Stacked stepping welding method for plastic package power device

Abstract

The invention provides a laminated stepping welding method for a plastic package power device, which comprises the following steps of: placing a second soldering lug on a first soldering lug to form a gap layer, and in the welding process of the plastic package power device and a heat dissipation bottom plate, transferring heat upwards in sequence through a welding clamp, and due to the existence of the gap layer, performing heat transfer between the plastic package power device and the heat dissipation bottom plate in a non-direct contact manner. The influence and the thermal shock of the temperature of the heating plate of the welding furnace on the plastic package power device are greatly reduced; meanwhile, in the welding process, the temperature is increased to the melting temperature of the first soldering lug, the first soldering lug is melted, then cooling is started, and welding is completed when heat is not completely conducted to influence a plastic packaging layer of the plastic packaging power device, so that an IMC layer is finally formed between a copper layer of a ceramic copper-clad plate at the bottom of the plastic packaging power device and a heat dissipation bottom plate through welding. And high temperature impact on the plastic package layer of the device is avoided, the deformation of the plastic package power device is reduced, the internal stress of a welding hole and the plastic package power device after welding is reduced, and a thinner solder layer is kept.

Description

Technical Field

[0001] The present invention relates to the technical field of power device welding, and in particular to a stacked step welding method for plastic-sealed power devices. Background Art

[0002] In the modern electronics industry, power devices, such as IGBTs (insulated-gate bipolar transistors), are core components for power conversion and control. Their performance and reliability play a crucial role in various electronic products and power systems. Plastic-encapsulated power devices, with their unique advantages, stand out in numerous applications, becoming a mainstream choice in the industry. From a performance perspective, plastic-encapsulated power devices offer excellent electrical insulation. Their encapsulation materials effectively isolate the device's internal live parts from the external environment, preventing leakage and short circuits. The cured plastic encapsulation material provides reliable mechanical protection for the chip inside, ensuring stable device operation under high-voltage, high-current conditions. Regarding cost, the plastic encapsulation process is mature and easily scalable. Compared to other encapsulation methods, the production process for plastic-encapsulated power devices can be highly automated, reducing labor costs and production cycles.

[0003] However, the actual manufacturing process for plastic-encapsulated power devices presents significant challenges in soldering. First, a thinner solder layer is desirable to reduce thermal resistance, typically requiring a thickness of 300 μm or less. Second, during product durability and reliability verification, low-temperature soldering materials can crack and peel under stress, necessitating the use of high-melting-point solder.

[0004] With the development of the power semiconductor industry, there is an increasing demand for soldering plastic-encapsulated products to copper heat sinks. Because the components being soldered are plastic-encapsulated, low-temperature solders, such as those with a melting point of approximately 217°C, are typically used to prevent the plastic-encapsulated products from deforming beyond industry standards due to internal stress caused by high-temperature expansion. However, some plastic-encapsulated products soldered with low-temperature solders failed high- and low-temperature reliability testing (-40°C to 175°C). After 500 cycles, cracking of the solder layer occurred, making them unsuitable for high-end applications. Analysis confirmed that the failure of the high- and low-temperature shock reliability tests was caused by the use of low-temperature solder. Therefore, the introduction of high-temperature solders, such as those with a melting point of approximately 230°C, is urgently needed to address the reliability issues associated with soldering plastic-encapsulated products to copper heat sinks. However, using high-temperature solders significantly deforms the plastic-encapsulated products, resulting in voids exceeding 10% of the solder layer area after soldering, significantly exceeding the industry standard of voids of less than 2% to 5% of the solder layer area. This makes it impossible to form a reliable solder layer between the plastic-encapsulated product and the copper heat sink. Among the material properties, the thermal expansion coefficient of encapsulation materials, such as epoxy molding compound (EMC), is 46, the thermal expansion coefficient of ceramic in active metal brazing (AMB) is 2.5, and the thermal expansion coefficient of the heat dissipation copper baseplate is 17. During high-temperature soldering, EMC rapidly deforms, creating a large gap between the half-bridge encapsulated product and the heat dissipation copper baseplate, preventing effective soldering and resulting in significantly higher voiding levels than permitted. Addressing the significant deformation and excessive voiding in the solder layer caused by the use of high-temperature solder between the encapsulated product and the heat dissipation copper baseplate has become a pressing issue within the industry. Summary of the Invention

[0005] In view of the shortcomings of the prior art described above, the purpose of the present invention is to provide a stacked step-by-step welding method for plastic-encapsulated power devices, which is used to solve the problem in the prior art that when plastic-encapsulated power devices are welded to a heat dissipation base plate using high-temperature solder, the plastic-encapsulated products have large deformation and the voids in the welding layer seriously exceed the standard.

[0006] To achieve the above-mentioned and other related purposes, the present invention provides a step-by-step welding method for stacking plastic-encapsulated power devices, the welding method comprising the following steps: S1. Installation process, including: S10, fix the heat dissipation base plate into the welding fixture; S11, fixing at least one first soldering piece on the heat dissipation base plate, with the fixing position of each first soldering piece corresponding to a soldering area of a plastic-encapsulated power device; S12. Place a second welding piece on the first welding piece, and place at least three second welding pieces evenly along the circumference of each first welding piece; wherein the first welding piece and the second welding piece are made of the same material, the area of the second welding piece is 0.05% to 0.1% of the area of the first welding piece, and the thickness of the second welding piece is 60% to 130% of the thickness of the first welding piece; S13, placing at least one of the plastic-encapsulated power devices on the second soldering pad on each of the first soldering pads, so that a gap layer is formed between each of the plastic-encapsulated power devices and its corresponding first soldering pad; S14, installing the deadweight of a welding fixture on the plastic-encapsulated power device; S2. Welding process, including: S20, placing the plastic-encapsulated power device mounted on the welding fixture into a preheating furnace for preheating; S21, sending the preheated plastic-encapsulated power device into a soldering furnace for soldering to form an intermetallic compound layer between the plastic-encapsulated power device and the heat dissipation base plate; wherein, during the soldering process, the soldering furnace heats until the first soldering piece melts and then cools to below the melting point of the first soldering piece and the second soldering piece; S22, sending the welded plastic-encapsulated power device into a cooling furnace for cooling.

[0007] Optionally, the second soldering piece placed on each of the first soldering pieces in step S12 is placed along an outer edge of the first soldering piece.

[0008] Optionally, in step S12, four second welding pieces are evenly placed circumferentially on each first welding piece.

[0009] Optionally, the melting points of the first soldering plate and the second soldering plate are greater than 225° C.; and the heat dissipation base plate is a copper heat dissipation base plate.

[0010] Furthermore, the first soldering sheet and the second soldering sheet are tin soldering sheets; the preheating temperature in step S20 is 200° C. to 220° C., and the preheating time is 650 seconds to 750 seconds.

[0011] Furthermore, the temperature reduction gradient in step S21 is (0.8-1.5)°C / second.

[0012] Optionally, the welding fixture is placed on a welding tray; the heat dissipation base plate and the welding fixture are matched and fixed through the first positioning hole on the heat dissipation base plate and the first positioning column on the welding fixture; the first welding plate and the heat dissipation base plate are matched and fixed through the second positioning hole on the first welding plate and the second positioning column on the heat dissipation base plate; the weight of the welding fixture is matched and fixed through the limiting column on it and the limiting hole on the welding fixture.

[0013] Furthermore, each of the first welding pieces has a length of 60 mm to 61 mm, a width of 56 mm to 57 mm, and a thickness of 0.25 mm to 0.35 mm; each of the second welding pieces has a length of 1.8 mm to 2.2 mm, a width of 1.0 mm to 1.4 mm, and a thickness of 0.22 mm to 0.28 mm.

[0014] Furthermore, each of the first welding tabs and each of the second welding tabs are rectangular; eight second positioning holes are provided on the four edges of the first welding tab, and four of the second positioning holes are provided at the four top corner edges of the first welding tab; in step S12, four second welding tabs are evenly placed circumferentially on each of the first welding tabs, and the four second welding tabs are correspondingly placed on the outside of the four second positioning holes at the four top corner edges of the first welding tab.

[0015] Furthermore, each second welding piece is flush with an outer edge of the adjacent second positioning hole, and a distance therebetween is 1.1 mm to 1.3 mm.

[0016] As described above, the present invention provides a method for step-by-step welding of stacked plastic-encapsulated power devices. During the installation process, a second welding sheet, which mainly serves as a support, is placed on a first welding sheet, which serves as the main welding material. Thus, when the plastic-encapsulated power device is placed on the second welding sheet, a gap layer is formed between the plastic-encapsulated power device and the first welding sheet. That is, the plastic-encapsulated power device is separated from the first welding sheet by the second welding sheet, and the area size of the second welding sheet is very small relative to the first welding sheet. Therefore, the second welding sheet mainly serves to support the plastic-encapsulated power device and does not serve as the main heat transfer path. Based on this installation method, during the welding process between the plastic-encapsulated power device and the heat dissipation base plate, including preheating and welding, the entire installed welding fixture is sent into the preheating furnace and the welding furnace and heated from the bottom heating plate. The heat is transferred upward through the welding fixture to the heat dissipation base plate, the first welding piece, the second welding piece and the plastic-encapsulated power device in sequence. At this time, due to the formation of a gap layer between the first welding piece and the plastic-encapsulated power device, the heat transfer between the two is non-direct contact heat transfer, but a heat transfer method of the gap layer in a vacuum environment or nitrogen, formic acid gas and other poor thermal conductivity. Therefore, no effective direct heat conduction channel is formed between the welding surface of the plastic-encapsulated power device and the first welding piece, which greatly reduces the impact of the welding furnace heating plate temperature on the plastic-encapsulated power device. Influence and thermal shock; at the same time, during the welding process, by controlling the heating temperature curve of the welding furnace, after the temperature is heated to the melting temperature of the first welding piece to melt the first welding piece, the temperature begins to drop. When the first welding piece is melting, the second welding piece will sink into the melt of the first welding piece due to its own gravity and will be melted by the temperature of the melt of the first welding piece because it is very small. The gap layer disappears, and the plastic-encapsulated power device and the first welding piece become in direct contact. However, the temperature starts to drop at this time so that the welding is completed before the heat is completely conducted to affect the plastic layer of the plastic-encapsulated power device, thereby finally forming an effective intermetallic compound (IMC) layer by welding between the copper layer of the ceramic copper-clad board at the bottom of the plastic-encapsulated power device and the heat dissipation base plate, and will not generate a high temperature shock on the plastic layer of the device, thereby reducing the deformation of the plastic-encapsulated power device, reducing the welding holes and the internal stress of the plastic-encapsulated power device after welding, and maintaining a thinner solder layer. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Schematic diagram of a soldering tray with a soldering fixture placed thereon used in the step-by-step soldering method for stacking plastic-encapsulated power devices of the present invention.

[0018] Figure 2 Display as Figure 1 Schematic diagram of a heat sink base plate, a first soldering piece, a second soldering piece and a plastic-encapsulated power device mounted on a soldering fixture.

[0019] Figure 3 It is a schematic diagram of a first soldering piece used in the step-by-step soldering method for stacking plastic-encapsulated power devices of the present invention.

[0020] Figure 4 Schematic diagram of the first soldering piece and the second soldering piece used in the step-by-step soldering method for stacking plastic-encapsulated power devices of the present invention.

[0021] Figure 5 It is a schematic diagram showing a heat dissipation base plate, a first soldering piece, a second soldering piece and a plastic-encapsulated power device installed on a soldering fixture in the step-by-step soldering method of the plastic-encapsulated power device stack of the present invention.

[0022] Figure 6 Display as Figure 5 Schematic diagram of the cross-section structure.

[0023] Figure 7 Displayed as Figure 6 Schematic diagram of the cross-sectional structure of the first soldering piece, the second soldering piece and the gap layer after enlargement.

[0024] Figure 8 It is a schematic diagram of the cross-sectional structure after the welding fixture is installed in the step-by-step welding method of the plastic-encapsulated power device stack of the present invention.

[0025] Component number description 10 heat dissipation base 100 First positioning hole 101 Second positioning column 11 welding fixture 110 First positioning column 111 Limiting hole 112 Fixture limit frame 12 Welding pallet 13 First soldering piece 130 Second positioning hole 14 Second soldering piece 15 Plastic packaged power devices 150 void layer 16 Welding fixture weight 160 Limit column DETAILED DESCRIPTION

[0026] The following describes the embodiments of the present invention through specific examples. Those skilled in the art will readily understand the other advantages and benefits of the present invention from the disclosure herein. The present invention may also be implemented or applied through various other specific embodiments, and the details in this specification may be modified or altered based on different viewpoints and applications without departing from the spirit of the present invention.

[0027] See also Figures 1 to 8 It should be noted that the diagrams provided in this embodiment are merely schematic illustrations of the basic concept of the present invention. Therefore, the diagrams only show components related to the present invention and are not drawn according to the number, shape, and size of components in actual implementation. In actual implementation, the type, quantity, and proportion of each component may be changed arbitrarily, and the component layout may also be more complex.

[0028] This embodiment provides a step-by-step welding method for stacking plastic-encapsulated power devices, the welding method comprising: S1 installation process and S2 welding process; wherein, The S1 installation process includes the following steps: S10, such as Figure 1 and Figure 2 As shown, the heat dissipation base plate 10 is fixed into the welding fixture 11; S11, such as Figure 2As shown, at least one first soldering piece 13 is fixed on the heat dissipation base plate 10, as shown in FIG. Figure 2 Combined with Figure 5 As shown, the fixed position of each first welding piece 13 corresponds to the welding area of a plastic-encapsulated power device 15; S12, such as Figure 4 As shown, the second welding tabs 14 are placed on the first welding tabs 13, and at least three second welding tabs 14 are evenly placed circumferentially on each first welding tab 13; wherein the first welding tabs 13 and the second welding tabs 14 are made of the same material, the area of the second welding tab 14 is 0.05% to 0.1% of the area of the first welding tab 13, and the thickness of the second welding tab 14 is 60% to 130% of the thickness of the first welding tab 13; S13, such as Figures 5 to 7 As shown, at least one of the plastic-encapsulated power devices 15 is placed correspondingly on the second soldering tab 14 on each of the first soldering tabs 13, so that a gap layer 150 is formed between each of the plastic-encapsulated power devices 15 and its corresponding first soldering tab 13; S14, such as Figure 8 As shown, a welding fixture deadweight 16 is installed on the plastic-encapsulated power device 15; The S2 welding process includes the following steps: S20, sending the plastic-encapsulated power device 15 mounted on the welding fixture 11 into a preheating furnace for preheating; S21, sending the preheated plastic-encapsulated power device 15 into a soldering furnace for soldering to form an intermetallic compound layer between the plastic-encapsulated power device 15 and the heat dissipation base plate 10; wherein, during the soldering process, the soldering furnace heats until the first soldering piece 13 melts and then cools to below the melting point of the first soldering piece 13 and the second soldering piece 14; S22, sending the welded plastic-encapsulated power device 15 into a cooling furnace for cooling.

[0029] In the step-by-step soldering method for stacking plastic-encapsulated power devices of this embodiment, a second soldering piece mainly used as a support is placed on a first soldering piece as a main soldering material during the installation process, so that when the plastic-encapsulated power device is placed on the second soldering piece, a gap is formed between the second soldering piece and the first soldering piece. Figure 7The gap layer 150 shown in the figure, that is, the plastic-encapsulated power device and the first soldering piece are separated by a second soldering piece, and the area size of the second soldering piece is very small compared to the first soldering piece, that is, the area of the second soldering piece is 0.05%~0.1% of the area of the first soldering piece, so that the second soldering piece mainly supports the plastic-encapsulated power device and does not serve as the main heat transfer path. Based on this installation method, during the welding process between the plastic-encapsulated power device and the heat sink base plate, including preheating and welding, the entire installed welding fixture is sent into the preheating furnace and the welding furnace and heated from the bottom heating plate. The heat is transferred upward to the heat sink base plate, the first soldering piece, the second soldering piece and the plastic-encapsulated power device in sequence through the welding fixture. At this time, due to the formation of a gap layer between the first soldering piece and the plastic-encapsulated power device, the heat transfer between the two is non-direct contact heat transfer, but a heat transfer method of the gap layer in a vacuum environment or nitrogen, formic acid gas and other poor thermal conductivity. Therefore, no effective direct heat conduction channel is formed between the welding surface of the plastic-encapsulated power device and the first soldering piece, which greatly reduces the impact of the welding furnace heating plate temperature on the plastic-encapsulated power device. Influence and thermal shock; at the same time, during the welding process, by controlling the heating temperature curve of the welding furnace, after the temperature is heated to the melting temperature of the first welding piece to melt the first welding piece, the temperature begins to drop. When the first welding piece is melting, the second welding piece will sink into the melt of the first welding piece due to its own gravity and will be melted by the temperature of the melt of the first welding piece because it is very small. The gap layer disappears, and the plastic-encapsulated power device and the first welding piece become in direct contact. However, the temperature starts to drop at this time so that the welding is completed before the heat is completely conducted to affect the plastic layer of the plastic-encapsulated power device, thereby finally forming an effective intermetallic compound (IMC) layer by welding between the copper layer of the ceramic copper-clad board at the bottom of the plastic-encapsulated power device and the heat dissipation base plate, and will not generate a high temperature shock on the plastic layer of the device, thereby reducing the deformation of the plastic-encapsulated power device, reducing the welding holes and the internal stress of the plastic-encapsulated power device after welding, and maintaining a thinner solder layer.

[0030] In a specific example, the first soldering piece 13 and the second soldering piece 14 are both selected to be high-temperature tin solder with a melting point of about 230°C. By adopting the welding method of this embodiment, the deformation of the plastic-encapsulated power device is reduced from the original 500μm~600μm to less than 300μm, a reduction of 40%~50%, effectively avoiding defects such as cracks and warping caused by deformation stress, and improving the structural stability and reliability of the product; after reliability testing, the product meets the reliability requirements of high and low temperature impact, and under the same conditions, it is increased from 500 times to more than 1000 times, which has achieved a very significant improvement, solving the technical bottleneck of the power semiconductor industry that cannot use high-temperature solder for welding.

[0031] The main function of the second soldering piece 14 is to support the plastic-encapsulated power device 15. Therefore, in order to ensure the stability of the second soldering piece 14 in supporting the plastic-encapsulated power device 15, as shown in FIG. Figure 4 As shown, the second soldering tabs 14 (framed in red in the figure) placed on each first soldering tab 13 are placed along the outer edge of the first soldering tab 13. Furthermore, since the second soldering tabs 14 also serve as solder during the soldering process and cool down after the first soldering tab 13 melts, to ensure that the second soldering tabs 14 are melted by the heat of the molten first soldering tab 13, the number of second soldering tabs 14 should be limited while ensuring support stability. Too many second soldering tabs 14 may not be fully melted, resulting in reduced soldering quality, while too few second soldering tabs 14 may result in poor support stability and an inability to form a good gap layer. Therefore, the number of second soldering tabs 14 is generally selected to be three or four. In this embodiment, four second soldering tabs 14 are preferred.

[0032] The welding method of this embodiment is particularly suitable for welding high-temperature solder, for example, the first soldering piece 13 and the second soldering piece 14 have a melting point greater than 225° C. In addition, the heat dissipation base plate 10 is generally a common copper heat dissipation base plate.

[0033] As a specific example, the first solder tab 13 and the second solder tab 14 are tin solder tabs with a melting point greater than 225°C. During the preheating process, the preheating temperature in step S20 is set to 200°C to 220°C for a preheating time of 650 seconds to 750 seconds. This preheating temperature does not reach the deformation temperature of the plastic layer of the plastic-encapsulated power device. Furthermore, during the cooling process in step S21, the cooling gradient is set to (0.8-1.5)°C / second.

[0034] As another specific example, during installation, if Figure 1 and Figure 2 As shown, the welding fixture 11 is placed on a welding tray 12; each welding tray 12 can be placed with one or more welding fixtures 11 according to actual needs, for example Figure 1 and Figure 2 In the embodiment, two welding fixtures 11 are placed on each welding tray 12. It should be noted that Figure 1 The welding fixture 11 is placed only on the right side of the welding tray 12, and no welding fixture 11 is placed on the left side. Of course, in practice, the required number of welding fixtures 11 can be placed on the welding tray 12 according to actual needs, and the full amount can be placed or the partial amount can be placed.

[0035] like Figure 2 As shown, the heat dissipation base plate 10 and the welding fixture 11 are matched and fixed through the first positioning hole 100 on the heat dissipation base plate 10 and the first positioning column 110 on the welding fixture 11. Figure 2A first positioning post 110 is provided at the upper and lower ends of each welding fixture 11, and a first positioning hole 100 is provided at the upper and lower ends of the heat dissipation base plate 10. The two first positioning holes 100 of the heat dissipation base plate 10 are respectively placed on the two first positioning posts 110 of the welding fixture 11 to achieve detachable fixation between the two. Figure 2 The welding fixture 11 placed on the left side of the welding tray 12 has fixed the heat dissipation base plate 10 and the first welding piece 13, and the welding fixture 11 placed on the right side has fixed the heat dissipation base plate 10, the first welding piece 13, the second welding piece 14, the plastic-encapsulated power device 15 and the welding fixture weight 16, that is, the installation is complete.

[0036] like Figure 2 and Figure 3 As shown, the first soldering piece 13 and the heat dissipation base plate 10 are matched and fixed with the second positioning hole 130 on the first soldering piece 13 and the second positioning column 101 on the heat dissipation base plate 10. Figure 2 In the welding fixture 11 on the left, three first welding pieces 13 are fixed on a heat dissipation base plate 10 (of course, the number of first welding pieces 13 fixed on a heat dissipation base plate 10 is set according to actual needs), and each of the first welding pieces 13 is provided with 8 second positioning holes 130, and the corresponding areas of the corresponding heat dissipation base plate 10 are provided with 8 second positioning columns 101. The 8 second positioning holes 130 of the first welding piece 13 are respectively correspondingly sleeved on the 8 second positioning columns 101 of the heat dissipation base plate 10 to realize detachable fixation between a first welding piece 13 and the heat dissipation base plate 10.

[0037] like Figure 2 and Figure 8 As shown, the weight of the welding fixture 16 is matched and fixed with the limiting hole 111 on the welding fixture 11 through the limiting column 160 thereon. Figure 2 A limiting hole 111 is respectively provided at the upper and lower ends of each welding fixture 11, and a limiting column 160 is respectively provided at the upper and lower ends of the welding fixture weight 16. The two limiting columns 160 of the welding fixture weight 16 are respectively correspondingly sleeved on the two limiting holes 111 of the welding fixture 11 to realize detachable fixation between the two.

[0038] The shapes of the first welding piece 13 and the second welding piece 14 are not particularly limited and can be regular or irregular. In a specific application, the length of each first welding piece 13 is selected to be 60mm~61mm, the width is selected to be 56mm~57mm, and the thickness is selected to be 0.25mm~0.35mm; the length of each second welding piece 14 is selected to be 1.8mm~2.2mm, the width is selected to be 1.0mm~1.4mm, and the thickness is selected to be 0.22mm~0.28mm. In this embodiment, Figure 3 and Figure 4 As shown, preferably, each of the first welding tabs 13 and each of the second welding tabs 14 are rectangular; eight second positioning holes 130 are provided on the four edges of the first welding tab 13, and four of the second positioning holes 130 are provided at the four vertex edges of the first welding tab 13; and in step S12, four second welding tabs 14 are evenly placed along the circumference on each of the first welding tabs 13, and the four second welding tabs 14 are correspondingly placed on the outside of the four second positioning holes 130 at the four vertex edges of the first welding tab 13, as shown in FIG. Figure 3 A second welding piece 14 is arranged on the upper side of the second positioning hole 130 at the upper left corner of the first welding piece 13, a second welding piece 14 is arranged on the lower side of the second positioning hole 130 at the lower left corner of the first welding piece 13, a second welding piece 14 is arranged on the upper side of the second positioning hole 130 at the upper right corner of the first welding piece 13, and a second welding piece 14 is arranged on the lower side of the second positioning hole 130 at the lower right corner of the first welding piece 13.

[0039] In this embodiment, it is most preferred that each of the first soldering tabs 13 and each of the second soldering tabs 14 are rectangular; the length of each of the first soldering tabs 13 is selected to be 60.8 mm, the width is selected to be 56.8 mm, and the thickness is selected to be 0.3 mm; the length of each of the second soldering tabs 14 is selected to be 2.0 mm, the width is selected to be 1.2 mm, and the thickness is selected to be 0.25 mm; the first soldering tab 13 is provided with 8 rectangular second positioning holes 130 along the edges of its four sides, and the connecting line of the second positioning holes 130 provided on each edge is parallel to the side length, wherein 4 of the second positioning holes 130 are provided at the four vertex edges of the first soldering tab 13, and the positions of the 8 second positioning holes 130 on the first soldering tab 13 are distributed as follows: Figure 3 As shown; and in step S12, four second welding pieces 14 are evenly placed on each first welding piece 13 along the circumferential direction, and the four second welding pieces 14 are correspondingly placed on the outside of the four second positioning holes 130 at the four vertex edges of the first welding piece 13, and each second welding piece 14 is flush with the outer edge of the adjacent second positioning hole 130, and the spacing between the two is 1.1mm~1.3mm, for example, 1.1mm, 1.2mm (for example Figure 4shown) or 1.3mm.

[0040] As an example, the plastic-encapsulated power device is preferably a half-bridge plastic-encapsulated power device.

[0041] In summary, the present invention provides a method for step-by-step welding of a stacked plastic-encapsulated power device. During the installation process, a second welding sheet, which mainly serves as a support, is placed on a first welding sheet, which serves as the main welding material. When the plastic-encapsulated power device is placed on the second welding sheet, a gap layer is formed between the plastic-encapsulated power device and the first welding sheet. That is, the plastic-encapsulated power device is separated from the first welding sheet by the second welding sheet, and the area size of the second welding sheet is very small relative to the first welding sheet. Therefore, the second welding sheet mainly serves to support the plastic-encapsulated power device and does not serve as the main heat transfer path. Based on this installation method, during the welding process between the plastic-encapsulated power device and the heat dissipation base plate, including preheating and welding, the entire installed welding fixture is sent into the preheating furnace and the welding furnace and heated from the bottom heating plate. The heat is transferred upward through the welding fixture to the heat dissipation base plate, the first welding piece, the second welding piece and the plastic-encapsulated power device in sequence. At this time, due to the formation of a gap layer between the first welding piece and the plastic-encapsulated power device, the heat transfer between the two is non-direct contact heat transfer, but a heat transfer method of the gap layer in a vacuum environment or nitrogen, formic acid gas and other poor thermal conductivity. Therefore, no effective direct heat conduction channel is formed between the welding surface of the plastic-encapsulated power device and the first welding piece, which greatly reduces the impact of the welding furnace heating plate temperature on the plastic-encapsulated power device. The invention also reduces the impact and thermal shock of the welding process by controlling the heating temperature curve of the welding furnace. After the first welding piece is heated to the melting temperature of the first welding piece and melted, the temperature begins to decrease. When the first welding piece is melting, the second welding piece sinks into the molten material of the first welding piece due to its own gravity. Since it is very small, it will be melted by the temperature of the molten material of the first welding piece, and the gap layer disappears. The plastic-encapsulated power device and the first welding piece are in direct contact. However, the temperature is decreased at this time to complete the welding before the heat is completely transferred to the plastic layer of the plastic-encapsulated power device. As a result, an effective intermetallic compound (IMC) layer is welded between the copper layer of the ceramic copper-clad plate at the bottom of the plastic-encapsulated power device and the heat dissipation base plate. The invention does not generate a high temperature shock to the plastic layer of the device, reduces the deformation of the plastic-encapsulated power device, reduces the welding holes and the internal stress of the plastic-encapsulated power device after welding, and maintains a relatively thin solder layer. Therefore, the invention effectively overcomes the various shortcomings of the prior art and has high industrial application value.

[0042] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the present invention. Anyone skilled in the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by one of ordinary skill in the art without departing from the spirit and technical principles disclosed herein are intended to be covered by the claims of the present invention.

Claims

1. A step-by-step welding method for stacked plastic-encapsulated power devices, characterized in that: The welding method comprises the following steps: S1. Installation process, including: S10, fix the heat dissipation base plate into the welding fixture; S11, fixing at least one first soldering piece on the heat dissipation base plate, with the fixing position of each first soldering piece corresponding to a soldering area of a plastic-encapsulated power device; S12. Place a second welding piece on the first welding piece, and place at least three second welding pieces evenly along the circumference of each first welding piece; wherein the first welding piece and the second welding piece are made of the same material, the area of the second welding piece is 0.05% to 0.1% of the area of the first welding piece, and the thickness of the second welding piece is 60% to 130% of the thickness of the first welding piece; S13, placing at least one of the plastic-encapsulated power devices on the second soldering pad on each of the first soldering pads, so that a gap layer is formed between each of the plastic-encapsulated power devices and its corresponding first soldering pad; S14, installing the deadweight of a welding fixture on the plastic-encapsulated power device; S2. Welding process, including: S20, placing the plastic-encapsulated power device mounted on the welding fixture into a preheating furnace for preheating; S21, sending the preheated plastic-encapsulated power device into a soldering furnace for soldering to form an intermetallic compound layer between the plastic-encapsulated power device and the heat dissipation base plate; wherein, during the soldering process, the soldering furnace heats until the first soldering piece melts and then cools to below the melting point of the first soldering piece and the second soldering piece; S22, sending the welded plastic-encapsulated power device into a cooling furnace for cooling.

2. The method for stacked step-by-step soldering of plastic-encapsulated power devices according to claim 1, characterized in that: In step S12, the second soldering piece placed on each of the first soldering pieces is placed along the outer edge of the first soldering piece.

3. The method for stacked stepwise soldering of plastic-encapsulated power devices according to claim 1 or 2, characterized in that: In step S12, four second welding pieces are evenly placed on each first welding piece along the circumferential direction.

4. The method for stacked step-by-step soldering of plastic-encapsulated power devices according to claim 1, wherein: The melting points of the first soldering piece and the second soldering piece are greater than 225° C.; the heat dissipation base plate is a copper heat dissipation base plate.

5. The method for stacked step-by-step soldering of plastic-encapsulated power devices according to claim 4, characterized in that: The first soldering piece and the second soldering piece are tin soldering pieces; the preheating temperature in step S20 is 200° C. to 220° C., and the preheating time is 650 seconds to 750 seconds.

6. The method for stacked stepwise soldering of plastic-encapsulated power devices according to claim 5, characterized in that: The temperature reduction gradient in step S21 is (0.8-1.5)°C / second.

7. The method for stacked step-by-step soldering of plastic-encapsulated power devices according to claim 1, wherein: The welding fixture is placed on the welding tray; the heat dissipation base plate and the welding fixture are matched and fixed through the first positioning hole on the heat dissipation base plate and the first positioning column on the welding fixture; the first welding piece and the heat dissipation base plate are matched and fixed through the second positioning hole on the first welding piece and the second positioning column on the heat dissipation base plate; the weight of the welding fixture is matched and fixed through the limiting column on it and the limiting hole on the welding fixture.

8. The method for stacked step-by-step soldering of plastic-encapsulated power devices according to claim 7, characterized in that: The length of each first welding piece is 60mm~61mm, the width is 56mm~57mm, and the thickness is 0.25mm~0.35mm; the length of each second welding piece is 1.8mm~2.2mm, the width is 1.0mm~1.4mm, and the thickness is 0.22mm~0.28mm.

9. The method for stacked stepwise soldering of plastic-encapsulated power devices according to claim 8, characterized in that: Each of the first welding tabs and each of the second welding tabs is rectangular; eight second positioning holes are provided on the four edges of the first welding tab, and four of the second positioning holes are provided at the four top corner edges of the first welding tab; in step S12, four second welding tabs are evenly placed circumferentially on each of the first welding tabs, and the four second welding tabs are correspondingly placed on the outside of the four second positioning holes at the four top corner edges of the first welding tab.

10. The method for stacked step-by-step soldering of plastic-encapsulated power devices according to claim 9, characterized in that: Each second welding piece is flush with the outer edge of the adjacent second positioning hole, and the distance between the two is 1.1 mm to 1.3 mm.

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