Vacuum Sintering of Electronic Devices

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In today’s era of rapid development of the electronics industry, vacuum sintering technology, also known as low-temperature vacuum brazing, is an important microelectronic interconnection technology. A large number of low-melting point metal-based alloy solders (also known as fillers) are used in modern electronic chip-level packaging and assembly. Connect, complete device packaging and board assembly”. The old process of packaging and assembling under nitrogen protection positive pressure in the past can no longer meet the requirements of modern electronic technology. Using the old process, the soldering surface of the chip and the substrate has a low penetration rate. There are gaps in the interior, which reduces the heat dissipation area, and the chip is easy to burn out due to junction temperature. Vacuum sintering technology is to solder and assemble electronic devices under vacuum negative pressure. The quality of the soldering surface of the chip and the substrate has undergone a qualitative change. The efficiency is greatly improved, and the compactness of the welding surface and the strength of the weld joint are also significantly improved.

1 Vacuum sintering process

1.1 Mechanism of vacuum sintering

Two different metals form a eutectic alloy in a certain proportion at a temperature much lower than their respective melting points, and this lower temperature is their eutectic point. The sintering process is to put an alloy sheet (usually called solder or solder) between the chip and the carrier (substrate or shell), and heat it to the eutectic point of the alloy in a certain vacuum environment to melt it and melt it into a liquid. The alloy solder infiltrates the welding layer metal of the entire chip substrate and the carrier welding surface under the action of vacuum hair suction, and the solder reacts physically and chemically with the welding layer metal and the metal of the carrier welding surface to form a certain amount of intermetallic compounds, and then cools to Below the eutectic point, the solder and the intermetallic compound will weld the chip and the carrier together to achieve good ohmic contact, thus completing the welding of the chip and the circuit carrier and functional devices.

Commonly used gold alloy flake solders for power hybrid integrated circuits include gold-tin alloy solder (melting point about 280C), gold-germanium alloy solder (melting point about 356C) and gold-silicon alloy solder (melting point about 370C).

1.2 Vacuum sintering process

Vacuum sintering mainly uses the vacuum technology of the vacuum sintering furnace to effectively control the environmental conditions in the furnace. Through the processes of preheating, exhausting, vacuuming, heating, cooling and gas charging, the appropriate temperature and protective gas control curve are set to achieve sintering. the whole process. The performance of the vacuum sintering furnace should meet the requirements of the vacuum sintering process. The key indicators of the vacuum sintering process are as follows:

1) Maximum temperature: 450 C;

2) Temperature uniformity: ±3 C (in the space area);

3) Heating rate: 15 C/min (average);

4) Cooling rate: 5 C/min (300 C~200 C);

5) Working vacuum: 6×10^-3Pa;

6) Vacuum pumping speed: 15 min~ 20 min to reach the working vacuum;

7) Vacuum leakage: ≤0.6 Pa/h;

8) Protective gas volume fraction: 99.9995%.

In power hybrid integrated circuits, the problem of simultaneous assembly of multiple chips is often encountered, requiring suitable tooling and fixture positioning. The performance of the tooling directly affects the welding quality. The position accuracy of the power chip and the substrate assembly and sintering is guaranteed by the tooling, and the thermal expansion pressure during welding is also guaranteed by the tooling. The structural size of the tooling changes with the device. Therefore, the vacuum sintering process has special requirements for the tooling. ,Summarized as follows:

1) Tooling materials are easy to choose stainless steel or high-purity reinforced graphite to prevent volatilization under vacuum and high temperature;

2) Heat treatment is required to eliminate processing stress during tooling processing, so as to ensure no deformation during use and ensure positioning accuracy;

3) The tooling has the function of adjusting the pressing force, which can effectively reduce the gap between the chip and the substrate, reduce welding voids, and ensure the sintering quality. Usually, elastic pressing (clamping) or self-weight pressing is used for pressing.

2 Vacuum sintering equipment

Vacuum sintering furnace is a special equipment for vacuum sintering and welding of power hybrid integrated circuits. Taking SIMUWU’s RVS-446 vacuum sintering furnace as an example, it is characterized by being suitable for mass production in the electronics industry. The main components include furnace body, heating tank, strong cooling system, vacuum system and electrical control system, etc.

2.1 furnace body

The furnace body is the main part of the vacuum sintering furnace. The cylinder body is composed of double-layer steel plates, with a water jacket in the middle and a heating liner inside. In order to meet the cleanliness requirements of component sintering, the inner wall of the cylinder is made of stainless steel, and the outer wall is made of carbon steel. The cylinder is equipped with devices such as water-cooled electrodes and thermocouple introduction. The furnace door adopts an oval standard head, and the door is equipped with an observation window. In order to ensure the vacuum in the furnace body, a dovetail sealing groove and an “O” rubber ring are used to seal between the boiler and the furnace door.

2.2 Forced cooling system

The forced cooling system is mainly designed to meet the sintering/welding cooling process. During the melting and solidification process of the solder, the cooling speed has a great influence on the quality of the sintered joint surface. Different solders and different devices have different cooling speed requirements. Vacuum sintering furnace has dual-function cooling mechanism, centrifugal fan and heat exchanger to achieve rapid cooling, can control the atmosphere to achieve natural cooling, and meet various process requirements.

3 Vacuum sintering quality

Carry out X-ray photography inspection for two kinds of metallization layer Ti-Ni-Ag and Ti-Ni-Au power chips, X-ray photography sampling inspection results, almost no voids, and the effective welding area of mass production has reached more than 95%.

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