A low void rate welding method for a chip heat sink assembly

Abstract

The present invention provides a welding method for a chip heat sink assembly with a low void ratio. Laser scribing is used to locate the welding position on the casing, effectively controlling the welding precision and preventing solder overflow. The welding area of the casing is pre-tinned by heating, melting and cooling a tin-silver-copper solder sheet. The welding surface of the chip heat sink assembly is tinned. The chip heat sink assembly is placed at the pre-tinned welding position of the casing, and the welding is completed in a vacuum sintering furnace without any tooling pressure. The present invention adopts a precise tin plating method of laser scribing + solder sheet pre-melting, avoiding the overflow of solder during post-treatment, preventing the overflow of solder from contaminating the surrounding mounting positions, and ensuring the high-precision welding of the chip heat sink assembly. At the same time, the "zero pressure" welding mode is successfully applied, not only obtaining a welding effect with a very low void ratio, but also eliminating the need for customized tooling, being easy to operate, solving the problems of surface damage and contamination of bare chips such as gallium arsenide and gallium nitride, and enabling the very low void ratio, high-efficiency and low-cost welding of multi-channel power chip heat sink assemblies at one time.

Description

Technical Field

[0001] The present invention belongs to the technical field of micro-assembly processes, and particularly relates to a method for welding a eutectic bare chip, mainly a power chip, to a heat sink assembly with a low void rate. Background Art

[0002] Products such as miniaturized power amplifiers have the characteristics of high integration and high power, and the heat flux density inside the chip (semiconductor materials such as GaAs, GaN, etc.) is very high. Therefore, it is particularly important for the chip to have a good heat dissipation channel.

[0003] Considering comprehensively from aspects such as thermal analysis, multi-temperature gradient, and solder thermal conductivity, the chip is generally welded to the heat sink by AuSn eutectic welding to achieve the first-stage welding. Then, the chip heat sink assembly is welded to the metal housing to achieve the second-stage welding. In view of the high thermal conductivity requirement, the total void rate of the first-stage and second-stage welding is required to be <10%. Therefore, the second-stage solder is required to have the characteristics of high thermal conductivity and high melting point, and the chip heat sink assembly after welding needs to achieve a low void rate.

[0004] In the industry, for the bare chip heat sink assembly, manual friction or assisted tooling pressurized vacuum sintering methods are usually used for welding. The former uses manual tweezers to clamp the heat sink for friction welding. The operation process is easy to damage the chip, with low efficiency, and it is even more impossible to achieve clamping friction welding for multi-channel chip heat sink assemblies or large-size chip heat sink assemblies. The latter is the vacuum sintering welding method commonly used in the industry. It is necessary to place a solder sheet in the welding area of the housing and perform welding by means of tooling pressurization. However, both the void rate and the yield are not ideal, and using tooling is extremely easy to damage and contaminate the chip, which also has a certain impact on the stability and safety of welding. Designing special tooling will also affect production efficiency and welding consistency. Summary of the Invention

[0005] The purpose of the present invention is to solve the deficiencies of the existing technology and provide a combined welding technology of "precision pre-tinning" and "zero pressure" to achieve low void rate and high yield welding of the chip heat sink assembly.

[0006] To achieve the above-mentioned invention purpose, the present invention provides the following technical solutions:

[0007] Step 1: According to the outer dimensions of the chip heat sink assembly, use ultraviolet laser to perform laser scribing on the housing welding position to locate the welding area;

[0008] Step 2: Cut the solder sheet used for welding according to the size of the chip heat sink assembly;

[0009] Step 3: Coat the surface of the solder sheet cut in Step 2 with flux and place it in the laser scribing groove of the housing;

[0010] Step 4: Place the housing with the solder pads placed in Step 3 on a hot plate pre-set to a certain temperature for heating; after the solder pads are melted, remove the housing from the hot plate and place it on a clean operation board to cool naturally;

[0011] Step 5: After the solder on the housing in Step 4 cools, use atomized cleaning to remove the flux and scrape off the surface oxides;

[0012] Step 6: Tin the welding surface of the chip heat sink assembly;

[0013] Spot-weld solder on the gold-plated metal tooling. After the solder melts, use metal tweezers to hold the heat sink and apply downward force to move it parallelly for tinning the welding surface of the chip heat sink assembly.

[0014] Step 7: Place the chip heat sink assembly in Step 6 at the pre-tinned welding position on the housing, place the housing in a vacuum sintering furnace, and use the pre-set welding curve to perform welding directly without placing a pressurizing tooling;

[0015] Step 8: Use atomized cleaning to remove the residual flux after welding for the welded parts in Step 7.

[0016] Further, the housing can be made of 6061 or 6063 aluminum alloy material, SiAl50 silicon-aluminum material, and the surface treatment method is: nickel plating and gold plating are used at the welding place of the housing.

[0017] Further, the heat sink in the chip heat sink assembly can be made of various materials such as tungsten copper, molybdenum copper, copper molybdenum copper, silicon aluminum, diamond copper, etc., and the surface treatment method is: nickel plating and gold plating are used at the welding place of the chip heat sink assembly.

[0018] Further, the preferred material of the solder pad is: tin-silver-copper, where the mass percentage of tin is 96.5%, the mass percentage of silver is 3%, the mass percentage of copper is 0.5%, and the thickness of the solder pad is 30um - 50um.

[0019] Further, in Step 1, according to the outer shape dimensions of the chip heat sink assembly, use ultraviolet laser to perform laser scribing on the welding area of the housing, specifically as follows:

[0020] When performing laser scribing on the welding area of the housing, scribe lines extending outward from the outer shape border of the chip heat sink assembly. The width of the scribe line is 30um - 60um, and the depth of the scribe line is 5um - 8um.

[0021] Further, in Step 1, use ultraviolet laser to perform laser scribing on the welding area of the housing, specifically as follows:

[0022] The laser scribing parameters are: laser power 2W - 5W, frequency 50KHz - 70KHz, scanning speed 500mm / s - 800mm / s, and the number of repetitions 3 - 7 times.

[0023] Further, two solder pads with the same size as the heat sink assembly need to be cut in Step 3. A thin layer of soldering flux is coated on the surface of each layer of solder pads, and the coating amount of the soldering flux on the surface of the topmost solder pad is 50% - 70% of the area of the solder pad.

[0024] Further, the hot stage temperature in Step 4 is set according to the structure of the casing cavity, and the temperature at the welding position is 15°C - 40°C higher than the melting point of the solder.

[0025] Further, in Step 5, the soldering flux is atomized and cleaned with a bromopropane atomizing cleaning solution or anhydrous ethanol, and the surface oxide is mechanically scraped off in one direction with a scalpel.

[0026] Further, in Step 7, the casing with the chip heat sink assembly installed is placed in a vacuum sintering furnace for welding without placing a pressurizing tooling, specifically as follows:

[0027] The chip heat sink assembly is welded to the welding area of the casing through the vacuum sintering furnace. The heating curve is set according to the structure of the casing cavity. The welding steps are: evacuating for 120 s - filling nitrogen for 60 s - evacuating for 120 s - filling nitrogen for 60 s - 200°C - 220°C, 200 s - 260 s - 200°C - 220°C, 100 s - 150 s - 240°C - 270°C, 30 s - 60 s - evacuating, heating at 240°C - 270°C, 60 s - 150 s - filling nitrogen, 60 s - 90 s, and the temperature drops to 60°C.

[0028] Further, after the welding in Step 8 is completed, the casing is taken out of the vacuum sintering furnace, and the residual soldering flux is cleaned with a bromopropane atomizing cleaning solution.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] (1) The present invention innovatively uses the laser scribing precise pre-tinning technology to accurately control the solder amount and overflow effect in the welding process of the chip heat sink assembly, avoiding the later manual cleaning of the overflowed solder after welding, and ensuring the welding requirements of narrow gaps and high precision;

[0031] (2) The present invention innovatively proposes and successfully applies the "zero pressure" welding mode, without the need to customize tooling, with high assembly efficiency, effectively reducing the tooling design cost, and also solving the problems of surface damage and contamination caused by pressurizing the bare chip;

[0032] (3) The present invention has good welding effect, good process consistency and high reliability. The welding method of "precise pre-tinning + zero pressure" obtains a welding effect with a low void rate of less than 5%, and the finished product rate is stably controlled above 95%;

[0033] (4) The present invention has strong versatility and operability, requires less manual intervention, can meet the welding of chip heat sink components for most power amplification products, does not require custom-made special tooling, is convenient to operate, and has high welding efficiency;

[0034] (5) The present invention can achieve high welding precision and is suitable for the assembly of power products with bare chips having a narrow mounting gap and high assembly precision.

[0035] (6) The present invention has high process adaptability and can achieve stable welding effects for products with different structural forms and multi-channel chip heat sink components with different quantities, and the process adaptability is extremely high. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 is a flowchart of a low void ratio welding method for a chip heat sink component of the present invention;

[0037] Figure 2 is a welding sectional view of the chip heat sink component welded on the casing of the present invention;

[0038] Figure 3 is a schematic diagram of the welding of the chip heat sink component of the present invention;

[0039] Figure 4 is a void ratio diagram of the welding of the chip heat sink component of the present invention;

[0040] In the drawings, each reference numeral represents:

[0041] 1, casing; 2, SAC305 solder sheet; 3, heat sink; 4, Au 80 Sn 20 solder sheet; 5, bare chip; 6, mounting gap; 7, welding void. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0042] To make the technical problems, technical solutions and advantages to be solved by the present invention clearer, the present invention will be further described below with reference to the accompanying drawings and specific embodiments. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention and cannot be used to limit the protection scope of the present invention.

[0043] As Figure 1 and Figure 2 shown, it is a low void ratio welding method for a chip heat sink component provided by an embodiment of the present invention. The bare chip 5 is pre-welded on the heat sink 3 with an Au 80 Sn 20 solder sheet 4 to form a chip heat sink component, and the chip heat sink component is welded on the casing 1 with an SAC305 solder sheet 2.

[0044] S1: For the housing 1 involved in the present invention, the materials selected are 6061 or 6063 aluminum alloy materials, SiAl50 silicon-aluminum materials, and the surface treatment method is nickel-gold plating. The nickel layer thickness is about 4 μm, and the gold layer thickness is 0.5 μm - 1 μm. For the heat sink in the chip heat sink assembly involved, various materials such as tungsten copper, molybdenum copper, copper molybdenum copper, silicon aluminum, diamond copper, etc. are used. The surface treatment method for the welding part is nickel-gold plating. The nickel layer thickness is 2 μm - 5 μm, the gold layer thickness is about 1.2 μm - 5.7 μm, and the heat sink thickness is about 0.25 mm. The chip uses gallium arsenide or gallium nitride chips, with a thickness of about 0.08 - 0.10 mm.

[0045] S2: For the primary welding, i.e., the solder for welding the chip on the heat sink, gold tin is selected. Among them, the mass percentage of gold is 80%, and the mass percentage of tin is 20%. The thickness of the solder sheet is 30 μm - 50 μm. For the secondary welding, i.e., the solder for welding the chip heat sink assembly on the housing, tin-silver-copper is selected. Among them, the mass percentage of tin is 96.5%, the mass percentage of silver is 3%, and the mass percentage of copper is 0.5%. The thickness of the solder sheet is 30 μm - 50 μm. According to the application temperature of the materials subsequently loaded during product assembly, solder sheets with other melting points can also be selected, such as Sn-3.5Ag, Pb37Sn63, In80Pb15Ag5, etc.

[0046] S3: To ensure high-precision welding, effectively prevent solder overflow during welding from affecting other mounting positions, and at the same time to precisely control the solder volume, in this embodiment, ultraviolet laser is used to perform laser scribing on the chip heat sink assembly before welding, and laser scribing is performed on the welding edge area of the chip heat sink assembly on the housing. The scribing is extended outward from the outer shape border of the chip heat sink assembly. The scribing width is 30 μm - 60 μm, and the scribing depth is 5 μm - 8 μm. The nickel layer and the gold layer need to be etched away to expose the housing body material. The laser scribing parameters are: laser power 2W - 5W, frequency 50KHz - 70KHz, scanning speed 500mm / s - 800mm / s, and the number of repetitions 3 - 7 times.

[0047] S4: Use a scalpel or a single-sided blade to cut the SAC305 solder sheet (two layers of solder sheets are required in this example) on a clean metal tooling. The size is the same as that of the chip heat sink assembly to be welded, and wipe it with a water-ethanol non-woven fabric and let it dry naturally.

[0048] S5: Place the housing under a microscope, use a small brush to dip the flux, and apply a thin layer of flux to the welding part in the laser scribed groove on the package. The flux should be evenly coated, and the area is equivalent to or slightly smaller than the heat sink area. Then place the SAC305 solder sheet and gently press it with forceps to ensure that the solder sheet is in close contact with the housing. Then, using the same method and requirements, apply a small amount of flux to the 50% - 70% area in the middle of the upper solder sheet.

[0049] S6: Set the temperature of the hot stage in advance so that the temperature of the welding part is 235°C to 250°C. Transfer the casing processed in S3 to the hot stage. After the solder is completely melted, remove it from the hot stage and place it on a clean operation board to cool. When removing the casing, it should be taken away from the welding hot stage horizontally to prevent the solder from tilting.

[0050] S7: After the casing cools, use bromopropane cleaning fluid to atomize and clean the welded parts to remove the flux; or use anhydrous ethanol solvent to clean the flux. After the solvent cleaning, use a scalpel to scrape off the oxide generated during the melting of the solder on the surface of the solder in one direction.

[0051] S8: Tin the welding surface of the chip heat sink assembly as follows: Set the temperature of the hot stage in advance to 240°C. Place the tinning metal tooling in the vacuum hole area of the welding hot stage. Coat the SAC305 solder sheet on the tooling. After the solder sheet melts, use tweezers to remove the oxide to expose the shiny solder. Hold the heat sink away from the chip end with tweezers, ensure that the heat sink is parallel to the tinning tooling, apply a downward force to the heat sink to move and tin it. The tinning time is 2 - 3s. When the heat sink is removed from the tinning tooling after tinning, it is necessary to ensure that the heat sink is parallel to the tinning tooling and apply a certain force to the heat sink to avoid too thick tinning on the back of the heat sink; after the heat sink is tinned, place it on a clean operation board and cool it to room temperature.

[0052] S9: Apply a thin layer of flux to the pre-tinned part in the laser engraved groove of the casing with a small brush. Place the chip heat sink assembly tinned in step S8 in the welding area, and then place the casing in a vacuum sintering furnace and perform welding using the pre-set SAC305 welding curve. The specific curve settings are as follows: Vacuum pumping for 120s - Nitrogen filling for 60s - Vacuum pumping for 120s - Nitrogen filling for 60s - Heating at 200°C - 220°C for 200s - 260s - Heating at 200°C - 220°C for 100s - 150s - Heating at 240°C - 270°C for 30s - 60s - Vacuum pumping, heating at 240°C - 270°C for 60s - 150s - Nitrogen filling for 60s - 90s, and the temperature drops to 60°C.

[0053] S10: Take out the casing from the vacuum sintering furnace and use bromopropane atomization to clean the remaining flux. The atomization cleaning conditions are: The parameters are steam cleaning for 60 - 120s, spray cleaning for 30 - 60s, and rinsing for 30 - 60s.

[0054] A low void ratio welding method for a chip heat sink assembly proposed by the present invention innovatively uses precise pre-tinning technology to accurately control the solder volume and overflow effect during the welding process of the chip heat sink assembly, avoiding the need for manual cleaning of the excess solder overflowed later, ensuring the welding accuracy of narrow gaps and high precision; breaking the commonly used pressure welding method in the industry, innovatively proposing and successfully applying the "zero pressure" welding mode, without the need for customized tooling, with high assembly efficiency, effectively reducing the tooling design cost, and also solving the problems of surface damage and contamination caused by pressurizing bare chips; the welding method of the present invention using "precise pre-tinning + zero pressure" has good process consistency, high reliability, good welding effect, meets the welding requirements of narrow pitch and high precision, obtains a low void ratio welding effect of less than 5%, and the physical welding schematic diagram is shown in Figure 3 as shown, the mounting gap is about 50um, and the X-ray photo shows ( Figure 4 as shown) almost no obvious welding voids, and the yield rate is stably controlled above 95%, effectively ensuring the high heat dissipation requirements of power chips. This method has been successfully applied to the development of multiple power amplifier products.

[0055] The present invention has been described in detail above in combination with specific embodiments and exemplary examples, but these descriptions should not be construed as limitations on the present invention. Those skilled in the art understand that without departing from the spirit and scope of the present invention, various equivalent substitutions, modifications or improvements can be made to the technical solutions and their implementation manners of the present invention, and these all fall within the scope of the present invention. The protection scope of the present invention is subject to the appended claims.

[0056] The content not detailedly described in the specification of the present invention belongs to the well-known technology of those skilled in the art.

Claims

1. A low void ratio welding method for a chip heat sink assembly, characterized in that it includes: Using laser scribing to position the welding area on the casing according to the size of the chip heat sink assembly; Cutting the solder sheet according to the size of the chip heat sink assembly; Coating the surface of the solder sheet with flux and placing it in the scribed groove of the casing; Placing the casing on a hot stage with a pre-set temperature for heating; after the solder sheet melts, remove the casing from the hot stage; After the solder on the casing cools, clean the flux and scrape off the surface oxide to complete the pre-tinning treatment of the welding area of the casing; Performing tinning treatment on the welding surface of the chip heat sink assembly; Coating the pre-tinned welding area of the casing with flux, placing the tinned chip heat sink assembly on the pre-tinned welding area of the casing, and placing the casing in a vacuum sintering furnace for welding without applying pressure using a pre-set welding curve; Atomizing and cleaning the remaining flux after welding; The chip heat sink assembly is a bare chip welded to a heat sink carrier using a gold-tin solder, where the bare chip is a gallium arsenide chip or a gallium nitride chip; Using ultraviolet laser to scribe the welding part of the casing to obtain a scribed groove; specifically as follows: the scribed area of the casing is the outer contour of the welding area of the chip heat sink assembly, cutting along the outside of the contour with a width of 30 - 60um and a cutting depth of etching off the gold layer and nickel layer to expose the body material of the casing; Performing tinning treatment on the welding surface of the chip heat sink assembly, specifically as follows: spot-welding solder on a gold-plated metal tooling, after the solder melts, use metal tweezers to hold the heat sink and apply downward force to move it parallelly, with a tinning time of 2s - 3s, and after tinning is completed, place it on a clean operating board to cool down; The welding steps corresponding to the pre-set welding curve include: pre-treatment - preheating - heat preservation - heating up - welding - cooling process.

2. A low void ratio welding method for a chip heat sink assembly according to claim 1, characterized in that the surfaces of the welding areas of the casing and the chip heat sink assembly are nickel-plated and gold-plated.

3. A low void ratio welding method for a chip heat sink assembly according to claim 1, characterized in that the solder sheet is a tin-silver-copper solder, and the thickness of the solder sheet is 30um - 50um.

4. A low void ratio welding method for a chip heat sink assembly according to claim 1, characterized in that the laser scribing parameters are: laser power 2W - 5W, frequency 50KHz - 70KHz, scanning speed 500mm / s - 800mm / s, and the number of repetitions 3 - 7 times.

5. A low void ratio welding method for a chip heat sink assembly according to claim 1, characterized in that one chip heat sink assembly needs to cut a solder sheet of the same size as the heat sink assembly; the number of solder sheets is determined according to the required solder amount; when there are multiple layers of solder sheets, flux is coated on the surface of each layer of solder sheet, and the coating amount of flux on the surface of the top layer of solder sheet is 50% - 70% of the solder sheet area.

6. A low void ratio welding method for a chip heat sink assembly according to claim 1, characterized in that After observing under a microscope that the solder sheet on the welding area of the casing on the hot stage has melted, immediately remove the casing from the hot stage and place it on a clean operating board to cool down.

7. A low void ratio welding method for a chip heat sink assembly according to claim 1, characterized in that The pretreatment process includes vacuum pumping and nitrogen filling in a cycle of not less than 2 times to purify the atmosphere in the purification furnace and reduce the oxygen content.

8. A method for welding a chip heat sink assembly with a low void rate according to claim 1, characterized in that, during the welding stage of the welding curve, vacuum pumping needs to be set, and the actual welding temperature should be 15°C to 40°C higher than the solder melting point, and the welding time above the melting point should be ensured to be 60s to 150s.

9. A method for welding a chip heat sink assembly with a low void rate according to claim 1, characterized in that, the curve settings for the preheating-holding-temperature rising stage of the welding curve are: 200°C to 220°C, 200s to 260s - 200°C to 220°C, 100s to 150s - 240°C to 270°C, 30s to 60s. Set the curve according to the housing structure, and it is necessary to ensure that the temperature at the welding point reaches the required value of the actual welding temperature after the temperature rising stage ends.

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