A method for soldering a printed circuit board for cryogenic low noise amplifiers

Abstract

The present application relates to the brazing processing technical field of microwave circuit, and particularly relates to a printed board brazing method of an extremely low temperature low noise amplifier. The method comprises the following steps: cleaning the printed board, the shell and the solder sheet, brushing the solder sheet with flux, putting the solder sheet and the printed board into the shell, putting a molybdenum copper heat sink into the chip hole of the printed board to obtain a workpiece to be welded; placing the workpiece to be welded in a thin-walled metal groove, weighing and pouring micro steel balls into the thin-walled metal groove, making the micro steel balls cover the workpiece to be welded, and flattening the uppermost micro steel ball; putting the thin-walled metal groove into a gas phase reflow furnace, setting a reflow welding curve, and brazing the printed board; after brazing, taking out the micro steel balls and the workpiece from the thin-walled metal groove in sequence, using an X-ray detector to inspect the welding quality of the workpiece, and finally using a gas phase cleaning machine to remove the residual flux on the surface of the workpiece. The present application reduces the steps of designing, processing and installing the pressing block tooling, greatly reduces the cost, and improves the production efficiency.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of brazing processing, in particular to a brazing method for printed board of cryogenic low-noise amplifier. BACKGROUND

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

[0003] For the cryogenic low-noise amplifier, the brazing penetration rate will affect its gain, and even the signal cannot be amplified and self-excitation occurs. At cryogenic temperature, due to the difference in the thermal expansion coefficient between the printed board and the shell, low-quality brazing can cause the risk of de-brazing, which seriously affects the performance indicators of the amplifier.

[0004] Currently, the common method for welding the printed board to the microwave amplifier cavity is as follows: according to the shape of the printed board, a pressing block tool is processed, and the size of the pressing block tool is appropriately reduced. The solder sheet is placed between the shell and the printed board, and the flux is brushed, then the pressing block tool is pressed on the printed board, the cover plate is covered on the pressing block tool, the long screw with a spring pad is used to lock the cover plate, the pressing block tool is provided with a downward pressure, so that the solder can spread and wet the welding surface during the welding process, and finally the whole workpiece is placed in a gas phase welding furnace for welding. The disadvantages of this brazing method are: 1) the pressing block tool needs to be processed, and if the shape of the printed board is complex, the processing cost is high and the time period is long; 2) the pressing block tool is a whole aluminum alloy flat plate processed, and the contact surface with the printed board is a flat surface, so the air holes on the printed board are blocked, and therefore the flux gas in the solder is not easy to be discharged from the grounding hole or the air hole, leaving a hollow in the welding surface, which reduces the penetration rate and leads to poor grounding effect; 3) when the cover plate screw is screwed, due to the difference between operators, the locking degree of screws in different batches is different, which will cause the difference in the downward pressure of the pressing block, affecting the batch consistency of the welding quality; 4) there are many chip holes on the printed board, and molybdenum copper heat sinks need to be installed in the chip holes, and since there is a height difference (≥0.1mm) between the molybdenum copper heat sink and the printed board, the pressing block tool cannot press the molybdenum copper heat sink, so the molybdenum copper heat sink usually needs to be adhered to these chip holes after brazing is completed, and the process is relatively complicated; 5) the whole brazing process needs to be screwed and unscrewed, which consumes manpower and time, and is low in efficiency.

[0005] Therefore, how to design a new microwave circuit brazing process method, so as to improve the grounding area of the printed board and the penetration rate, improve the high-frequency signal transmission quality, at the same time, realize the integrated welding of the printed board and the molybdenum copper heat sink, reduce the process, reduce the production cost, and improve the production efficiency, become the technical problems to be solved. SUMMARY

[0006] In order to solve the problems in the prior art, the purpose of the present application is to provide a printed board brazing method for a cryogenic low-noise amplifier.

[0007] In order to achieve the above-mentioned purpose, the following technical scheme is adopted in the present application:

[0008] A printed board brazing method for a cryogenic low-noise amplifier, the method comprising the following steps:

[0009] S1, cleaning the printed board, the shell and the solder sheet, brushing the soldering flux on the solder sheet, putting the solder sheet, the printed board into the shell in turn, putting the molybdenum copper heat sink into the chip hole of the printed board, and obtaining the workpiece to be welded;

[0010] S2, placing the workpiece to be welded in a thin-walled metal tank, weighing a certain weight of micro steel balls and pouring them into the thin-walled metal tank, so that the micro steel balls are above the workpiece to be welded, and the uppermost micro steel balls are flattened;

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

[0012] S4, after brazing, first taking out the micro steel balls from the thin-walled metal tank, then taking out the workpiece after welding, using an X-ray detector to inspect the welding quality of the workpiece, and finally using a gas phase cleaning machine to remove the residual soldering flux on the surface of the workpiece.

[0013] According to the present application, in step S3, the temperature sensor is a component of the gas phase reflow furnace, which is used to collect the actual temperature of the workpiece to be welded, and according to the received actual temperature data, the heating power output of the gas phase reflow furnace is adjusted, so that the welding temperature in the furnace corresponds to the reflow welding curve. The computer system of the gas phase reflow furnace adjusts the heating power output of the gas phase reflow furnace, so that the welding temperature in the furnace is kept near the preset reflow welding curve.

[0014] According to the present application, in step S3, the reflow welding curve is divided into four temperature stages: preheating treatment stage, holding treatment stage, reflow treatment stage and cooling treatment stage.

[0015] The temperature of the preheating stage is room temperature to 125 DEG C, the temperature increasing rate is less than 2 DEG C / S, the preheating time is 70S to 110S; the temperature of the holding stage is 125 DEG C to 160 DEG C, the temperature increasing rate is less than 1 DEG C / S, the holding time is 60S to 120S; the peak temperature of the reflow stage is 205 DEG C to 215 DEG C, the peak time is 10±1S, the time above liquidus is 30S to 60S; the temperature of the cooling stage is 215 DEG C to 75 DEG C, the cooling speed is 5 to 6 DEG C / S; during the whole soldering process, the temperature from room temperature to the highest temperature does not exceed 5 minutes. The purpose of the holding stage is to make the flux inside the soldering sheet fully play a role, remove the oxides on the surface of the microstrip board and the substrate and timely discharge, so as to improve the welding quality. The use of micro steel beads makes the flux discharge easy, so the time of this stage can be set shorter, improving the efficiency.

[0016] According to the application, preferably, the shape of the solder sheet is the same as that of the printed board, and the solder sheet is prepared by using a laser sheet cutting machine.

[0017] According to the application, preferably, in the step S1, the cleaning of the printed board, the shell and the solder sheet comprises:

[0018] The soldering surface of the printed board and the shell is cleaned by using a dust-free cloth dipped in anhydrous ethanol, and the solder sheet is ultrasonically cleaned by using an ultrasonic cleaner to remove oil film and stains.

[0019] According to the application, preferably, in the step S1, the flux is applied on the solder sheet by using a brush.

[0020] According to the application, preferably, the printed board is a gold-plated copper-clad printed board; the shell is made of aluminum alloy plated with silver or gold; and the molybdenum copper heat sink is made of molybdenum copper alloy plated with gold.

[0021] According to the application, preferably, the solder sheet is Sn63Pb37, and the thickness is 0.1mm.

[0022] According to the application, preferably, the flux is rosin flux, and the rosin flux is in liquid state.

[0023] According to the application, preferably, the micro steel bead is made of 440C stainless steel, the diameter of the micro steel bead is 0.5-2mm, and the total weight of the micro steel bead is 2-5kg.

[0024] According to the application, preferably, the wall thickness of the thin-wall metal groove is 0.5mm, and the material of the thin-wall metal groove is iron.

[0025] Compared with the prior art, the application has the following advantages:

[0026] The innovation of the brazing method for the extremely low-temperature low-noise amplifier printed board lies in that the micro steel ball is used to replace the pressing block tooling, and the brazing process of the extremely low-temperature microwave amplifier printed board can be realized with high quality. The micro steel ball is used to replace the pressing block tooling for pressing, the tiny gap between the micro steel balls makes the flux gas in the solder volatilize and discharge in time, the welding cavity is reduced, the grounding area and the brazing penetration rate of the printed board are improved, the high-frequency signal transmission quality is improved, the holding time can be set shorter, and the efficiency is improved. Since the diameter of the micro steel ball is small, and the weight can be accurately controlled, the uniformity of the pressing force and the pressure distribution of different batches of welding can be ensured during production, and the welding consistency of the amplifier batch production can be ensured. In addition, since there is a height difference (≥0.1 mm) between the surface of the printed board and the molybdenum copper heat sink, the micro steel ball can simultaneously give the printed board and the molybdenum copper heat sink a pressing force, realize the integrated welding of the printed board and the molybdenum copper heat sink, and reduce the process. Finally, considering that the amplifier is used at an extremely low temperature, the Sn63Pb37 solder sheet with good wettability, better plasticity after welding and small welding stress is selected, the solder sheet is not easy to crack at an extremely low temperature, and reliable connection at low temperature is ensured. In summary, the present application reduces the design, processing and installation of the pressing block tooling, greatly reduces the cost, improves the production efficiency, and has a strong guiding role for the design, processing and production of other microwave modules. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 The brazing method for the extremely low-temperature low-noise amplifier printed board in the present application is a flow chart;

[0028] Figure 2 The brazing assembly structure diagram of the extremely low-temperature low-noise amplifier printed board in the present application is shown.

[0029] Among them:

[0030] 1, thin-walled metal groove, 2, shell, 3, printed board, 4, molybdenum copper heat sink, 5, solder sheet, 6, micro steel ball, 7, temperature sensor. DETAILED DESCRIPTION

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

[0032] The present application belongs to the brazing processing technical field 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 improves the grounding area and the brazing penetration rate of the printed board, improves the high-frequency signal transmission quality, simultaneously realizes the integrated welding of the printed board and the molybdenum copper heat sink, reduces the process, reduces the production cost, and improves the production efficiency.

[0033] The processing method of the conventional soldering microstrip board comprises the following operation steps: cleaning the printed board and the shell, placing the solder sheet, brushing the flux, installing the microstrip board, fixing the pressing block, welding, disassembling the pressing block, cleaning and testing. The soldering method of the printed board of the ultra-low temperature low noise amplifier improves the process of fixing the pressing block, welding and disassembling the pressing block.

[0034] In the implementation process of the soldering method of the printed board of the ultra-low temperature low noise amplifier, the following materials, equipment and tools are used:

[0035] (1) solder sheet (Sn63Pb37, thickness 0.1mm); (2) rosin flux; (3) anhydrous ethanol (CH3CH2OH); (4) dust-free cloth; (5) laser scribing machine; (6) gas phase reflow furnace; (7) micro steel ball (brand 440C, diameter 1mm); (8) thin-walled metal groove; (9) gas phase cleaning machine; (10) X-ray detector; (11) weighing balance; (12) ultrasonic cleaning machine; (13) brush.

[0036] As shown in Figure 1 The soldering method of the printed board of the ultra-low temperature low noise amplifier comprises the following steps:

[0037] S1, cleaning the printed board 3, the shell 2 and the solder sheet 5, brushing the flux on the solder sheet 5, placing the solder sheet 5, the printed board 3 into the shell 2 in turn, placing the molybdenum copper heat sink 4 into the chip hole of the printed board 3, and obtaining the workpiece to be welded. The shell 2 is the shell of the ultra-low temperature low noise amplifier.

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

[0039] In step S1, the cleaning of the printed board 3, the shell 2 and the solder sheet 5 comprises:

[0040] The welding surfaces of the printed board 3 and the shell 2 are cleaned by using the dust-free cloth to dip the anhydrous ethanol, and the solder sheet 5 is ultrasonically cleaned by using the ultrasonic cleaning machine to remove the oil film and stains.

[0041] In step S1, the flux is brushed on the solder sheet 5 by using the brush, and the flux is rosin flux, which is in liquid state.

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

[0043] S2, the above to be welded workpiece is placed in the thin-walled metal groove 1, a certain weight of micro steel ball 6 is weighed by a weighing balance, the micro steel ball 6 is slowly poured into the thin-walled metal groove 1, the to-be-welded workpiece is covered, and the uppermost micro steel ball 6 is flattened. The material of the micro steel ball 6 is 440C stainless steel, the diameter of the micro steel ball 6 is 0.5-2 mm, and the total weight of the micro steel ball 6 is A kg. Preferably, the value of A is 2-5 kg. The wall thickness of the thin-walled metal groove 1 is 0.5 mm. The material of the thin-walled metal groove 1 is iron.

[0044] S3, the thin-walled metal groove 1 provided with the micro steel ball 6 and the to-be-welded workpiece is placed in a gas phase reflow furnace, the temperature sensor 7 in the gas phase reflow furnace is inserted into the micro steel ball 6 to collect the temperature of the to-be-welded workpiece, a reflow soldering curve is set, the heating power output of the gas phase reflow furnace is adjusted according to the temperature of the to-be-welded workpiece collected by the temperature sensor 7, the welding temperature in the furnace corresponds to the reflow soldering curve, and the printed board is soldered by using the gas phase reflow furnace.

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

[0046] The temperature of the preheating treatment stage is room temperature-125 DEG C, the temperature rising rate is less than 2 DEG C / S, and the preheating time is 70 S-110 S. 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 be raised too fast.

[0047] The temperature of the holding treatment stage is 125 DEG C-160 DEG C, the temperature rising rate is less than 1 DEG C / S, and the holding time is 60 S-120 S. The purpose of the holding treatment stage is to keep the temperature of the printed board, the shell and the steel ball the same, reduce the relative temperature difference, and make the flux fully work to remove the oxides on the surface of the printed board and the shell, thereby improving the welding quality. The purpose of the holding treatment stage is to make the flux in the solder sheet fully work to remove the oxides on the surface of the microstrip board and the substrate in time, thereby improving the welding quality. The use of the micro steel ball makes the flux discharge easy, so the time of this stage can be set shorter, and the efficiency is improved.

[0048] The peak temperature of the reflow treatment stage is 205 DEG C-215 DEG C, the peak time is 10±1 S, and the time above the liquidus is 30 S-60 S. The purpose of the reflow treatment stage is to raise the temperature to above the melting point of the solder sheet and maintain a certain welding time, so that the intermetallic compound is formed, and the welding of the printed board and the shell is completed.

[0049] The temperature of the cooling treatment stage is 215 DEG C-75 DEG C, and the cooling speed is 5-6 DEG 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. The welding quality of the workpiece is inspected using an X-ray detector, and finally the residual flux on the surface of the workpiece is removed using a gas phase cleaning machine.

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

[0053] (1) The penetration rate of the ultra-low temperature low noise amplifier after welding was tested by X-RAY inspection instrument, and it was found that the penetration rate of the ultra-low temperature low noise amplifier could reach about 90%, which was 15 percentage points higher than that of the pressing block tooling scheme, and met 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. The penetration rates of the 10 amplifiers were all around 90%, and the voids were small and evenly distributed. This indicates 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 scheme 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 uses micro-steel balls for pressurization to achieve uniform and precisely controllable downward pressure, and the gas on the welding surface is easily discharged, thereby increasing the ground contact area and brazing penetration of the printed circuit board and improving the quality of high-frequency signal transmission. At the same time, the present invention can achieve integrated welding of the printed circuit board and the molybdenum-copper heat sink, reducing the number of steps, reducing production costs, and improving 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 support 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 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 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. Clean the printed circuit board, the housing, and the solder sheet, apply flux on the solder sheet, place the solder sheet and the printed circuit board into the housing in sequence, place a molybdenum-copper heat sink into the chip hole of the printed circuit board, and obtain a workpiece to be welded; S2. Place the workpiece to be welded in a thin-walled metal tank, and pour a certain weight of micro steel balls 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 balls and the workpiece to be welded into a vapor phase reflow furnace, inserting a temperature sensor in the vapor phase reflow furnace into the micro steel balls to collect the temperature of the workpiece to be welded, setting a reflow profile, and brazing the printed circuit board using the vapor phase reflow furnace based on the reflow profile; 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 vapor 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 holding treatment stage is between 125°C and 160°C, the heating rate is less than 1°C / S, and the holding 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 a 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 rises from room temperature to the maximum temperature in 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, wherein: 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 solder sheet is Sn63Pb37 and has a thickness of 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 0.5-2 mm, and the total weight of the micro steel ball is 2-5 kg.

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

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