Application of vacuum sintering and tunnel furnace sintering

In terms of power chip assembly, the existing main problems are: the assembled chips, due to the bottom of the hollow area is more, thermal resistance is bigger, work have a lot of heat when not through the effective way to transfer to the shell, resulting in junction temperature is exorbitant, while working reliability, reduce the life of the power device, even 1 thermal breakdown failure by junction temperature is too high. In recent years, with the rapid development of the electronics industry, vacuum sintering process has been widely used, and the voidage rate, thermal resistance, connection strength and reliability have been significantly improved, which may become one of the key technologies for power chip assembly.

1.     The sintering mechanism

Two different metals can form eutectic alloys in proportion at temperatures well below their respective melting points. This lower temperature is their low eutectic point. Sintering process is in between the chip and carrier (substrate) or pipe shell of a metal sheet (solder), in a certain vacuum or protective atmosphere heating to the alloy melting point make it melt, melt into liquid alloy infiltrating the whole chip welding metal and carrier surface of substrate, solder welding the weld metal and carrier with the metal physical and chemical reaction, generate a certain amount of intermetallic compound, and then, in the process of cooled to below the melting point of solder and intermetallic compound chip and carrier welded together, form good ohmic contact, thus complete the chip and carrier welding drawing.

In this paper, the tunnel sintering furnace and vacuum sintering furnace are selected respectively to weld the chip and DBC board, and the cavity rate of the two technologies is compared and analyzed through X-ray scanning, which proves that the actual effect of vacuum sintering process is better.

2.  Sintering process test of tunnel furnace

The use of tunnel furnace heating, in DBC board and chip add solder piece, when the solder piece by heat melting, with the help of it on DBC board coated copper part of mutual melting and the formation of metal compound, achieve new alloy surface will chip and DBC board solidly welded together. The test results are as follows:

FIG. 1 (a) shows the control curve of sintering temperature in the tunnel furnace. FIG. 1 (b) is the reflow furnace in FIG. 1, which is a multi-purpose "cold wall" process welding furnace. Under the condition of observation 1 < a), X-ray scan diagram of the back side of the chip completed by the tunnel sintering furnace and benefit window are provided on the top cover of the chamber. Through it, real-time recording of each stage of the sintering process can be achieved. Then, the cavity rate of the chip after sintering can be obtained by the computer as 8, 7%.Adjust the temperature curve in time.

3.  Vacuum sintering furnace process test

Internally open vacuum pump to extract air (vacuum degree 1 × 10 × 5 mbar);Then the vacuum sintering furnace used in the charge test is a kind of welding nitrogen dilution residual air with the function of rapid annealing. When the vacuum reaches its limit and there is basically no residual gas in the chamber, the filled hydrogen begins to heat up. At high temperature using hydrogen reduction, DBC board, solder and chip reduction, remove oxide, improve the penetration rate of solder. After a period of high temperature, the chip has been welded to the DBC board, but there are still many bubbles on the welding surface. After the vacuum pump is started for the third time, residual bubbles on the welding surface can be extracted as much as possible after a period of time, thus reducing the voids in the chip welding and the voids rate.

The whole process can be programmed and controlled by computer. The time, gas flow and temperature of each program section can be precisely set, and the operation is convenient. In addition, because of the use of solder sheet, do not have flux welding, so that the sintered chip can be directly sent to the next process, reducing the cleaning link, reduce production costs. The test results are as follows:

FIG. 2 (a) shows the control curve of vacuum sintering temperature. FIG. 2 (b) is the X-ray scan diagram of the back of the chip completed by vacuum sintering under the conditions of FIG. 2 (). The computer shows that the cavity rate of the chip after sintering is only 0 coincidence 2%.

4.      Interpretation of result

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

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

2)     Sintering cavity rate: the vacuum sintering cavity rate (0· 52%) is only 6% of the tunnel sintering furnace cavity rate (8 · 7%), which reduces by 16 times and significantly improves the effective welding area.

From the comparison of the above test results, it can be seen that vacuum sintering can obtain higher quality sintering results than tunnel furnace sintering process, and the production efficiency and product reliability can be connected, so that the sintered chips can be directly sent to the next process, reducing the cleaning link and reducing the production cost. The test results are as follows:

To significant improvement; And the whole sintering process is precisely controlled by computer programming to avoid errors caused by human operation.

5.     Conclusion

With the continuous improvement of semiconductor manufacturing process, vacuum sintering process and related equipment will be updated and improved day by day, gaining wider recognition and market application. However, there are many factors that affect the quality of power chip sintering, which need to be further recognized and explored.

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Innovation of industrial furnace combustion technology

Industrial furnaces are mainly divided into smelting casting and heat treatment furnace two types. The former is used for refining and smelting casting, while the latter is used for heating and heat treatment of billets before processing. Most of the fuel used for metal heating is natural gas or city gas, which must be fed into the air by air blower to support combustion. 

As we all know, the air is composed of nitrogen, which accounts for 78%, and oxygen, which accounts for 21% (and about 1% of the noble gases). Nitrogen, which has no combustion effect, becomes a drag, which not only delays the heating speed, but also takes away the heat in the flue gas, adding to the environmental problems. After years of research and development, The Linde Group of Germany has invented The low-temperature pure oxygen combustion technology (LTOF), which is suitable for smelting, melting casting and heat treatment of non-ferrous metals such as steel, copper and aluminum.

1. Introduction of low temperature pure oxygen combustion technology (LTOF) in industrial furnace

The main effects of pure oxygen combustion technology are:

(1) No nitrogen enters the furnace;

(2) No nitrogen removes heat in the flue gas;

(3) Iincrease the efficiency of radiation heat transfer;

(4) The flue gas stays longer in the furnace, so as to improve the heat transfer effect;

(5) Since there is no nitrogen in combustion supporting gas, it has little impact on environmental protection when discharged;

(6) The combustion supporting effect of pure oxygen reaches 77%, while the atmospheric combustion supporting rate is only 23%.

2. Advantages of low temperature pure oxygen burner (LTOF) flame in industrial furnace

The objective of low temperature pure oxygen combustion (LTOF) technology development is to improve the melting capacity and efficiency of reflector. The unique LTOF burner allows the flue gas from the entrainment furnace to enter the burner mixing zone, diluting the oxygen concentration at the front of the burner and slowing down the rate of combustion reaction, resulting in a lower flame temperature that is close to the flame of air-fuel combustion. 

This LTOF pure oxygen combustion flame characteristics make the temperature in the furnace more uniform, eliminate local hot spots, reduce fuel consumption, reduce flue gas emissions and improve metal recovery rate. Comparison between air combustion and low-temperature pure oxygen combustion, among which, air combustion, burner power is 311kW, water cooling is 231kW, the average constantly measured in molten aluminum is 1131℃, and heat flux is 79kW/m2.Low-temperature pure oxygen combustion, burner power 257kW, water cooling 66kW, continuous measurement of aluminum in the average value of 1152℃, heat flux 79kW/m2.

3. Flame form of LTOF low-temperature pure oxygen burner in industrial furnace, flame free combustion is the most effective

(1) Can reduce NOx emission by 90%;

(2) Compared with regenerative or ordinary pure oxygen combustion, flameless combustion is the least sensitive to air leakage, and there is no significant change in NOx production. The average energy consumption and thermal balance of LTOF pure oxygen combustion technology in 28t aluminum alloy melting reflector.

Powder Metallurgy Pressed Sintering Integrative Furnace

The physical and mechanical properties of the metal powder can be obtained by sintering. For multicomponent liquid phase sintering, the sintering temperature is generally lower than that of refractory metals, but higher than that of fusible metals.

1.    Powder metallurgy sintering process

After the powder forming process, sintering and post-treatment (such as finishing) are required. Sintering and finishing are generally done separately. That is, sintering finished, and then put on the press finishing. For multicomponent liquid phase sintering, this separate process makes the quality of products difficult to be guaranteed. The sintering temperature is low, the fusible metal is not melted, and the bonding enhancement effect is not obtained. High sintering temperature, fusible metal melting, and easy loss and re-accumulation, will also affect the quality of products. The ideal method is to press in the furnace immediately when the sintering temperature is reached. The following describes the structural principles of some pressurized and sintered integrated equipment used in aviation powder metallurgy for high temperature sealing materials of copper or steel in the range of 500 ~ 900 ℃, pressing in the range of 50 ~ 2 000 kg and controlling accuracy of ±1.5% of the set value.

In order to achieve pressure sintering of powder metallurgy, the following problems need to be solved in the equipment:

(1)  when the products are heated in the reducing atmosphere, the sealing of the furnace cover and the furnace tank must be ensured first, and the gas cannot be leaked. When the furnace tank is expanded by heat, when the tank body extends, there should be expansion and contraction space, can not be gored;

(2)  pressurization is carried out in the 900 ℃ high temperature furnace tank, and the extrusion mechanism is set in the furnace tank. Firstly, the high temperature strength of metal materials should be considered, and secondly, the sealing of sports pairs should be considered. They cannot produce creep, cannot be bitten to death, and the friction resistance should not be too large;

(3)  the melting point of copper alloy is generally more than 750 ℃. Once the sintering temperature reaches, the pressure should be followed up immediately, and the pressure should be given as much as it is set, followed up dynamically, and the pressure should be kept stable.

(4)  the heating process and pressurizing process are controlled by PLC (programmable control) to ensure the repeatability of sintering process.

2.  Equipment structure features

The invention relates to a steel - based product with copper alloy powder as adhesive, pressurized and sintering furnace.

In this furnace, there is a fixed pressure head of metal embedded in the furnace body, furnace frame, elevator, furnace cover trolley, furnace tank and pressure cover. The upper end of the fixed pressure head is connected with the beam of the furnace body, and only the pressure plane inside the furnace is exposed. The furnace body, furnace frame, beam and fixed pressure head are rigidly connected. When the bottom plate of the furnace frame is welded with the ground embedded iron, a rigid frame member is formed. The vertical distance between the bearing plane of the upper constant pressure head and the ground is fixed.

The elevator is set in the pit below the furnace. The upper plane of the elevator is welded with two tracks, flush with the ground track, to facilitate the access of the furnace cover trolley. The surrounding plane of the furnace cover trolley is equipped with two sealing grooves, the inner ring is sand seal, and the outer ring is liquid seal (emulsion).There are N pressure springs in the liquid seal tank, which can carry the weight of the furnace.