technology, and there are also some manufacturers that provide nano-silver solder and related materials.

The main problems encountered in the development of silver sintering technology

1.The cost of silver paste used in silver sintering technology is much higher than that of solder paste. The cost of silver paste increases as the size of silver particles decreases, and the cost of precious metal coating on the copper layer of the substrate also increases;

2.Silver sintering technology requires a certain amount of auxiliary pressure, and high auxiliary pressure can easily cause chip damage; the whole process of silver sintering preheating and sintering lasts for more than 60 minutes, and the production efficiency is low;

3.The connection layer obtained by silver sintering technology generally has internal voids at the micron or submicron level, and there is no effective detection method at present.

In fact, the sintering temperature, sintering pressure, and sintering atmosphere all have a great impact on the silver sintering process, so high-quality equipment is needed to control these problems.