A kind of non-cleaning ball shrinking method

Abstract

The invention belongs to the technical field of infrared array detector manufacturing, and specifically relates to a cleaning-free shrinking method. The application places the readout circuit after indium stripping in a formic acid solution to remove the oxide on the surface of the indium column; after the oxide is removed, the formic acid solution remaining on the surface of the readout circuit is quickly cleaned off; and the readout circuit without obvious formic acid solution is placed in a vacuum reflow furnace for shrinking. The indium column is prevented from contacting the solution during the heating process. After shrinking, the circuit surface is clean and free of foreign matter, and no cleaning is required again. The generation of impurities on the circuit surface during the shrinking process is avoided, repeated cleaning after shrinking is reduced, and process efficiency is improved. The cost of a corrosion-resistant reflow furnace is relatively high, and this shrinking method has low requirements for the equipment of the reflow furnace, saving production costs.

Description

Technical Field

[0001] The invention belongs to the technical field of manufacturing infrared array detectors, and specifically relates to a cleaning-free ball shrinking method. Background Art

[0002] The maturity of the manufacturing process of infrared array detectors has made them widely used in aerospace, security, detection, military reconnaissance and other fields. The structure of the infrared detector chip from top to bottom includes: material chip, interconnection layer, electrode material and silicon readout circuit. The indium bumps on the readout circuit correspond one-to-one to the unit pixels of the optoelectronic material chip. The substrate electrode is connected to the chip through indium bump welding to achieve independent electrical connection between the chip and the corresponding units of the substrate. Several independent electrical pathways formed with indium solder as a bridge can transmit photoelectric signals in a timely and effective manner. Therefore, preparing indium columns with a certain height and good uniformity on the readout circuit is conducive to the interconnection and bonding of the chip and the circuit. In the flip-chip structure, the silicon chip is connected to the readout circuit through solder indium bumps, which effectively reduces the thermal mismatch between materials, limits the relative movement between materials, and increases the service life of the materials.

[0003] The preparation process of indium column for silicon-based readout circuit mainly includes UBM deposition, photolithography, indium deposition, indium stripping, and indium ball shrinking. Among them, ball shrinking refers to the melting of indium column at high temperature, and the shrinking of indium column into ball through wetting and surface tension. After ball shrinking, the surface of indium ball is smoother and more uniform; the height of indium column is also improved. The ball shrinking process increases the fault tolerance of interconnection, reduces welding pressure, reduces the lateral shear tension after welding, and greatly improves the reliability of the device. Since indium column is easily oxidized when in contact with air, and the melting point of the formed oxide In2O3 is higher than that of indium, it will be wrapped in solid form on the surface of indium column during ball shrinking process, inhibiting ball formation and hot melt welding. Therefore, the surface oxide of indium needs to be removed before ball shrinking. Two points must also be guaranteed during ball shrinking process: there is no oxide layer on the surface of indium column itself; indium column is isolated from air (oxygen) during heating process to avoid oxidation reaction.

[0004] The commonly used shrinking method is the lower-cost wet shrinking method, which uses chemical reagents such as formic acid and dilute hydrochloric acid to pre-treat the indium column surface oxides, and then puts the circuit into an organic solvent and heats it evenly to make it pill. Alternatively, the flux ZnCl2 is directly added to the organic reagent, and the flux does not react with the solution. Heating in the solution can effectively isolate the air and prevent the indium from being oxidized.

[0005] However, the solution of the prior art increases the chemical reagent contamination during the process of placing the readout circuit into the shrinking liquid for heating after being corroded by chemical reagents, resulting in more involved media in the solution. During the wet shrinking heating process, impurities and foreign objects may be retained on the circuit surface. After shrinking, it was observed that the solution impurities are mainly divided into two parts: one part of the solution impurities will float on the surface of the indium column; the other part is the indium column melting process combined with the surrounding impurities. After cooling and solidification, the impurities may be wrapped at the bottom of the indium ball or cover the surface of the indium ball. Wet shrinking has higher requirements for the cleanliness of the circuit, and the circuit needs to be cleaned before and after shrinking. After shrinking, the residual liquid and foreign objects in the circuit need to be washed away, and the circuit needs to be cleaned repeatedly. The process is cumbersome and time-consuming. Solidified impurities and foreign objects are difficult to clean and remove on the surface of the substrate. The presence of impurities and foreign objects affects the interconnection of the chip and the transmission of electrical signals of independent circuits. Summary of the invention

[0006] In response to the above technical problems, the present invention proposes a cleaning-free ball shrinking method. The present application places the readout circuit after indium stripping in a formic acid solution to remove the oxide on the surface of the indium column; after the oxide is removed, the formic acid solution remaining on the surface of the readout circuit is quickly cleaned off; the readout circuit without obvious formic acid solution is placed in a vacuum reflow furnace for ball shrinking. This avoids the indium column from contacting the solution during the heating process. After ball shrinking, the circuit surface is clean and free of foreign matter, and no further cleaning is required. The generation of impurities on the circuit surface during the ball shrinking process is avoided, repeated cleaning after ball shrinking is reduced, and process efficiency is improved. The cost of a corrosion-resistant reflow furnace is relatively high, and this ball shrinking method has low requirements for the equipment of the reflow furnace, saving production costs.

[0007] In order to solve the above technical problems, the technical solution adopted by the present invention is a cleaning-free ball shrinking method, comprising: placing the readout circuit after indium stripping in a formic acid solution to remove the oxide on the surface of the indium column; after the oxide removal is completed, taking out the readout circuit from the formic acid solution, and quickly cleaning off the formic acid solution remaining on the surface of the readout circuit; placing the readout circuit without obvious formic acid solution in a vacuum reflow furnace for ball shrinking.

[0008] In some embodiments, the rapidly cleaning the residual formic acid solution on the surface of the readout circuit comprises: rapidly drying the readout circuit by means of an air blowing device, so that no obvious formic acid solution remains on the surface of the readout circuit.

[0009] In some embodiments, placing the readout circuit without obvious formic acid solution in a vacuum reflow furnace for ball shrinkage includes: gradually heating the readout circuit in the vacuum reflow furnace so that the temperature of the readout circuit is raised from room temperature to a first preset temperature; maintaining the first preset temperature until the indium column on the readout circuit completes ball shrinkage.

[0010] In some embodiments, the method further includes: before heating the readout circuit, evacuating the vacuum reflow furnace.

[0011] In some embodiments, the first preset temperature is 200°C.

[0012] In some embodiments, the formic acid concentration of the formic acid solution is 25%-35%.

[0013] In some embodiments, placing the readout circuit after indium stripping in a formic acid solution to remove oxides on the surface of the indium column includes: placing the readout circuit after indium stripping in a formic acid solution for 8-12 minutes to remove oxides on the surface of the indium column.

[0014] In some embodiments, placing the readout circuit without obvious formic acid solution in a vacuum reflow oven for shrinking the balls further includes: heating the readout circuit in the vacuum reflow oven for 80-120 minutes to shrink the indium pillars on the readout circuit.

[0015] In some embodiments, the method further includes: when heating the readout circuit, filling the vacuum reflow furnace with a reducing gas so that an oxide layer on the surface of the indium column is reduced.

[0016] In some embodiments, the reducing gas is hydrogen.

[0017] The beneficial effects created by the present invention are as follows: the present application removes the oxide on the surface of the indium column by placing the readout circuit after indium stripping in a formic acid solution; after the oxide removal is completed, the readout circuit is taken out from the formic acid solution, and the formic acid solution remaining on the surface of the readout circuit is quickly cleaned off; the readout circuit without obvious formic acid solution is placed in a vacuum reflow furnace for ball shrinking. This avoids the contact of the indium column with the solution during the heating process. After ball shrinking, the circuit surface is clean and free of foreign matter, and no further cleaning is required. The generation of impurities on the circuit surface during ball shrinking is avoided, repeated cleaning after ball shrinking is reduced, and process efficiency is improved. The cost of a corrosion-resistant reflow furnace is relatively high, and this ball shrinking method has low requirements for the equipment of the reflow furnace, saving production costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The scope of the present disclosure may be better understood by reading the following detailed description of exemplary embodiments in conjunction with the accompanying drawings. The drawings included are:

[0019] Figure 1 An overall flow chart of a no-cleaning ball shrinking method provided in an embodiment of the present application;

[0020] Figure 2 A process flow chart of a no-cleaning ball shrinking method provided in an embodiment of the present application. DETAILED DESCRIPTION

[0021] In order to make the purpose, technical solutions and advantages of the present application clearer, the present application will be further described in detail below in conjunction with the accompanying drawings. The described embodiments should not be regarded as limiting the present application. All other embodiments obtained by ordinary technicians in the field without making creative work are within the scope of protection of this application.

[0022] In the following description, reference is made to “some embodiments”, which describe a subset of all possible embodiments, but it will be understood that “some embodiments” may be the same subset or different subsets of all possible embodiments and may be combined with each other without conflict.

[0023] If similar descriptions of "first\second\third" appear in the application documents, the following instructions will be added. In the following description, the terms "first\second\third" involved are merely used to distinguish similar objects and do not represent a specific ordering of the objects. It can be understood that "first\second\third" can be interchanged in a specific order or sequence where permitted, so that the embodiments of the present application described herein can be implemented in an order other than that illustrated or described herein.

[0024] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application belongs. The terms used herein are only for the purpose of describing the embodiments of this application and are not intended to limit this application.

[0025] Embodiment 1:

[0026] In the solution of the prior art, during the process of placing the readout circuit into the shrinking liquid for heating after being corroded by chemical reagents, chemical reagent contamination is increased, resulting in more involved media in the solution. During the wet shrinking heating process, impurities and foreign objects may be retained on the circuit surface. After shrinking, it is observed that the solution impurities are mainly divided into two parts: one part of the solution impurities will float on the surface of the indium column; the other part is the indium column melting process combined with the surrounding impurities. After cooling and solidification, the impurities may be wrapped at the bottom of the indium ball or covered on the surface of the indium ball. Wet shrinking has higher requirements for the cleanliness of the circuit, and the circuit needs to be cleaned before and after shrinking. After shrinking, the residual liquid and foreign objects in the circuit need to be washed away, and the circuit needs to be cleaned repeatedly, which is a cumbersome and time-consuming process. Solidified impurities and foreign objects are difficult to clean and remove on the surface of the substrate. The presence of impurities and foreign objects affects the interconnection of the chip and the transmission of electrical signals of independent circuits.

[0027] In view of the problems existing in the prior art, such as Figure 1 and Figure 2 As shown, the present application provides a cleaning-free ball shrinking method, the method comprising:

[0028] Step S1: placing the readout circuit after indium stripping in a formic acid solution to remove oxides on the surface of the indium column.

[0029] In the process of indium precipitation, indium will also be precipitated in other areas where indium columns are not needed, so it is necessary to strip the indium in other areas where indium columns are not needed, so there is a link of indium stripping. After the indium stripping is completed, the indium column can be sphericalized. However, since the indium column is easily oxidized when in contact with air, and the melting point of the formed oxide In2O3 is higher than that of indium, it will be wrapped in a solid form on the surface of the indium column during the sphericalization process, inhibiting sphering and hot-melt welding. Therefore, the surface oxide of indium needs to be removed before sphericalization. Therefore, the readout circuit needs to be immersed in a formic acid solution to remove the oxide on the surface of the indium column.

[0030] In some embodiments, the formic acid concentration of the formic acid solution is 25%-35%.

[0031] In some embodiments, step S1 of "placing the readout circuit after indium stripping in a formic acid solution to remove oxides on the surface of the indium column" includes:

[0032] Step S11: placing the readout circuit after indium stripping in a formic acid solution for 8-12 minutes to remove oxides on the surface of the indium column.

[0033] It takes a certain amount of time to remove the oxide on the surface of the indium column, so the readout circuit that has completed the indium stripping needs to be immersed in a formic acid solution with a concentration of 25%-35% for 8-12 minutes to ensure that the oxide on the surface of the indium column is completely removed.

[0034] Step S2: After the oxide removal is completed, the readout circuit is taken out from the formic acid solution, and the formic acid solution remaining on the surface of the readout circuit is quickly cleaned off.

[0035] After the oxide removal is completed, when the indium column is fished out from the formic acid solution, it is inevitable that there will be a large amount of formic acid residue. Because indium is easily oxidized in the air, in order to avoid the indium column from forming a thicker oxide layer again, the fished out readout circuit can be directly placed in a vacuum reflow furnace. However, since a large amount of formic acid is attached to the fished out readout circuit, it is easy to corrode the vacuum reflow furnace, which greatly affects the service life of the vacuum reflow furnace. Therefore, in this application, it is necessary to quickly clean the readout circuit taken out from the formic acid solution, which can greatly reduce the residual amount of formic acid solution and avoid the large amount of oxide generation.

[0036] Therefore, in some embodiments, step S2 “after the oxide removal is completed, the readout circuit is taken out from the formic acid solution, and the formic acid solution remaining on the surface of the readout circuit is quickly cleaned off” includes:

[0037] Step S21: The readout circuit is quickly dried by a blowing device so that no obvious formic acid solution remains on the surface of the readout circuit.

[0038] By using a high-power air blower to quickly dry the readout circuit at room temperature or lower, the residual formic acid solution on the readout circuit can be quickly reduced, so that there is no obvious formic acid solution remaining on the surface of the readout circuit. In addition, the readout circuit is dried at room temperature or lower because a strong wind flow is formed when the high-power air blower blows the readout circuit, which will lower the surface temperature of the readout circuit and shorten the contact time between indium and oxygen, which can effectively inhibit the oxidation of indium and reduce the generation of oxides, so that the generated oxides will not affect the shrinkage of the ball.

[0039] Step S3: placing the readout circuit without obvious formic acid solution in a vacuum reflow furnace for ball shrinking.

[0040] In some embodiments, step S3 of "placing the readout circuit without obvious formic acid solution in a vacuum reflow oven for shrinking" includes:

[0041] Step S31: gradually increasing the temperature of the readout circuit in the vacuum reflow furnace so that the temperature of the readout circuit increases from room temperature to a first preset temperature.

[0042] Step S32: maintaining the first preset temperature until the indium column on the readout circuit has completely shrunk.

[0043] In some embodiments, before step S31 of "gradually heating the readout circuit in the vacuum reflow furnace so that the temperature of the readout circuit is raised from room temperature to a first preset temperature", the method further includes:

[0044] Step S33: Before heating the readout circuit, evacuate the vacuum reflow furnace.

[0045] In some embodiments, the first preset temperature is 220°C.

[0046] In some embodiments, step S3 of "placing the readout circuit without obvious formic acid solution in a vacuum reflow oven for shrinking" further includes:

[0047] The readout circuit is heated in the vacuum reflow oven for 80-120 minutes to shrink the indium pillars on the readout circuit.

[0048] A vacuum environment can be achieved in a vacuum reflow furnace, so in this application, before shrinking the ball, the vacuum reflow furnace needs to be evacuated to prevent the problem of indium oxidation when the readout circuit is heated. After the vacuum reflow furnace is evacuated, the readout circuit can be heated gradually until it reaches 220°C and maintained at 220°C for a period of time, so that the indium column shrinks into a ball by slow heating. The heating temperature set in the vacuum reflow furnace is slightly higher than the melting point of indium to ensure that the indium column is completely melted, shrunk and pilled. The duration of the entire heating process is 80-120 minutes to ensure that the indium column can be completely dissolved. Moreover, since the readout circuit is soaked in formic acid, a small amount of formic acid will still remain on the surface after drying, and a small amount of formic acid does not directly contact the vacuum cavity and will not corrode the equipment. Residual formic acid will decompose into carbon dioxide and water at high temperature, so no new solid impurities will be formed on the surface of the circuit. When the reflow is completed, the temperature drops to room temperature and the circuit can be taken out. The circuit cooled to room temperature is dry and clean, and there is no need to clean the circuit again.

[0049] Although the generation of oxides is reduced as much as possible during the drying process, oxides are still inevitably generated, which has a smaller impact on the shrinkage of the balls. Therefore, in order to achieve a better shrinkage effect, in some embodiments, the method further includes:

[0050] Step S4: When heating the readout circuit, a reducing gas is filled into the vacuum reflow furnace to reduce the oxide layer on the surface of the indium column.

[0051] In some embodiments, the reducing gas is hydrogen.

[0052] In order to improve the ball shrinking effect, and because there are fewer oxides on the surface of the indium column, we can use the method of filling the vacuum reflow furnace with reducing gas, such as hydrogen, to reduce the oxides on the surface of the indium column to improve the ball shrinking effect of the indium column. Of course, the air in the vacuum reflow furnace can also be removed by filling with reducing gas, so that the vacuum reflow furnace is in an oxygen-free environment.

[0053] Therefore, the shrinking ball completed by the method of the present application avoids the contact between the indium column and the solution during the heating process. After the shrinking ball, the circuit surface is clean and free of foreign matter, and no further cleaning is required. The generation of impurities on the circuit surface during the shrinking ball process is avoided, and repeated cleaning after the shrinking ball is reduced, thereby improving the process efficiency. The cost of the corrosion-resistant reflow furnace is relatively high, and this shrinking ball method has low requirements for the equipment of the reflow furnace, saving production costs.

[0054] It should be understood that "one embodiment" or "an embodiment" mentioned throughout the specification means that specific features, structures or characteristics related to the embodiment are included in at least one embodiment of the present application. Therefore, "in one embodiment" or "in an embodiment" appearing throughout the specification does not necessarily refer to the same embodiment. In addition, these specific features, structures or characteristics can be combined in one or more embodiments in any suitable manner. It should be understood that in various embodiments of the present application, the size of the sequence number of the above-mentioned processes does not mean the order of execution, and the execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application. The above-mentioned sequence numbers of the embodiments of the present application are only for description and do not represent the advantages and disadvantages of the embodiments.

[0055] It should be noted that, in this article, the terms "include", "comprises" or any other variations thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, an element defined by the sentence "comprises a ..." does not exclude the existence of other identical elements in the process, method, article or device including the element.

[0056] A person of ordinary skill in the art can understand that: all or part of the steps of implementing the above-mentioned method embodiment can be completed by hardware related to program instructions, and the aforementioned program can be stored in a computer-readable storage medium. When the program is executed, it executes the steps of the above-mentioned method embodiment; and the aforementioned storage medium includes: mobile storage devices, read-only memories (ROM, Read Only Memory), disks or optical disks, etc. Various media that can store program codes.

[0057] Alternatively, if the above-mentioned integrated unit of the present application is implemented in the form of a software function module and sold or used as an independent product, it can also be stored in a computer-readable storage medium. Based on this understanding, the technical solution of the embodiment of the present application can essentially or in other words, the part that contributes to the prior art can be embodied in the form of a software product, which is stored in a storage medium and includes a number of instructions for a controller to execute all or part of the methods described in each embodiment of the present application. The aforementioned storage medium includes: various media that can store program codes, such as mobile storage devices, ROMs, magnetic disks, or optical disks.

[0058] The above is only an implementation method of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art who is familiar with the present technical field can easily think of changes or substitutions within the technical scope disclosed in the present application, which should be included in the protection scope of the present application. Therefore, the protection scope of the present application should be based on the protection scope of the claims.

Claims

1. A no-cleaning ball shrinking method, characterized in that: include: Placing the readout circuit after indium stripping in a formic acid solution to remove oxides on the surface of the indium column; After the oxide removal is completed, the formic acid solution remaining on the surface of the readout circuit is quickly cleaned off; placing the readout circuit without obvious formic acid solution residue in a vacuum reflow oven for ball shrinking; The step of placing the readout circuit without obvious formic acid solution in a vacuum reflow furnace for shrinking comprises: Gradually heating the readout circuit in the vacuum reflow furnace so that the temperature of the readout circuit is raised from room temperature to a first preset temperature; The first preset temperature is maintained until the indium column on the readout circuit has completed shrinking.

2. The method according to claim 1, characterized in that The method of quickly cleaning the residual formic acid solution on the surface of the readout circuit comprises: The readout circuit is quickly dried by a blowing device so that no obvious formic acid solution remains on the surface of the readout circuit.

3. The method according to claim 1, characterized in that The method further comprises: Before heating the readout circuit, the vacuum reflow furnace is first subjected to a vacuum treatment.

4. The method according to claim 1, characterized in that: The first preset temperature is 220°C.

5. The method according to claim 1, characterized in that The formic acid concentration of the formic acid solution is 25%-35%.

6. The method according to claim 5, characterized in that The step of placing the readout circuit after indium stripping in a formic acid solution to remove oxides on the surface of the indium column comprises: The readout circuit after indium stripping is placed in a formic acid solution for 8-12 minutes to remove oxides on the surface of the indium column.

7. The method according to claim 1, characterized in that The step of placing the readout circuit without obvious formic acid solution in a vacuum reflow oven for shrinking the circuit further comprises: The readout circuit is heated in the vacuum reflow oven for 80-120 minutes to shrink the indium pillars on the readout circuit.

8. The method according to claim 1, characterized in that The method further comprises: When the readout circuit is heated, reducing gas is filled into the vacuum reflow furnace so that the oxide layer on the surface of the indium column is reduced.

9. The method according to claim 8, characterized in that The reducing gas is hydrogen.

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