A method for soldering a semiconductor chip

Abstract

The application belongs to the field of semiconductor chip assembly, and particularly relates to a method for eutectic sintering of semiconductor chips. The method uses formic acid as a protective agent during eutectic sintering, and scientifically sets the eutectic sintering temperature rising and constant temperature programs, so that heat can be effectively and uniformly transferred to the solder and the chip, the solder can be stably melted, the formation of voids is reduced, and the adverse effects of the voids on the qualified rate of the solder piece are reduced.

Description

Technical Field

[0001] This invention belongs to the field of semiconductor chip assembly, and specifically relates to a method for eutectic sintering of semiconductor chips. Background Technology

[0002] Eutectic bonding is a common process in high-power devices, but it can easily create voids on the back of the chip, leading to thermal failure. This is because there are gaps between the solder pad, the chip, and the thermal layer. At the point of contact with the thermal layer, the solder melts first, and under the action of surface tension, it pulls the unmelted solder away, drying out the original area and creating a void on the back of the chip. Because of the presence of these voids, the heat generated by the chip during operation cannot be conducted to the air through the thermal layer in time. This is especially true for high-power chips or those used for switching, which can generate a large amount of heat instantaneously. If the heat cannot be dissipated, the chip will fail due to overheating. Furthermore, when a cavity forms on the back of the chip, the ultrasonic waves applied to the bonding die during bonding exert pressure under high-frequency vibration, continuously rubbing the chip. This high-frequency ultrasonic pressure forms a fulcrum at the edge of the cavity, generating a large torque. This torque significantly increases the stress at the fulcrum. Due to the ultrasonic vibration, a pulse impact force is continuously applied to the fulcrum. Since the chip material is very brittle, the combined effect of the force may cause microcracks inside the chip, leading to a decrease in chip performance or even damage. For these reasons, the chip yield rate is greatly reduced, making the micro-assembly process of high-power products a problem that restricts our development.

[0003] Eutectic bonding, due to its high sintering temperature (generally above 280℃), allows for subsequent product processing. The lower temperature during reflow soldering creates a temperature gradient, preventing secondary melting of the solder in the semiconductor chip and ensuring product reliability. Because of the high eutectic temperature, lead-tin-silver solder or gold-antimony solder are commonly used. When using lead-tin-silver solder for eutectic bonding, the common process involves sintering with a nitrogen-hydrogen mixture. While nitrogen provides good protection, this method has several drawbacks, such as uncontrollable furnace temperature and solder melting state. Another method is reflow soldering, but reflow soldering typically uses nitrogen protection, which only provides physical protection and lacks reducing properties. Common reflow soldering uses ordinary mechanical pumps, which have limited capacity, only reaching a few Pascals, insufficient to effectively protect the solder and chip from oxidation. However, the hydrogen sintering furnace cannot precisely control the heating or cooling time of the product, which causes serious solder overflow on the chip. The overflowing solder may cause the back of the chip to lose solder, exacerbating the formation of voids.

[0004] Patent publication number CN108461380A discloses a control structure and control method for the sintering void ratio of a large-area integrated circuit chip. The control method includes (1) raw material processing: first, pre-baking the shell at a temperature of 100-150°C for 1-4 hours; then, plasma cleaning of the shell, chip, and solder sheet to remove oxides and contaminants from their surfaces; (2) pre-assembly: stacking the weight, transition sheet, chip, solder sheet, shell, and carrier together to obtain the control structure and control method. Structure; (3) Alloy sintering and bonding: The alloy sintering and bonding process specifically includes the following steps: (A) Filling with protective gas: Place the pre-assembled control structure into the welding furnace, evacuate at room temperature to make the vacuum degree in the cavity 0.1mbar~1.5mbar, then fill with nitrogen until the vacuum degree in the cavity is 3mbar~20mbar; finally fill with protective gas; (B) Preheating process: Heat the welding cavity from room temperature to the preheating temperature T1, T1=230~290℃, and the heating rate of the heating process to T1 is 20~80℃ / m (C) Heat preservation process: Heat to the preheating temperature and hold for more than 100 seconds; (D) Heating process: Heat the cavity from the preheating temperature T1 to the welding temperature T2, T2 = 310~400℃, and the heating rate of the process from T1 to T2 is 20~80℃ / min; (E) Reaction process: Maintain the temperature in the welding cavity at T2, the solder sheet melts completely, and completes the eutectic reaction with the metal on the back of the chip and the gold on the back of the shell to form a eutectic; the reaction time is 60s~400s; (F) Exhaust process: First, exhaust the welding cavity. (G) Rapid cooling process: The cavity temperature is reduced from T2 to T3, where T3 = 100 to 300 ℃; the cooling rate during the process of reducing the cavity temperature from T2 to T3 is 20 to 80 ℃ / min; (H) Slow cooling process: The cavity temperature is reduced from T3 to T4, where T4 = room temperature to 100 ℃; the cooling rate during the slow cooling process is 5 to 60 ℃ / min. In step (A) of step (3) above, the protective gas introduced is nitrogen and formic acid, with a volume ratio of nitrogen to formic acid of 10:1 to 2:1. However, this technical solution requires the use of weights and transition sheets to make the solder pads flat, thereby ensuring the sintering effect. Increasing the weights will reduce production efficiency and is only suitable for larger integrated circuit chips. The chip size is generally too large and not suitable for small-sized chips. Increasing the weights will also increase the risk of squeezing the solder. After the internal solder is squeezed out, the solder overflows, resulting in uneven solder on the back of the chip and causing voids.

[0005] Currently, the bare chips used in miniaturized, high-power solid-state relay products require micro-assembly processing, and the low yield has greatly hindered the development of high-power devices. Therefore, it is crucial to study a eutectic welding process with high yield and excellent sintering quality. Summary of the Invention

[0006] This invention addresses the shortcomings of existing technologies by proposing a method for eutectic sintering of semiconductor chips.

[0007] Specifically, this is achieved through the following technical solutions:

[0008] A method for eutectic sintering of semiconductor chips, wherein the eutectic sintering involves connecting one side of a preformed solder pad to a hot-layer sheet by spot welding, then stacking the chip on the other side of the preformed solder pad to form an assembly, placing the assembly into a preheated sintering equipment, evacuating the vacuum and heating it to 120-130°C, and then performing eutectic sintering using formic acid as a protective agent; the eutectic sintering adopts a combination of isothermal sintering and temperature-increasing sintering.

[0009] Furthermore, the method for eutectic sintering of the semiconductor chip includes the following steps:

[0010] 1) The silver-based solder is cut and trimmed to form a preformed solder sheet with a thickness of 0.025 mm;

[0011] 2) Flatten the preformed welding sheet to ensure that it is free from warping, arching, and unevenness;

[0012] 3) First, one side of the preformed solder sheet is connected to the hot layer sheet by spot welding. Then, the chip is stacked on the other side of the preformed solder sheet to form an assembly. Then, the assembly is placed in a preheated sintering equipment, vacuumed and heated to 120-130℃, and then eutectic sintering is carried out with formic acid as a protective agent.

[0013] 4) After the eutectic sintering is completed, high-purity nitrogen gas is introduced into the sintering equipment for cooling. When the temperature drops to 150°C, the sintering equipment is turned on for natural cooling.

[0014] The silver-based solder is Pb92.5Sn5Ag2.5.

[0015] The area of ​​the preformed solder pad is the same size as the chip area.

[0016] The preheating temperature of the sintering equipment is 25°C.

[0017] The time for heating to 120-130℃ in step 3) is 3-5 minutes.

[0018] The formic acid is introduced at a rate of 5 L / min.

[0019] The eutectic sintering is performed using isothermal sintering and temperature-increasing sintering.

[0020] The isothermal sintering process is divided into four stages: the first stage is to hold at 120-130℃ for 3-5 minutes, the second stage is to hold at 170-190℃ for 3-5 minutes, the third stage is to hold at 270-290℃ for 3-5 minutes, and the fourth stage is to hold at 320-340℃ for 1-2 minutes.

[0021] The heating and sintering process is divided into three stages: the first stage is heating from 120-130℃ to 170-190℃; the second stage is heating from 170-190℃ to 270-290℃; and the third stage is heating from 270-290℃ to 320-340℃.

[0022] The first stage of heating and sintering takes 2-3 minutes.

[0023] The second stage of heating and sintering takes 2-3 minutes.

[0024] The third stage of heating and sintering takes 2-3 minutes.

[0025] The amount of high-purity nitrogen introduced is controlled to ensure that the assembly temperature drops from 320-340℃ to 150℃ within 5-10 minutes.

[0026] Beneficial effects:

[0027] This invention utilizes a Pb92.5Sn5Ag2.5 solder with a melting point of 296℃ and a solidus of 287℃ to prepare pre-shaped solder pads. The size of these pads is comparable to the chip area, and the pad thickness is 0.025mm. A flattening fixture ensures the solder is free of warping, arching, and unevenness, allowing for complete adhesion to the chip without back-side air bubbles. During eutectic sintering, formic acid is used as a protective agent to effectively protect the silver-based material, converting the oxidized silver-based epoxy into silver ions, thus improving the wetting ability between the solder and the chip. Furthermore, by scientifically setting the eutectic sintering temperature rise and isotherm programs, heat can be effectively and evenly transferred to the solder and chip during the eutectic sintering process, ensuring stable solder melting without expansion or overflow. This also helps to reduce excessive flow during solder melting and the phenomenon of drying out the solder on the back of the chip, reducing void formation and thus mitigating the adverse effects of voids on the solder pad yield.

[0028] This invention improves the situation where solder pads are prone to bulging and unevenness during operation, which can easily lead to air bubbles, by spot welding solder pads onto the hot layer, thus reducing void formation. Furthermore, this invention eliminates the need for weights or other materials to ensure sufficient contact between the solder pads and the chip, resulting in lower preparation costs and reducing the adverse effects of weights on the other surface of the solder pads or chip during eutectic sintering.

[0029] This invention uses formic acid as a protective gas, which is simple to control and provides a good reduction effect. In contrast, traditional methods use only nitrogen or inert gas as protective agents, which cannot achieve a good reduction effect. Using a mixture of formic acid and nitrogen or inert gas as a protective agent is difficult to control and can easily cause system instability, thus affecting sintering quality and yield.

[0030] The method of this invention is applicable to the micro-assembly production of small, small-area semiconductor chips. Attached Figure Description

[0031] Figure 1 The eutectic sintering temperature control curve of the present invention. Detailed Implementation

[0032] The specific embodiments of the present invention will be described in further detail below, but the present invention is not limited to these embodiments. Any improvements or substitutions based on the basic spirit of these embodiments shall still fall within the scope of protection claimed by the claims of the present invention.

[0033] Example 1

[0034] A method for eutectic sintering of a semiconductor chip includes the following steps:

[0035] 1) Silver-based solder Pb92.5Sn5Ag2.5 is cut and shaped into preformed solder sheets with the same specifications as the chip area and a thickness of 0.025mm.

[0036] 2) Flatten the preformed welding sheet to ensure that it is free from warping, arching, and unevenness;

[0037] 3) First, one side of the preformed solder sheet is connected to the hot layer sheet by spot welding. Then, the bare chip is stacked on the other side of the preformed solder sheet to form an assembly. Then, the assembly is placed in a sintering equipment with a preheating temperature of 25°C. After vacuuming and heating to 120°C within 3 minutes, eutectic sintering is performed with formic acid as a protective agent. The formic acid filling rate is 5L / min.

[0038] 4) After the eutectic sintering is completed, high-purity nitrogen gas is introduced into the sintering equipment to reduce the temperature of the assembly from 320°C to 150°C within 5 minutes. Then the sintering equipment is turned on for natural cooling.

[0039] The eutectic sintering adopts a combination of isothermal sintering and temperature-increasing sintering. The isothermal sintering is divided into four stages: the first stage is holding at 120°C for 3 minutes, the second stage is holding at 170°C for 3 minutes, the third stage is holding at 270°C for 3 minutes, and the fourth stage is holding at 320°C for 1 minute. The temperature-increasing sintering is divided into three stages: the first stage is increasing the temperature from 120°C to 170°C within 2 minutes; the second stage is increasing the temperature from 170°C to 270°C within 2 minutes; and the third stage is increasing the temperature from 270°C to 320°C within 2 minutes.

[0040] The chip in this embodiment was fabricated into a JGX-1H10DMA high-power solid-state relay using a micro-assembly process. The statistics of qualified products are shown in the table below:

[0041] model Assembly quantity Quantity of non-conforming non-compliance rate pass rate JGX-1H10DMA 50 2 4％ 96％

[0042] In this embodiment, under the same testing conditions, chips prepared by the traditional method were used to fabricate JGX-1H10DMA high-power solid-state relays using a micro-assembly process. The number of qualified products was counted, and the result showed that the pass rate was 40%. Considering the differences between chips prepared by the traditional method and those of this embodiment, the protective agent formic acid was replaced with nitrogen gas, and the solder and hot-layer sheet were not connected by spot welding, but the bare chip, solder, and hot-layer sheet were directly stacked from top to bottom and clamped with a fixture to obtain the assembly. Other conditions were the same as in this embodiment. Considering that there are pores between the solder, the hot-layer sheet, and the bare chip in the traditional method, and that the solder in contact with the hot-layer sheet will melt first, the surface tension will pull the unmelted solder, drying out the original position and forming a void, and since nitrogen gas does not have reducing properties, it will also aggravate the oxidation of the solder, thus reducing the pass rate.

[0043] Example 2

[0044] A method for eutectic sintering of a semiconductor chip includes the following steps:

[0045] 1) Silver-based solder Pb92.5Sn5Ag2.5 is cut and shaped into preformed solder sheets with the same specifications as the chip area and a thickness of 0.025mm.

[0046] 2) Flatten the preformed welding sheet to ensure that it is free from warping, arching, and unevenness;

[0047] 3) First, one side of the preformed solder sheet is connected to the hot layer sheet by spot welding. Then, the bare chip is stacked on the other side of the preformed solder sheet to form an assembly. Then, the assembly is placed in a sintering equipment with a preheating temperature of 25°C. After vacuuming and heating to 130°C within 5 minutes, eutectic sintering is performed using formic acid as a protective agent. The formic acid filling rate is 5L / min.

[0048] 4) After the eutectic sintering is completed, high-purity nitrogen gas is introduced into the sintering equipment to reduce the temperature of the assembly from 340℃ to 150℃ within 5-10 minutes. Then, the sintering equipment is turned on for natural cooling.

[0049] The eutectic sintering adopts a combination of isothermal sintering and temperature-increasing sintering. The isothermal sintering is divided into four stages: the first stage is holding at 130°C for 5 minutes, the second stage is holding at 190°C for 5 minutes, the third stage is holding at 290°C for 5 minutes, and the fourth stage is holding at 340°C for 2 minutes. The temperature-increasing sintering is divided into three stages: the first stage is increasing the temperature from 130°C to 190°C within 3 minutes; the second stage is continuing to increase the temperature to 290°C within 3 minutes; and the third stage is continuing to increase the temperature to 340°C within 3 minutes.

[0050] The chip in this embodiment was fabricated into a JZC-1H20DMC high-power solid-state relay using a micro-assembly process. The statistics of qualified products are shown in the table below:

[0051] model Assembly quantity Quantity of non-conforming non-compliance rate pass rate JZC-1H20DMC 40 1 2.5％ 97.5％

[0052] In this embodiment, under the same testing conditions, chips prepared by the traditional method were used to fabricate JZC-1H20DMC high-power solid-state relays using a micro-assembly process. The number of qualified products was counted, and the result showed that the pass rate was 15%. Considering the differences between the chips prepared by the traditional method and those of this embodiment, the protective agent formic acid was replaced with nitrogen gas, and the solder and the heat spreader were not connected by spot welding, but the bare chip, solder, and heat spreader were directly stacked from top to bottom and then pressed with a heavy block to obtain the assembly. Other conditions were the same as in this embodiment. Considering that there are pores between the solder, the heat spreader, and the bare chip in the traditional method, and that the solder in contact with the heat spreader will melt first, the surface tension will pull the unmelted solder, drying out the original position and forming a void, and since nitrogen gas does not have reducing properties, it will also aggravate the oxidation of the solder, thus reducing the pass rate.

[0053] Example 3

[0054] A method for eutectic sintering of a semiconductor chip includes the following steps:

[0055] 1) Silver-based solder Pb92.5Sn5Ag2.5 is cut and shaped into preformed solder sheets with the same specifications as the chip area and a thickness of 0.025mm.

[0056] 2) Flatten the preformed welding sheet to ensure that it is free from warping, arching, and unevenness;

[0057] 3) First, one side of the preformed solder sheet is connected to the hot layer sheet by spot welding. Then, the bare chip is stacked on the other side of the preformed solder sheet to form an assembly. Then, the assembly is placed in a sintering equipment with a preheating temperature of 25°C. After vacuuming and heating to 125°C within 4 minutes, eutectic sintering is performed with formic acid as a protective agent. The formic acid filling rate is 5L / min.

[0058] 4) After the eutectic sintering is completed, high-purity nitrogen gas is introduced into the sintering equipment to reduce the temperature of the assembly from 330°C to 150°C within 8 minutes. Then the sintering equipment is turned on for natural cooling.

[0059] The eutectic sintering process specifically involves: first holding at 125°C for 4 minutes, then raising the temperature by 180°C within 150 seconds and holding for 4 minutes, then raising the temperature to 280°C within 150 seconds and holding for 4 minutes, and finally raising the temperature to 330°C within 150 seconds and holding for 90 seconds.

[0060] The chips in this embodiment were fabricated into high-power solid-state relays JGX-1H10DMA and JZC-1H20DMC using a micro-assembly process, and the yield rate was significantly better than that of traditional methods.

[0061] Example 4

[0062] A method for eutectic sintering of a semiconductor chip includes the following steps:

[0063] 1) Silver-based solder Pb92.5Sn5Ag2.5 is cut and shaped into preformed solder sheets with the same specifications as the chip area and a thickness of 0.025mm.

[0064] 2) Flatten the preformed welding sheet to ensure that it is free from warping, arching, and unevenness;

[0065] 3) First, one side of the preformed solder sheet is connected to the hot layer sheet by spot welding. Then, the bare chip is stacked on the other side of the preformed solder sheet to form an assembly. Then, the assembly is placed in a sintering equipment with a preheating temperature of 25°C. After vacuuming and heating to 120°C within 5 minutes, eutectic sintering is performed with formic acid as a protective agent. The formic acid filling rate is 5L / min.

[0066] 4) After the eutectic sintering is completed, high-purity nitrogen gas is introduced into the sintering equipment to reduce the temperature of the assembly from 340°C to 150°C within 5 minutes. Then the sintering equipment is turned on for natural cooling.

[0067] The eutectic sintering process specifically involves: first holding at 120°C for 5 minutes, then raising the temperature by 190°C over 2 minutes and holding for 3 minutes, then raising the temperature to 270°C over 3 minutes and holding for 5 minutes, and finally raising the temperature to 340°C over 2 minutes and holding for 1 minute.

[0068] The chips in this embodiment were fabricated into high-power solid-state relays JGX-1H10DMA and JZC-1H20DMC using a micro-assembly process, and the yield rate was significantly better than that of traditional methods.

[0069] Example 5

[0070] A method for eutectic sintering of a semiconductor chip includes the following steps:

[0071] 1) Silver-based solder Pb92.5Sn5Ag2.5 is cut and shaped into preformed solder sheets with the same specifications as the chip area and a thickness of 0.025mm.

[0072] 2) Flatten the preformed welding sheet to ensure that it is free from warping, arching, and unevenness;

[0073] 3) First, one side of the preformed solder sheet is connected to the hot layer sheet by spot welding. Then, the bare chip is stacked on the other side of the preformed solder sheet to form an assembly. Then, the assembly is placed in a sintering equipment with a preheating temperature of 25°C. After vacuuming and heating to 130°C within 3 minutes, eutectic sintering is performed with formic acid as a protective agent. The formic acid filling rate is 5L / min.

[0074] 4) After the eutectic sintering is completed, high-purity nitrogen gas is introduced into the sintering equipment to reduce the temperature of the assembly from 330°C to 150°C within 10 minutes. Then the sintering equipment is turned on for natural cooling.

[0075] The eutectic sintering process specifically involves: first holding at 130°C for 3 minutes, then raising the temperature at a rate of 170°C within 3 minutes and holding for 4 minutes, then raising the temperature to 290°C within 2 minutes and holding for 3 minutes, and finally raising the temperature to 330°C within 3 minutes and holding for 1 minute.

[0076] The chips in this embodiment were fabricated into high-power solid-state relays JGX-1H10DMA and JZC-1H20DMC using a micro-assembly process, and the yield rate was significantly better than that of traditional methods.

Claims

1. A method for eutectic sintering of a semiconductor chip, characterized in that, The eutectic sintering process involves connecting one side of a preformed solder pad to a hot-layer sheet via spot welding, then stacking the chip on the other side of the preformed solder pad to form an assembly. After the assembly is formed, it is placed in a preheated sintering equipment, vacuumed, and heated to 120-130°C, and then eutectic sintering is performed using formic acid as a protective agent. The eutectic sintering process combines isothermal sintering and temperature-increasing sintering methods. The isothermal sintering is divided into four stages: the first stage is to hold at 120-130℃ for 3-5 minutes, the second stage is to hold at 170-190℃ for 3-5 minutes, the third stage is to hold at 270-290℃ for 3-5 minutes, and the fourth stage is to hold at 320-340℃ for 1-2 minutes.

2. The method for eutectic sintering of a semiconductor chip as described in claim 1, characterized in that, The time required to raise the temperature to 120-130℃ is 3-5 minutes.

3. The method for eutectic sintering of a semiconductor chip as described in claim 1, characterized in that, The heating sintering is divided into three stages: the first stage is heating from 120-130℃ to 170-190℃; the second stage is heating from 170-190℃ to 270-290℃; and the third stage is heating from 270-290℃ to 320-340℃.

4. The method for eutectic sintering of a semiconductor chip as described in claim 3, characterized in that, The first stage of the heating and sintering process takes 2-3 minutes.

5. The method for eutectic sintering of a semiconductor chip as described in claim 3, characterized in that, The second stage of the heating and sintering process takes 2-3 minutes.

6. The method for eutectic sintering of a semiconductor chip as described in claim 3, characterized in that, The third stage of the heating and sintering process takes 2-3 minutes.

7. The method for eutectic sintering of a semiconductor chip as described in claim 1, characterized in that, Includes the following steps: 1) The silver-based solder is cut and trimmed to form a preformed solder sheet with a thickness of 0.025 mm; 2) Flatten the preformed welding sheet to ensure that it is free from warping, arching, and unevenness; 3) First, one side of the preformed solder sheet is connected to the hot-layer sheet by spot welding. Then, the chip is stacked on the other side of the preformed solder sheet to form an assembly. After the assembly is formed, it is placed in a preheated sintering equipment, vacuumed and heated to 120-130℃, and then eutectic sintering is performed with formic acid as a protective agent. The eutectic sintering is performed by holding the temperature at 120-130℃ for 3-5 minutes, then raising the temperature to 170-190℃ within 2-3 minutes, holding for 3-5 minutes, then raising the temperature to 270-290℃ within 2-3 minutes, holding for 3-5 minutes, and then raising the temperature to 320-340℃ within 2-3 minutes and holding for 1-2 minutes. 4) After the eutectic sintering is completed, high-purity nitrogen gas is introduced into the sintering equipment for cooling. When the temperature drops to 150°C, the sintering equipment is turned on for natural cooling.

8. The method for eutectic sintering of a semiconductor chip as described in claim 7, characterized in that, The silver-based solder is Pb92.5Sn5Ag2.

5.

9. The method for eutectic sintering of a semiconductor chip as described in claim 1, characterized in that, The preheating temperature of the sintering equipment is 25°C.

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