Ultra-low-temperature low-noise amplifier printed board brazing method

Abstract

The invention relates to the technical field of brazing processing of microwave circuits, in particular to a brazing method for a printed board of an extremely-low-temperature low-noise amplifier. The method comprises the steps that a printed board, a shell and a solder piece are cleaned, scaling powder is brushed on the solder piece, the solder piece and the printed board are placed in the shell, a molybdenum-copper heat sink is placed in a chip hole of the printed board, and a workpiece to be welded is obtained; a to-be-welded workpiece is placed in the thin-wall metal groove, the micro steel balls are weighed and poured into the thin-wall metal groove, the micro steel balls are made to submerge the to-be-welded workpiece, and the micro steel balls on the uppermost layer are laid flat; the thin-wall metal groove is placed in a gas phase reflow oven, a reflow soldering curve is set, and the printed board is brazed; and after brazing is completed, the micro steel balls and the workpiece are sequentially taken out of the thin-wall metal groove, the welding quality of the workpiece is detected through an X-ray detector, and finally residual scaling powder on the surface of the workpiece is removed through a gas phase cleaning machine. The links of designing, machining and installing the pressing block tool are reduced, the cost is reduced to a large extent, and the production efficiency is improved.

Description

Technical Field

[0001] The invention relates to the technical field of brazing processing, and in particular to a brazing method for an ultra-low temperature low noise amplifier printed circuit board. Background Art

[0002] The ultra-low-noise amplifier is a low-noise microwave amplifier that works in a low-temperature environment. Its typical operating temperature range includes the liquid nitrogen temperature range (typical temperature: 77K) and the liquid hydrogen temperature range (typical temperature: 20K). The most notable feature of the cryogenic low-noise amplifier (hereinafter referred to as the ultra-low-noise amplifier) ​​compared to the microwave amplifier in the normal temperature range is that the noise temperature of the device is significantly reduced. For the amplifier, thermal noise is a function of temperature. As the operating temperature decreases, the noise temperature of the amplifier decreases significantly. Therefore, the performance of the cryogenic amplifier has been greatly improved. It has the characteristics of low noise, good gain stability, and high reliability. It is a key component of the receiving front end of high-sensitivity radar and communication systems.

[0003] For ultra-low temperature low noise amplifiers, the solder penetration rate will affect its gain, and even the signal cannot be amplified and self-excitation occurs. At extremely low temperatures, due to the difference in thermal expansion coefficients between the printed circuit board and the shell, low-quality soldering will lead to the risk of desoldering, seriously affecting the performance indicators of the amplifier.

[0004] At present, the common method of welding a printed circuit board to a microwave amplifier cavity is: according to the shape of the printed circuit board, a pressing block tool is processed, and the size of the pressing block tool is appropriately reduced. A solder sheet is placed between the shell and the printed circuit board, and flux is applied. Then the pressing block tool is pressed on the printed circuit board, and then the cover plate is covered on the pressing block tool. The cover plate is locked with a long screw with a spring washer to provide downward pressure to the pressing block tool so that the solder can spread and wet the welding surface during the welding process. Finally, the entire workpiece is placed in a vapor phase welding furnace for welding. The disadvantages of this brazing method are: 1) It is necessary to process the block fixture. If the shape of the printed circuit board is complex, the processing cost is high and the time cycle is long; 2) The block fixture is processed from a whole piece of aluminum alloy flat plate, and the contact surface with the printed circuit board is a fitted plane. The pores on the printed circuit board are blocked. Therefore, during welding, the flux gas in the solder is not easy to be discharged from the grounding hole or pores, and remains in the welding surface to form voids, which reduces the penetration rate and leads to poor grounding effect; 3) When screwing the cover screws, due to differences in operators, the locking degree of screws in different batches is different, which will cause differences in the pressure under the block, affecting the batch consistency of welding quality; 4) There are many chip holes on the printed circuit board, and molybdenum-copper heat sinks need to be installed in the chip holes. Due to the height difference between the molybdenum-copper heat sink and the printed circuit board (≥0.1mm), the block fixture cannot press the molybdenum-copper heat sink, so usually after brazing is completed, the molybdenum-copper heat sink needs to be bonded to these chip holes, and the process is relatively cumbersome; 5) The entire brazing process requires screwing and disassembling screws, which consumes manpower and time and is inefficient.

[0005] Therefore, how to design a new microwave circuit brazing process method to increase the grounding area and solder penetration of the printed circuit board, improve the quality of high-frequency signal transmission, and at the same time, realize the integrated welding of the printed circuit board and the molybdenum-copper heat sink, reduce the process, reduce the production cost, and improve the production efficiency has become a technical problem that needs to be solved urgently. Summary of the invention

[0006] In order to solve the deficiencies in the prior art, the object of the present invention is to provide a method for soldering a printed circuit board of an ultra-low temperature low noise amplifier.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] A method for brazing a printed circuit board of an ultra-low temperature low noise amplifier comprises the following steps:

[0009] S1, cleaning the printed circuit board, the housing and the solder sheet, applying flux on the solder sheet, sequentially placing the solder sheet and the printed circuit board into the housing, placing a molybdenum-copper heat sink into the chip hole of the printed circuit board, and obtaining a workpiece to be welded;

[0010] S2. Place the workpiece to be welded in a thin-walled metal tank, weigh a certain weight of micro steel balls and pour them into the thin-walled metal tank so that the micro steel balls cover the workpiece to be welded, and level the top layer of micro steel balls;

[0011] S3, placing the thin-walled metal tank containing the micro steel ball and the workpiece to be welded into a vapor phase reflow furnace, inserting the temperature sensor in the vapor phase reflow furnace into the micro steel ball to collect the temperature of the workpiece to be welded, setting a reflow curve, and based on the reflow curve, using the vapor phase reflow furnace to braze the printed circuit board;

[0012] S4. After brazing is completed, first remove the micro steel balls from the thin-walled metal groove, then take out the welded workpiece, use an X-ray detector to check the welding quality of the workpiece, and finally use a gas phase cleaning machine to remove the residual flux on the surface of the workpiece.

[0013] Preferably, in step S3, the temperature sensor is a component of the vapor phase reflow furnace, and is used to collect the actual temperature of the workpiece to be welded, and adjust the heating power output of the vapor phase reflow furnace according to the received actual temperature data, so that the welding temperature in the furnace corresponds to the reflow curve. The computer system of the vapor phase reflow furnace adjusts the heating power output of the vapor phase reflow furnace so that the welding temperature in the furnace is maintained near the preset reflow curve.

[0014] Preferably, according to the present invention, in step S3, the reflow soldering curve is divided into four temperature stages: a preheating treatment stage, a heat preservation treatment stage, a reflow treatment stage and a cooling treatment stage;

[0015] The temperature of the preheating treatment stage is room temperature to 125°C, the heating rate is less than 2°C / S, and the preheating time is 70S to 110S; the temperature of the insulation treatment stage is between 125°C and 160°C, the heating rate is less than 1°C / S, and the insulation time is 60S to 120S; the peak temperature of the reflow treatment stage is 205°C to 215°C, the peak time is 10±1S, and the time above the liquidus line is 30S to 60S; the temperature of the cooling treatment stage is the peak temperature of 215°C to 75°C, and the cooling rate is 5 to 6°C / S; during the entire brazing process, the temperature does not exceed 5 minutes from room temperature to the highest temperature. The purpose of the insulation treatment stage is to make the flux inside the solder sheet fully play its role, remove the oxides on the surface of the microstrip board and the substrate and discharge them in time, thereby improving the welding quality. The use of micro steel balls makes it easier to discharge the flux, so the time of this stage can be set shorter, which improves efficiency.

[0016] Preferably according to the present invention, the shape of the solder sheet is the same as that of the printed circuit board, and the solder sheet is prepared by a laser scribing machine.

[0017] Preferably, according to the present invention, in step S1, the cleaning of the printed circuit board, the housing and the solder sheet comprises:

[0018] Use a dust-free cloth dipped in anhydrous ethanol to clean the soldering surfaces of the printed circuit board and the shell, and use an ultrasonic cleaner to ultrasonically clean the solder sheet to remove oil film and stains.

[0019] Preferably according to the present invention, in step S1, the soldering flux is applied on the solder sheet using a brush.

[0020] Preferably according to the present invention, the printed circuit board is a gold-plated copper-clad printed circuit board; the shell is made of aluminum alloy with silver or gold plating; and the molybdenum-copper heat sink is made of molybdenum-copper alloy with gold plating on the surface.

[0021] Preferably according to the present invention, the grade of the solder sheet is Sn63Pb37 and the thickness is 0.1 mm.

[0022] Preferably according to the present invention, the soldering flux is rosin soldering flux, and the rosin soldering flux is in liquid state.

[0023] Preferably according to the present invention, the material of the micro steel balls is 440C stainless steel, the diameter of the micro steel balls is 0.5-2 mm, and the total weight of the micro steel balls is 2-5 kg.

[0024] Preferably according to the present invention, the wall thickness of the thin-walled metal groove is 0.5 mm; and the material of the thin-walled metal groove is iron.

[0025] Compared with the prior art, the advantages of the present invention are:

[0026] The invention discloses a method for brazing a printed circuit board of a very low temperature low noise amplifier. The innovation point is that a micro steel ball is used to replace a pressing block tool, which can realize the brazing process of a printed circuit board of a very low temperature microwave amplifier with high quality. The invention adopts micro steel balls to replace the pressing block tool for pressurization. The tiny gaps between the micro steel balls allow the flux gas in the solder to volatilize and discharge in time, thereby reducing welding voids, increasing the grounding area and brazing penetration rate of the printed circuit board, improving the transmission quality of high-frequency signals, and the insulation time can also be set shorter to improve efficiency. Since the diameter of the micro steel balls is small and the weight can be precisely controlled, the uniformity of the pressure and pressure distribution during welding of different batches during production is consistent, which can ensure the welding consistency of the amplifier mass production. In addition, since there is a height difference (≥0.1mm) between the surface of the printed circuit board and the molybdenum-copper heat sink, the use of micro steel balls can simultaneously apply pressure to the printed circuit board and the molybdenum-copper heat sink, thereby realizing the integrated welding of the printed circuit board-molybdenum-copper heat sink and reducing the process. Finally, considering that the amplifier is used at extremely low temperatures, a Sn63Pb37 solder sheet with good wettability, better plasticity after welding, and low welding stress is selected. The solder sheet is not easy to crack at extremely low temperatures, ensuring reliable connection at low temperatures. In summary, the present invention reduces the steps of designing, processing, and installing the block tooling, greatly reduces costs, improves production efficiency, and has a strong reference and guiding role in the design, processing, and production of other microwave modules. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 This is a flow chart of the brazing method of the ultra-low temperature low noise amplifier printed circuit board in the present invention;

[0028] Figure 2 The figure is a schematic diagram of the brazing assembly structure of the ultra-low temperature low noise amplifier printed circuit board in the present invention.

[0029] in:

[0030] 1. Thin-walled metal slot, 2. Shell, 3. Printed circuit board, 4. Molybdenum-copper heat sink, 5. Solder sheet, 6. Micro steel ball, 7. Temperature sensor. DETAILED DESCRIPTION

[0031] The present invention will be further described below in conjunction with the accompanying drawings:

[0032] The present invention belongs to the technical field of brazing processing of microwave circuits, and specifically relates to a brazing process method for a microstrip board and a substrate (or a shell) in a microwave circuit. The method increases the ground contact area and the soldering penetration rate of a printed circuit board, improves the transmission quality of high-frequency signals, and realizes the integrated welding of a printed circuit board and a molybdenum-copper heat sink, thereby reducing the number of procedures, reducing the production cost, and improving the production efficiency.

[0033] The conventional processing method of brazing microstrip boards includes the following steps: cleaning the printed board and the housing, placing solder sheets, applying flux, installing the microstrip board, fixing the pressing block, welding, removing the pressing block, cleaning and testing. The ultra-low temperature low noise amplifier printed board brazing method described in the present invention is improved in the fixing the pressing block, welding, and removing the pressing block process.

[0034] In the implementation process of the ultra-low temperature low noise amplifier printed circuit board brazing method described in the present invention, the following materials, equipment and tools are used:

[0035] (1) Solder sheet (Sn63Pb37, thickness 0.1 mm); (2) Rosin flux; (3) Anhydrous ethanol (CH3CH2OH); (4) Dust-free cloth; (5) Laser scribing machine; (6) Vapor phase reflow furnace; (7) Micro steel ball (brand 440C, diameter 1 mm); (8) Thin-walled metal groove; (9) Vapor phase cleaning machine; (10) X-ray detector; (11) Weighing balance; (12) Ultrasonic cleaning machine; (13) Brush.

[0036] like Figure 1 As shown, the ultra-low temperature low noise amplifier printed circuit board brazing method comprises the following steps:

[0037] S1, cleaning the printed circuit board 3, the housing 2 and the solder sheet 5, applying flux on the solder sheet 5, sequentially placing the solder sheet 5 and the printed circuit board 3 into the housing 2, placing a molybdenum copper heat sink 4 into the chip hole of the printed circuit board 3, and obtaining a workpiece to be welded. The housing 2 is a housing of an extremely low temperature low noise amplifier.

[0038] The shape of the solder sheet is the same as that of the printed circuit board, and the solder sheet is prepared by a laser scribing machine.

[0039] In step S1, the cleaning of the printed circuit board 3, the housing 2 and the solder sheet 5 includes:

[0040] Use a dust-free cloth dipped in anhydrous ethanol to clean the soldering surfaces of the printed circuit board 3 and the housing 2, and use an ultrasonic cleaning machine to ultrasonically clean the solder sheet 5 to remove oil film and stains.

[0041] In step S1, the soldering flux is applied on the solder sheet 5 by using a brush. The soldering flux is rosin soldering flux, which is in liquid state.

[0042] Preferably, the printed circuit board 3 is a gold-plated copper-clad printed circuit board; the housing 2 is made of aluminum alloy with silver or gold plating; the molybdenum-copper heat sink 4 is made of molybdenum-copper alloy with gold plating on the surface. The solder sheet 5 is Sn63Pb37 with a thickness of 0.1 mm. The soldering flux is liquid rosin soldering flux.

[0043] S2. Place the workpiece to be welded in the thin-walled metal tank 1, use a weighing balance to weigh a certain weight of micro steel balls 6, slowly pour the micro steel balls 6 into the thin-walled metal tank 1, cover the workpiece to be welded, and level the top layer of micro steel balls 6. The material of the micro steel balls 6 is 440C stainless steel, the diameter of the micro steel balls 6 is 0.5-2mm, and the total weight of the micro steel balls 6 is A kg. Preferably, the value of A is 2-5kg. The wall thickness of the thin-walled metal tank 1 is 0.5mm. The material of the thin-walled metal tank 1 is iron.

[0044] S3. Place the thin-walled metal tank 1 containing the micro steel ball 6 and the workpiece to be welded into a vapor phase reflow furnace, and insert the temperature sensor 7 in the vapor phase reflow furnace into the micro steel ball 6 to collect the temperature of the workpiece to be welded, set the reflow curve, and adjust the heating power output of the vapor phase reflow furnace according to the temperature of the workpiece to be welded collected by the temperature sensor 7, so that the welding temperature in the furnace corresponds to the reflow curve, and use the vapor phase reflow furnace to braze the printed circuit board.

[0045] The reflow soldering curve is divided into four temperature stages: a preheating treatment stage, a heat preservation treatment stage, a reflow treatment stage and a cooling treatment stage.

[0046] The temperature of the preheating treatment stage is room temperature to 125°C, the heating rate is less than 2°C / S, and the preheating time is 70S to 110S. The purpose of the preheating treatment stage is to raise the temperature of the workpiece from room temperature to the active temperature required for the flux to work, and the temperature should not rise too fast.

[0047] The temperature of the insulation treatment stage is between 125°C and 160°C, the heating rate is less than 1°C / S, and the insulation time is 60S to 120S. The purpose of the insulation treatment stage is to keep the temperatures of the printed circuit board, shell and steel balls the same, reduce the relative temperature difference, and allow the flux to fully play its role, remove the oxides on the surface of the printed circuit board and shell, thereby improving the welding quality. The purpose of the insulation treatment stage is to allow the flux inside the solder sheet to fully play its role, remove the oxides on the surface of the microstrip board and substrate and discharge them in time, thereby improving the welding quality. The use of micro steel balls makes it easier to discharge the flux, so the time of this stage can be set shorter, which improves efficiency.

[0048] The peak temperature of the reflow treatment stage is 205°C to 215°C, the peak time is 10±1S, and the time above the liquidus line is 30S to 60S. The purpose of the reflow treatment stage is to raise the temperature above the melting point of the solder sheet and maintain a certain welding time to form an intermetallic compound and complete the welding of the printed circuit board and the shell.

[0049] The temperature of the cooling treatment stage is a peak temperature of 215° C. to 75° C., and the cooling rate is 5 to 6° C. / S. The purpose of the cooling treatment stage is to rapidly cool the solder and solidify the solder.

[0050] During the entire brazing process, the temperature rises from room temperature to the maximum temperature within no more than 5 minutes.

[0051] S4. After the brazing process is completed, the micro steel ball 6 is first taken out from the thin-walled metal groove 1, and then the welded workpiece is taken out, and the welding quality of the workpiece is inspected by an X-ray detector, and finally the residual flux on the surface of the workpiece is removed by a gas phase cleaning machine.

[0052] After testing, the ultra-low temperature low noise amplifier welded by the above method can achieve the following welding effects:

[0053] (1) The penetration rate of the extremely low temperature low noise amplifier after welding was tested by X-RAY inspection instrument. It was found that the penetration rate of the extremely low temperature low noise amplifier can reach about 90%, which is 15 percentage points higher than the briquetting tooling scheme, meeting the 70% penetration rate requirement for large-area printed circuit boards specified by military and aerospace products.

[0054] (2) In this embodiment, 10 amplifiers were welded using the method described in the present invention, and the penetration rate was about 90%. The voids were small and evenly distributed. It can be seen that the batch production consistency of the product was good.

[0055] (3) The present invention significantly improves welding efficiency, reduces costs, and eliminates the processes of briquetting tooling design, processing, and screw installation and removal. Compared with the briquetting tooling solution in the prior art, the efficiency can be increased by more than 3 times.

[0056] In summary, the method for brazing the ultra-low temperature low noise amplifier printed circuit board described in the present invention adopts micro steel balls for pressurization to achieve uniform and precisely controllable downward pressure, and the gas on the welding surface is easy to discharge, thereby increasing the ground contact area and brazing penetration rate of the printed circuit board and improving the quality of high-frequency signal transmission. At the same time, the present invention can realize the integrated welding of the printed circuit board and the molybdenum-copper heat sink, reducing the process steps, reducing the production cost, and improving the production efficiency. After the successful application of this process method on the ultra-low temperature microwave amplifier module, it has a strong reference and guiding role in the design, processing, and production of other microwave modules. The successful expansion of this process will greatly promote the level of microwave circuit design and manufacturing. The application of the present invention also provides technical guarantee for the research and production of high-frequency microwave circuits and complex microwave circuits.

[0057] The above-described embodiments are merely descriptions of preferred implementations of the present invention and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the scope of protection determined by the claims of the present invention.

Claims

1. A method for brazing a printed circuit board of an ultra-low temperature low noise amplifier, characterized in that: The method comprises the following steps: S1, cleaning the printed circuit board, the housing and the solder sheet, applying flux on the solder sheet, sequentially placing the solder sheet and the printed circuit board into the housing, placing a molybdenum-copper heat sink into the chip hole of the printed circuit board, and obtaining a workpiece to be welded; S2. Place the workpiece to be welded in a thin-walled metal tank, weigh a certain weight of micro steel balls and pour them into the thin-walled metal tank so that the micro steel balls cover the workpiece to be welded, and level the top layer of micro steel balls; S3, placing the thin-walled metal tank containing the micro steel ball and the workpiece to be welded into a vapor phase reflow furnace, inserting the temperature sensor in the vapor phase reflow furnace into the micro steel ball to collect the temperature of the workpiece to be welded, setting a reflow curve, and based on the reflow curve, using the vapor phase reflow furnace to braze the printed circuit board; S4. After brazing is completed, first remove the micro steel balls from the thin-walled metal groove, then take out the welded workpiece, use an X-ray detector to check the welding quality of the workpiece, and finally use a gas phase cleaning machine to remove the residual flux on the surface of the workpiece.

2. The method for soldering a printed circuit board of an ultra-low temperature low noise amplifier according to claim 1, characterized in that: In step S3, the temperature sensor is used to collect the actual temperature of the workpiece to be welded, and adjust the heating power output of the vapor phase reflow furnace according to the received actual temperature data, so that the welding temperature in the furnace corresponds to the reflow curve; In step S3, the reflow soldering curve is divided into four temperature stages: a preheating treatment stage, a heat preservation treatment stage, a reflow treatment stage and a cooling treatment stage; The temperature of the preheating treatment stage is room temperature to 125°C, the heating rate is less than 2°C / S, and the preheating time is 70S to 110S; the temperature of the insulation treatment stage is between 125°C and 160°C, the heating rate is less than 1°C / S, and the insulation time is 60S to 120S; the peak temperature of the reflux treatment stage is 205°C to 215°C, the peak time is 10±1S, and the time above the liquidus line is 30S to 60S; the temperature of the cooling treatment stage is a peak temperature of 215°C to 75°C, and the cooling rate is 5 to 6°C / S; during the entire brazing welding process, the temperature rises from room temperature to the maximum temperature within no more than 5 minutes.

3. The method for soldering a printed circuit board of an ultra-low temperature low noise amplifier according to claim 1, characterized in that: The shape of the solder sheet is the same as that of the printed circuit board, and the solder sheet is prepared by a laser scribing machine.

4. The method for soldering a printed circuit board of an ultra-low temperature low noise amplifier according to claim 1, characterized in that: In step S1, the cleaning of the printed circuit board, the housing and the solder sheet includes: Use a dust-free cloth dipped in anhydrous ethanol to clean the soldering surfaces of the printed circuit board and the shell, and use an ultrasonic cleaner to ultrasonically clean the solder sheet to remove oil film and stains.

5. The method for soldering a printed circuit board of an ultra-low temperature low noise amplifier according to claim 1, characterized in that: In step S1, the soldering flux is applied on the solder sheet using a brush.

6. The method for soldering a printed circuit board of an ultra-low temperature low noise amplifier according to claim 1, characterized in that: The printed circuit board adopts a gold-plated copper-clad printed circuit board; The shell is made of silver-plated or gold-plated aluminum alloy; The molybdenum-copper heat sink is made of a molybdenum-copper alloy with a gold-plated surface.

7. The method for soldering a printed circuit board of an ultra-low temperature low noise amplifier according to claim 1 or 3, characterized in that: The grade of the solder sheet is Sn63Pb37, and the thickness is 0.1 mm.

8. The method for soldering a printed circuit board of an ultra-low temperature low noise amplifier according to claim 1 or 5, characterized in that: The soldering flux is rosin soldering flux, and the rosin soldering flux is in liquid state.

9. The method for soldering a printed circuit board of an ultra-low temperature low noise amplifier according to claim 1, characterized in that: The material of the micro steel ball is 440C stainless steel, the diameter of the micro steel ball is 0.5-2mm, and the total weight of the micro steel ball is 2-5kg.

10. The method for soldering a printed circuit board of a very low temperature low noise amplifier according to claim 1, characterized in that: The wall thickness of the thin-walled metal groove is 0.5 mm; the material of the thin-walled metal groove is iron.

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