Laser bar packaging structure and welding fixture and method

Abstract

The present invention provides a laser bar packaging structure and a welding fixture and method. The welding fixture includes: an upper pressing plate and a lower pressing plate aligned in parallel, the upper pressing plate and the lower pressing plate are locked by a screw shaft perpendicular to the upper and lower pressing plates, the lower pressing plate is provided with a narrow groove for accommodating packaging materials, the surface of the upper pressing plate opposite to the narrow groove is provided with a balanced clamping structure, the upper pressing plate and the lower pressing plate have the same area and material, and the thermal expansion coefficient of the screw shaft is smaller than the thermal expansion coefficient of the upper pressing plate and the lower pressing plate. The present invention uses a specially designed welding fixture combined with a low-temperature welding method to weld two heat sinks and laser bars together, thereby reducing the thermal stress caused by high-temperature welding.

Description

Technical Field

[0001] The present invention relates to the field of laser bar packaging technology, and in particular to a laser bar packaging structure and a welding fixture and method. Background Art

[0002] In recent years, high-power semiconductor lasers have been widely used in laser pumping, medical / cosmetics, display lighting, industrial processing and other fields. The light-emitting chips of high-power semiconductor lasers are mostly in the form of bars. The typical size of a laser bar is 10×1×0.11 cubic millimeters in the three directions of length×width×thickness. One of the two sides of a laser bar perpendicular to the thickness is the light-emitting surface. The two sides of a laser bar perpendicular to the width are the P pole and the N pole. The surface of the P pole has a texture structure, which corresponds to the division and arrangement of nearly 20 light-emitting areas; the N pole is a plane, which is equivalent to the short circuit of the N pole of nearly 20 light-emitting areas. There is a gold-plated layer on the surface of the P pole and the N pole. The electrical power of a laser bar is more than 100 watts (the driving current is about 50A and the voltage is about 2V). Given that the luminous efficiency of the chip is about 50%, about 50% of the electrical power is converted into thermal power, that is, 50 watts of heat. If the heat cannot be effectively transferred from such a small chip volume, the temperature of the chip will rise rapidly, which will have a direct and fatal impact on the luminous efficiency and life of the chip.

[0003] In order to provide a current of about 50A to the laser bar of the above typical size and transmit about 50 watts of heat, two heat sinks sandwich a laser bar in the middle to form a sandwich structure. The parts of the laser bar and the heat sink that contact each other are the P pole and N pole of the laser bar respectively. The surface of the P pole has a texture structure, which corresponds to the division and arrangement of nearly 20 light-emitting areas; the N pole is a plane, which is equivalent to the short circuit of the N pole of nearly 20 light-emitting areas. There is a gold-plated layer on the surface of the P pole and the N pole. One of the two sides of a laser bar perpendicular to the thickness is the light-emitting surface, such as Figure 1 As shown. If the thermal expansion coefficients of the laser bar and the heat sink materials do not match, thermal stress will be generated between the laser bar and the heat sink after high-temperature welding and cooling to room temperature. This thermal stress may cause mechanical damage to the laser bar, especially when the bar is a flat long strip, which is prone to cracks or even brittle fracture.

[0004] Many low-temperature soldering alloys currently used for optoelectronic component welding, such as the indium-bismuth-tin alloy system, have melting points between 110-230°C, but these alloys have a thick oxide layer on the surface, and chemical flux is generally required in the welding process. For laser bars, the residual chemical flux after the welding process may cause a short circuit or breakdown of the laser chip under high current driving. Summary of the invention

[0005] In response to the above technical problems, the present invention provides a laser bar packaging structure and a welding fixture and method, which uses a specially designed welding fixture combined with a low-temperature welding method to weld two heat sinks and laser bars together, thereby reducing the thermal stress caused by high-temperature welding.

[0006] In order to achieve the technical purpose of the present invention, the technical solution adopted by the present invention is:

[0007] The present invention provides a welding fixture for a laser bar packaging structure, comprising: an upper pressing plate and a lower pressing plate aligned in parallel, the upper pressing plate and the lower pressing plate are locked by a screw shaft perpendicular to the upper and lower pressing plates, the lower pressing plate is provided with a narrow groove for accommodating packaging materials, the surface of the upper pressing plate opposite to the narrow groove is provided with a balanced clamping structure, the upper pressing plate and the lower pressing plate have the same area and material, and the thermal expansion coefficient of the screw shaft is smaller than the thermal expansion coefficient of the upper pressing plate and the lower pressing plate.

[0008] Preferably, the screw shaft is arranged at both ends of the upper pressing plate and the lower pressing plate.

[0009] More preferably, the screw shaft includes a Kovar screw that passes through the upper pressing plate and the lower pressing plate, and a Kovar nut that is located above the upper pressing plate and matches the Kovar screw.

[0010] Preferably, the balanced compression structure includes but is not limited to an interconnected spring array structure or a two-layer steel ball structure, and the interconnected spring array structure includes but is not limited to an interconnected multi-circular ring structure or a multi-diamond structure.

[0011] Preferably, the packaging material includes a heat sink and a laser bar.

[0012] The present invention also provides a welding method for a laser bar packaging structure, which is achieved by clamping the packaging material in the middle of the above-mentioned welding fixture and performing low-temperature welding (110-180°C), wherein the packaging material includes a heat sink and a laser bar, and the welding surface of the heat sink is plated with a welding alloy or a welding sheet of the welding alloy is added between the heat sink and the laser bar.

[0013] Preferably, the material of the heat sink includes but is not limited to tungsten-copper alloy, with the tungsten content being between 70% and 90% (by weight), more preferably 80-90%; or molybdenum-copper alloy, with the molybdenum content being between 60-90% (by weight), more preferably 85-90%; or a ternary alloy of tungsten-molybdenum-copper; or a laminated structure of copper sheets and molybdenum sheets.

[0014] Preferably, the welding surface of the heat sink is plated with precious metals that are easy to weld, and the precious metals include but are not limited to gold, platinum, palladium, and silver; the thickness of the coating is generally about 0.5-1.0 μm; the coating can be made by electroplating, chemical plating or vacuum plating; or the welding surface of the heat sink is plated with a low-temperature welding alloy, and the low-temperature welding alloy can be an alloy of indium (In), bismuth (Bi), and tin (Sn), with a melting point between 119-232°C; the low-temperature welding alloy can be plated on the welding surface of the heat sink by vacuum plating, and the method of plating each metal in turn can be adopted; the total thickness of the coating is greater than 3 μm, preferably 5-10 μm.

[0015] Preferably, the welding sheet of the welding alloy can be pressed, formed and cut after melting the alloy; the thickness of the welding sheet is controlled between 5-20 μm, preferably 10 μm; and the plane size of the welding sheet is smaller than the welding surface of the laser bar.

[0016] The present invention further provides a laser bar packaging structure obtained by the above method.

[0017] The beneficial effects of the present invention are:

[0018] The present invention designs a welding fixture for a laser bar packaging structure, and the welding fixture has two characteristics. One is that the pressure between the fixtures increases with the increase of temperature; the other is that the fixture surface has a balanced pressing structure, which can automatically adjust the pressure between the fixtures to be uniform; combined with the fixture, the present invention completes the welding of the laser bar and the heat sink under low temperature and flux-free conditions, thereby reducing the thermal stress generated by high-temperature welding, and the formed packaging structure welding interface is uniform along the length direction of the bar without cold solder joints; due to the low-temperature welding, the thermal stress between the chip and the bar is low, there is no chip damage or crack phenomenon, the yield of the produced products is high, and it has broad application prospects. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 It is the packaging structure of the laser bar.

[0020] Figure 2 The present invention is a schematic diagram of the structure of a welding fixture for a laser bar packaging structure, wherein: 101, heat sink, 102, welding piece, 103, laser bar, 104, welding piece, 105, heat sink, 106, balanced clamping structure, 107, Kovar alloy nut, 108, Kovar alloy screw, 109, guide positioning pin, 110, upper pressure plate, 111, lower pressure plate.

[0021] Figure 3 It is a schematic diagram of a multi-ring structure connected to each other in the welding fixture of the present invention.

[0022] Figure 4 It is a schematic diagram of the balanced compression principle of a multi-ring structure connected to each other. DETAILED DESCRIPTION

[0023] In order to explain the present invention more clearly, the present invention is further described in detail below in conjunction with embodiments and with reference to the accompanying drawings. It should be understood by those skilled in the art that the content described below is illustrative rather than restrictive, and should not be used to limit the scope of protection of the present invention.

[0024] The structure of a welding fixture for a laser bar packaging structure provided by the present invention is as follows: Figure 2 As shown, it includes: an upper pressing plate 110 and a lower pressing plate 111 aligned in parallel, the upper pressing plate 110 and the lower pressing plate 111 can be aligned through a guide positioning pin 109, and locked through a screw shaft perpendicular to the upper and lower pressing plates, the screw shaft is arranged at both ends of the upper pressing plate 110 and the lower pressing plate 111, including a Kovar alloy screw 108 penetrating the upper pressing plate 110 and the lower pressing plate 111, and a Kovar alloy nut 107 located above the upper pressing plate 110 and matching with the Kovar alloy screw 108. A narrow groove for accommodating packaging materials (including heat sinks and laser bars) is provided on the lower pressing plate 111, and a balanced clamping structure 106 is provided on the surface of the upper pressing plate 110 opposite to the narrow groove.

[0025] The welding fixture has three functions: first, it facilitates the alignment of the heat sink / solder pad / laser bar / solder pad / heat sink; second, it pressurizes the layered structure of the heat sink / solder pad / laser bar / solder pad / heat sink during the welding process; third, it adjusts the uneven pressure caused by the inconsistent thickness of the heat sink during the welding process.

[0026] The first function of the welding fixture, namely the alignment function, is completed by cutting grooves, and the heat sink, solder piece and laser bar are placed in a narrow groove with slightly larger size.

[0027] The second function of the welding fixture is to apply pressure to the welding piece when it melts. At this time, the inside of the welding piece has melted, but the oxide layer on the outer surface has not melted, blocking the contact between the molten welding alloy and the laser bar welding surface. At this time, just a little pressure can break the solid oxide layer on the surface, and the molten internal alloy reacts with the gold plating on the laser bar welding surface to complete the welding. It can be seen that the pressure at the melting moment is crucial to the chemical reaction of low-temperature flux-free welding.

[0028] We have also tried to use springs or screws to tighten the pressure, but the results were not good. The pressure of the spring gradually decreases as the temperature rises; the pressure of the screw tightening method also gradually decreases as the temperature rises. If the initial pressure at room temperature is too high, it will cause mechanical damage to the chip.

[0029] Based on this, we designed a special welding fixture. The initial pressure at room temperature is not high, but as the temperature increases, the pressure gradually increases.

[0030] Figure 2 In the structure, the upper pressing plate 110 and the lower pressing plate 111 have the same area and material, and the thermal expansion coefficient of the screw shaft is smaller than that of the upper pressing plate 110 and the lower pressing plate 111. Therefore, when the temperature rises, the upper and lower pressing plates expand more than the screw shaft, and the screw shaft has been locked, resulting in pressure on the heat sink / solder sheet / laser bar / solder sheet / heat sink structure in the middle of the fixture.

[0031] The calculation formula of the clamp pressure F is as follows:

[0032] F≈ΔαΔT S 螺 E 螺 Formula 1

[0033] Where Δα is the difference in thermal expansion coefficient between the upper platen / lower platen and the Kovar alloy screw, ΔT is the difference between the melting point of the solder and the ambient temperature, S 螺 and E 螺 are the cross-sectional area and Young's modulus of Kovar alloy screw.

[0034] The third function of the fixture is to adjust and balance the uneven pressure caused by inconsistent heat sink thickness. Figure 3 As shown, there is a multi-ring structure connected to each other on the surface of the upper pressure plate as a balanced pressing structure. This structure is similar to a spring array structure connected to each other. The balanced pressing principle is as follows: Figure 4 As shown:

[0035] When points 1 and 2 in the figure are subjected to forces in the direction of the arrows, the circular rings at points 1 and 2 are deformed, and the forces at points 3 and 4 are used to squeeze the middle circular ring, causing it to deform toward point 5, resulting in a displacement of 6, and achieving balanced pressure. Due to the local unevenness of the heat sink, the pressure at a certain point is higher than that at other adjacent points, and the circular ring at the high pressure point is flattened and deformed, squeezing the adjacent circular rings to make them rise, thereby achieving a pressure balance.

[0036] In addition to the interconnected multi-ring structure, there are similar structures that can achieve the same effect, such as an interconnected multi-rhombus structure, or a two-layer steel ball structure.

[0037] The present invention provides a welding method for a laser bar packaging structure, which is achieved by clamping the packaging material in the middle of the above-mentioned welding fixture and performing low-temperature welding, and the packaging material mainly includes a heat sink and a laser bar.

[0038] The heat sink material must be a metal with good thermal and electrical conductivity, and its thermal expansion coefficient must match the material of the laser bar. The thermal performance parameters of some related materials are shown in Table 1:

[0039] Table 1. Thermal properties of relevant materials

[0040]

[0041] The best thermally conductive metals in the table are copper and silver, but their thermal expansion coefficients do not match the laser material AlGaAs. The material that best matches the thermal expansion coefficient of AlGaAs is Kovar, but its thermal conductivity is not high. The thermal conductivity and thermal expansion coefficients that are close to those of laser chip materials are tungsten and molybdenum, but the mechanical processing performance of these two materials is not good, and the material price is much higher than copper.

[0042] In the present invention, the material of the heat sink can be a tungsten copper alloy, and the tungsten content is between 70% and 90% (by weight), and more preferably 80-90%. The thermal expansion coefficient and thermal conductivity of the tungsten copper alloy are listed in Table 2:

[0043] Table 2. Thermal properties of tungsten copper alloy (selected from Zhuzhou Jiabang Refractory Metal Co., Ltd.)

[0044]

[0045] The heat sink may also be a molybdenum-copper alloy, with the molybdenum content being between 60-90% (by weight), more preferably between 85-90%. The thermal expansion coefficient and thermal conductivity of the molybdenum-copper alloy are shown in Table 3:

[0046] Table 3. Thermal properties of molybdenum-copper alloys (selected from Antai Technology Co., Ltd.)

[0047]

[0048] The heat sink may also be a ternary alloy of tungsten-molybdenum-copper; or may be a laminated structure of a copper sheet and a molybdenum sheet.

[0049] The welding surface of the heat sink and the laser bar can be in two forms: (1) coated with precious metals that are easy to weld, such as gold, platinum, palladium, and silver. The thickness of the coating is generally about 0.5-1.0μm. The coating can be made by electroplating, chemical plating, or vacuum plating. (2) Coated with low-temperature welding alloys with a thickness greater than 3μm.

[0050] The solder for welding the heat sink and the laser bar uses a low-temperature alloy. The low-temperature alloy can be an alloy of indium (In), bismuth (Bi), and tin (Sn), with a melting point between 119°C and 232°C. Commonly used low-temperature alloys generally contain lead, which is not considered here because it is considered to have a damaging effect on the human nervous system.

[0051] The eutectic composition of tin and indium is 52In-48Sn (wt%), and the eutectic melting point is 117°C. The eutectic composition of tin and bismuth is 58Bi-42Sn (wt%), and the eutectic melting point is 138°C. These two low-temperature soldering alloys have been used in electronic packaging soldering materials for more than 50 years. 52In-48Sn is relatively soft and has excellent ductility; 58Bi-42Sn is relatively hard, with high strength but low ductility. Based on 58Bi-42Sn, a small amount of indium is added; or based on 52In-48Sn, a small amount of bismuth is added, thus forming an infinite number of alloy compositions. In addition, trace amounts of other elements, especially trace amounts of silver, can improve the strength and ductility of the material. Table 4 lists some alloy compositions reported in the literature, as well as the corresponding mechanical properties:

[0052] Table 4. Composition and mechanical properties of some low temperature soldering alloys

[0053]

[0054] The welding alloy can be plated on the welding surface of the heat sink by vacuum plating. The method of plating each metal in turn can be adopted. For example, 40Sn-55Bi-5In can be plated with Sn, Bi, and finally In in turn to form a layered structure of Sn / Bi / In. During the welding process, the three metals will melt, mix, and form an alloy. Sn / Bi / In can also be repeatedly superimposed, so that the thickness of each layer is reduced, and the time for melting and mixing to form an alloy is correspondingly shortened. The total thickness of the coating should be greater than 3μm, preferably 5-10μm.

[0055] The welding alloy can also be sandwiched between the laser and the heat sink by means of a welding sheet. The welding sheet of the alloy can be pressed into shape and cut after melting the alloy. The thickness of the welding sheet should be controlled between 5-20μm, preferably 10μm. The plane size of the welding sheet should be slightly smaller than the welding surface of the laser bar to avoid excessive solder flow causing a short circuit between the P pole and the N pole.

[0056] In the present application, the low-temperature soldering alloy replaces the gold-tin alloy, which greatly reduces the thermal stress in the soldering process. The reasons why thermal stress is generated in welding are as follows: at the melting point of the solder, the solder becomes liquid and has no stress; after the soldering is completed, the laser bar and the heat sink are integrated; in the process of dropping from the melting point temperature to room temperature, due to the difference in thermal expansion coefficient between the laser bar material AlGaAs and the heat sink, stress is generated at the junction of the bar and the heat sink. This thermal stress may cause mechanical damage to the chip. The melting point of the gold-tin alloy is 280°C, and the temperature of the low-temperature soldering alloy is, for example, 140°C. The temperature difference for the thermal stress generated by the gold-tin alloy is (280°C-25°C)=255°C. The temperature difference for the thermal stress generated by the low-temperature soldering alloy is (140°C-25°C)=115°C. The thermal stress generated by the low-temperature soldering alloy is only 45% of the thermal stress generated by the gold-tin alloy.

[0057] In addition, the elastic modulus and yield strength of low-temperature soldering alloys, especially indium alloys, are much lower than those of eutectic gold-tin alloys, and thermal stress can be released through plastic deformation. In addition, low-temperature alloys will creep at room temperature, which can also produce plastic deformation to release thermal stress.

[0058] Example

[0059] The laser bar adopts Huaguang Optoelectronics' 808-nanometer laser bar with a size of 10×1×0.11 square millimeters. Each bar has 19 light-emitting areas, with a normal driving current of 50A and a voltage of 2.1V. The heat sink adopts 90W-10Cu tungsten-copper alloy, whose thermal expansion coefficient is 6.5e-6 / K, which is only 0.7e-6 / K larger than the thermal expansion coefficient of the AlGaAs material of the laser bar of 5.8e-6 / K. The surface of the heat sink is plated with chemically plated nickel and chemically plated gold, with a coating thickness of 5μm for nickel and 0.5μm for gold. In this embodiment, the indium-tin eutectic alloy of 52In-48Sn is used, and its melting point is 117°C.

[0060] The upper and lower clamps of the welding fixture are similar in length and width, 6mm wide and 25mm long. The upper clamp has a spring structure and is about 25mm thick, while the lower clamp is 20mm thick. The clamp material is selected from Mg-containing aluminum alloy, whose thermal expansion coefficient is about 20E-6 / ℃. The screw material of the fixture is 304L stainless steel, whose thermal expansion coefficient is about 16E-6 / ℃. Through this welding fixture, the welding of laser bars and heat sinks can be completed at low temperature and without flux, thereby reducing the thermal stress generated by high-temperature welding. The formed packaging structure welding interface is uniform along the length of the bar, without cold welding; due to the low-temperature welding, the thermal stress between the chip and the bar is low, there is no chip damage or crack phenomenon, and the yield of the production product is high.

[0061] Obviously, the above embodiments of the present invention are merely examples to more clearly illustrate the present invention, and are not limitations on the implementation methods of the present invention. For ordinary technicians in the relevant field, other different forms of changes or modifications can be made on the basis of the above description. It is impossible to list all the implementation methods here. All obvious changes or modifications derived from the technical solution of the present invention are still within the protection scope of the present invention.

Claims

1. A welding method for a laser bar packaging structure, which is achieved by clamping the packaging material in the middle of a welding fixture and performing low-temperature welding, wherein the welding fixture comprises: The upper pressing plate and the lower pressing plate are aligned in parallel, and the upper pressing plate and the lower pressing plate are locked by a screw shaft perpendicular to the upper and lower pressing plates, and the screw shaft is arranged at both ends of the upper pressing plate and the lower pressing plate, and the screw shaft includes a Kovar alloy screw that passes through the upper pressing plate and the lower pressing plate, and a Kovar alloy nut that is located above the upper pressing plate and matches the Kovar alloy screw. The lower pressing plate is provided with a narrow groove for accommodating packaging materials, and the surface of the upper pressing plate opposite to the narrow groove is provided with a balanced clamping structure, and the balanced clamping structure includes a spring array structure or a two-layer steel ball structure that are connected to each other, and the spring array structure that is connected to each other includes a multi-ring structure or a multi-rhombus structure that is connected to each other. The product has the same area and material, the thermal expansion coefficient of the screw shaft is smaller than the thermal expansion coefficient of the upper pressing plate and the lower pressing plate, the packaging material includes a heat sink and a laser bar, the material of the heat sink includes a tungsten-copper alloy, the tungsten content is between 70% and 90%; or a molybdenum-copper alloy, the molybdenum content is between 60% and 90%; or a three-element alloy of tungsten-molybdenum-copper; or a laminated structure of copper sheets and molybdenum sheets, the welding surface of the heat sink is plated with a welding alloy or a welding sheet of a welding alloy is added between the heat sink and the laser bar, the welding solder of the heat sink and the laser bar adopts a low-temperature alloy, the low-temperature alloy is an alloy of indium, bismuth and tin, and the melting point is between 119°C and 232°C; the temperature of the low-temperature welding is 110-180°C.

2. A welding method for a laser bar packaging structure according to claim 1, characterized in that: The welding surface of the heat sink is plated with a precious metal that is easy to weld, and the precious metal includes gold, platinum, palladium or silver; the thickness of the coating is 0.5-1.0µm; the manufacturing method of the coating includes electroplating, chemical plating or vacuum plating; or the welding surface of the heat sink is plated with a low-temperature welding alloy, and the low-temperature welding alloy includes an alloy of indium, bismuth and tin, and the melting point is between 119-232°C; the low-temperature welding alloy is plated on the welding surface of the heat sink by a vacuum plating method, and each metal is plated in turn; the total thickness of the coating is greater than 3 µm.

3. The welding method of a laser bar packaging structure according to claim 1, characterized in that: The welding sheet of the welding alloy is pressed into shape and cut after the alloy is smelted; the thickness of the welding sheet is controlled between 5-20 μm; and the plane size of the welding sheet is smaller than the welding surface of the laser bar.

4. A laser bar packaging structure obtained by the welding method of a laser bar packaging structure according to claim 1.

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