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Comparison of Vacuum Sintering and Tunnel Furnace Sintering Technology

In terms of power chip assembly, the main problems at present are: after the assembled chip, due to the large void area at the bottom and the large thermal resistance, a large amount of heat generated during operation cannot be transmitted to the shell through an effective way, resulting in the junction temperature during operation. If it is too high, the reliability will be reduced, the working life of the power device will be reduced, and even the thermal breakdown will fail due to the high junction temperature. In recent years, with the rapid development of the electronic industry, vacuum sintering process has been widely used, and the void ratio, thermal resistance, connection strength, reliability, etc. have been significantly improved, or it will become one of the key technologies for power chip assembly.

1 Sintering mechanism

Two different metals can form eutectic alloys at a temperature far below their respective melting points, and this lower temperature is their eutectic point. The sintering process is to put an alloy sheet (solder) between the chip and the carrier (substrate or tube shell), heat it to the eutectic point of the alloy in a certain vacuum or protective atmosphere to melt it, and the molten alloy infiltrates the entire chip. The solder layer metal of the substrate and the carrier solder surface, the solder has a physical and chemical reaction with the solder layer metal and the metal on the carrier solder surface to generate a certain amount of intermetallic compounds, and then in the process of cooling below the eutectic point, through the solder and metal The intermetallic compound solders the chip and the carrier together to form a good ohmic contact, thereby completing the soldering of the chip and the carrier.

In this paper, a tunnel sintering furnace and a vacuum sintering furnace are respectively used to weld the chip and the DBC board, and the void ratio of the two processes is compared and analyzed by X-ray scanning, which proves that the actual effect of the vacuum sintering process is better.

2 Tunnel furnace sintering process test

Using tunnel furnace heating, a solder piece is added between the DBC board and the chip. When the solder piece is heated and melted, the intermetallic compound is formed by the mutual melting of the part of the DBC board covered with the pot to realize a new alloy surface to connect the chip and the chip. The DBC boards are securely soldered together. The test results are as follows:

Figure 1(a) is the temperature control curve of tunnel furnace sintering; under the conditions of Figure 1(a), the X-ray scan of the back side of the chip completed by the tunnel sintering furnace shows that the void ratio of the chip after sintering is 8.7% by computer.

3 Process test of vacuum sintering furnace

The vacuum sintering furnace used in the experiment is a soldering reflow furnace with rapid annealing function, which is a multi-purpose “cold wall” process soldering furnace. The top cover of the chamber is equipped with an observation window, through which real-time recording of each stage of the sintering process can be achieved, and the temperature curve can be adjusted conveniently and timely.

Process control process: place the chip and DBC board in a well-sealed chamber, turn on the vacuum pump to extract air (vacuum degree 1×10-3 mbar): then fill in nitrogen to dilute the residual air; open the vacuum pump again to extract nitrogen; wait for the vacuum degree When the limit is reached and there is basically no residual gas in the chamber, the hydrogen gas is charged and the temperature starts to rise. At high temperature, the reduction effect of hydrogen is used to reduce the DBC board, solder and chip, remove oxides, and improve the wettability of solder. After a period of high temperature, the chip and the DBC board have been welded together, but there are still many air bubbles on the welding surface. The vacuum pump is turned on for the third time, and after a period of time, the residual air bubbles on the welding surface can be extracted as much as possible, so that the voids in the chip welding are reduced, and the void ratio is also reduced.

The whole process can be programmed and controlled by a computer, and the time, gas flow and temperature of each program segment can be accurately set, and the operation is convenient. In addition, due to the use of solder sheets, flux-free soldering is achieved, so that the sintered chips can be directly sent to the next process, which reduces cleaning links and reduces production costs. The test results are as follows:

Figure 2(a) is the temperature control curve of vacuum sintering; under the conditions of Figure 2(a), the X-ray scan of the back of the chip after vacuum sintering shows that the void ratio of the chip after sintering is only 0.52%.

4 Results Analysis

By analyzing and comparing the results of the above two different sintering processes, it can be concluded that:

1) Sintering time: The vacuum sintering time is only 1/4 of the time of the tunnel sintering furnace, which greatly improves the production efficiency.

2) Sintering void rate: The vacuum sintering void rate (0.52%) is only 6% of the tunnel sintering furnace void rate (8.7%), which is 16 times lower, and the effective welding area is significantly improved.

From the comparison of the above test results, it can be seen that vacuum sintering can obtain higher quality sintering results than the tunnel furnace sintering process, and the production efficiency and product reliability have been significantly improved; and the entire sintering process is completely controlled by computer programming to avoid manual operation. error to come.

With the continuous improvement of the semiconductor manufacturing process, the vacuum sintering process and related equipment will be updated and perfected day by day, and will be more widely recognized and applied in the market. However, there are still many factors that affect the sintering quality of power chips, which need to be further understood. and explore.

Selection of vacuum sintering equipment: The RVS vacuum sintering furnace provided by SIMUWU is a high-quality product for this kind of process. It has the characteristics of good temperature uniformity, high temperature control accuracy, and efficient glue removal system. SIMUWU specializes in the manufacture of vacuum furnaces, has more than ten years of relevant experience, and has a good reputation in the field of vacuum furnace manufacturing.