Power module reflow soldering method and power module

Abstract

The invention provides a power module reflow soldering method and a power module, and the method comprises the following steps: a soldering assembly is provided, the soldering assembly comprises a copper-clad ceramic substrate, a solder layer and a to-be-soldered object, the copper-clad ceramic substrate comprises a ceramic chip and a copper foil located on the surface of the ceramic chip, and an organic solderability preservative is arranged on one side, away from the ceramic chip, of the copper foil; the solder layer is located between the organic solderability preservative and the to-be-soldered object, and the solder layer contains scaling powder; and the welding assembly is placed in a reflow soldering furnace, so that the to-be-welded object and the copper-clad ceramic substrate are welded. According to the invention, the copper surface of the copper-clad ceramic substrate is provided with the organic solderability preservative film, so that the oxidation reaction of oxygen and the copper foil can be resisted, and the copper-clad ceramic substrate is prevented from being oxidized in the reflow soldering process; besides, by reducing the number of the welding assemblies in the carrying disc, the temperature rise degree caused by gathering of the welding assemblies is reduced, meanwhile, the nitrogen protection degree of the single welding assembly is increased, and the copper-clad ceramic substrate is further prevented from being oxidized in the reflow soldering process.

Description

Technical Field

[0001] The invention belongs to the technical field of semiconductors and relates to a power module reflow soldering method and a power module. Background Art

[0002] Ultrasonic wire bonding technology is simple in process and low in cost. It is one of the key interconnection technologies in power module packaging. The bonding strength has an important impact on the reliability of the power module. Copper-clad ceramic substrates have excellent thermal conductivity, electrical insulation and good mechanical strength. They are key electronic packaging materials in power modules. Usually, the copper surface used to connect to the chip in the copper-clad ceramic substrate is divided into several areas. The first side of the chip is welded to the copper surface of the preset area by reflow soldering, and then the second side of the chip is bonded to the copper surface of other areas by ultrasonic wire bonding. The state of the copper surface is a key parameter affecting the bonding strength. The oxide layer produced by copper surface oxidation will hinder interface bonding and atomic diffusion, thereby reducing the bonding strength. However, increasing the ultrasonic power may lead to over-welding.

[0003] During the reflow soldering process, the copper surface of the non-welding area will be oxidized. The current methods to prevent the copper surface oxidation of the copper-clad ceramic substrate during the reflow soldering process include reducing the residual oxygen in the reflow furnace (increasing the amount of nitrogen), lowering the furnace temperature, using formic acid reduction or using a more reducing cleaning fluid. However, increasing the amount of nitrogen and replacing the cleaning fluid will increase the manufacturing cost of the power module; lowering the furnace temperature will cause the risk of cold welding; using formic acid reduction requires the purchase of a formic acid reflow furnace separately and the need to change the solder and process, which will result in more equipment, manpower, and material costs.

[0004] Therefore, how to provide a power module reflow soldering method and a power module to reduce the cost while ensuring that the copper surface is not oxidized has become a technical problem that needs to be solved urgently by those skilled in the art. Summary of the invention

[0005] In view of the above-mentioned shortcomings of the prior art, an object of the present invention is to provide a power module reflow soldering method and a power module, so as to solve the problem of high soldering cost of the power module in the prior art.

[0006] To achieve the above-mentioned object and other related objects, the present invention provides a power module reflow soldering method, comprising the following steps: A welding assembly is provided, the welding assembly comprising a copper-clad ceramic substrate, a solder layer and an object to be welded, wherein the copper-clad ceramic substrate comprises a ceramic sheet and a copper foil sheet located on the surface of the ceramic sheet, an organic solder-protective film is provided on a side of the copper foil sheet away from the ceramic sheet, the solder layer is located between the organic solder-protective film and the object to be welded, and the solder layer contains a flux; The welding assembly is placed in a reflow oven to weld the object to be welded and the copper-clad ceramic substrate, wherein during the welding process, the flux removes the organic solderability film between the copper foil and the solder layer, and retains the organic solderability film in the non-welding area to protect the copper foil in the non-welding area from being oxidized during the welding process.

[0007] Optionally, the soldering assembly is placed in a carrier, and the soldering assembly is transferred to the reflow oven via the carrier.

[0008] Optionally, based on a reference number, the distance between the welding assemblies is increased by reducing the number of the welding assemblies in the carrier.

[0009] Optionally, the organic solderability film includes a rosin-based organic solderability film, an active resin-based organic solderability film or an imidazole-based organic solderability film.

[0010] Optionally, the thickness of the organic solderability film ranges from 0.2 to 0.5 microns.

[0011] Optionally, the object to be soldered includes one or more of a chip and a heat sink.

[0012] Optionally, the copper-clad ceramic substrate adopts a double-sided copper-clad ceramic substrate, the copper foil includes a first copper foil and a second copper foil, and the first copper foil and the second copper foil are separately arranged on two opposite sides of the ceramic sheet; the organic solder protection film includes a first organic solder protection film and a second organic solder protection film, the first organic solder protection film is located on a side of the first copper foil away from the ceramic sheet, and the second organic solder protection film is located on a side of the second copper foil away from the ceramic sheet; the solder layer includes a first solder layer and a second solder layer, the first solder layer is located on a side of the first organic solder protection film away from the ceramic sheet, and the second solder layer is located on a side of the second organic solder protection film away from the ceramic sheet; the object to be soldered includes a chip and a heat sink, the chip is located on a side of the first solder layer away from the ceramic sheet, and the heat sink is located on a side of the second solder layer away from the ceramic sheet.

[0013] Optionally, the heat sink is made of copper.

[0014] Optionally, the solder layer includes a solder paste layer.

[0015] The present invention also provides a power module, wherein the power module is obtained by welding using any of the power module reflow welding methods described above.

[0016] As described above, in the power module reflow soldering method and power module of the present invention, the copper surface of the copper-clad ceramic substrate is provided with an organic solder protection film, which can resist the oxidation reaction between oxygen and the copper surface of the copper-clad ceramic substrate, thereby preventing the copper-clad ceramic substrate from being oxidized during the reflow soldering process; in addition, on the basis of the reference number, by reducing the number of welding assemblies in the carrier and increasing the distance between the welding assemblies, the temperature rise caused by the aggregation of the welding assemblies is reduced, and the degree of nitrogen protection of the single welding assembly is also increased, thereby further preventing the copper-clad ceramic substrate from being oxidized during the reflow soldering process. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Shown is a flow chart of a power module reflow soldering method in an embodiment of the present invention.

[0018] Figure 2 Shown is a schematic diagram of a welding assembly according to an embodiment of the present invention.

[0019] Figure 3 Shown is a schematic diagram of a power module according to an embodiment of the present invention.

[0020] Description of the accompanying drawings: 1-copper-clad ceramic substrate, 100-ceramic sheet, 101-first copper foil sheet, 102-second copper foil sheet; 200-first organic solderability film, 201-second organic solderability film; 300-first solder layer, 301-second solder layer; 400-chip, 401-heat sink; S1~S2-steps. DETAILED DESCRIPTION

[0021] The following describes the embodiments of the present invention through specific examples, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed in various ways based on different viewpoints and applications without departing from the spirit of the present invention.

[0022] See also Figures 1 to 3 It should be noted that the illustrations provided in this embodiment are only used to illustrate the basic concept of the present invention in a schematic manner, and the drawings only show components related to the present invention rather than being 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 complicated.

[0023] This embodiment provides a power module reflow soldering method. Figure 1 , including the following steps: S1: providing a welding assembly, the welding assembly comprising a copper-clad ceramic substrate, a solder layer and an object to be welded, wherein the copper-clad ceramic substrate comprises a ceramic sheet and a copper foil sheet located on the surface of the ceramic sheet, an organic solder-protective film is provided on a side of the copper foil sheet away from the ceramic sheet, the solder layer is located between the organic solder-protective film and the object to be welded, and the solder layer contains a flux; S2: placing the welding assembly in a reflow oven to weld the object to be welded and the copper-clad ceramic substrate, wherein during the welding process, the flux removes the organic solderability film between the copper foil and the solder layer, and retains the organic solderability film in the non-welding area to protect the copper foil in the non-welding area from being oxidized during the welding process.

[0024] The power module reflow soldering method of this embodiment is described in detail below with reference to the specific drawings.

[0025] First, see Figure 2 , perform step S1: provide a welding assembly, the welding assembly includes a copper-clad ceramic substrate 1, a solder layer and an object to be welded, wherein the copper-clad ceramic substrate 1 includes a ceramic sheet 100 and a copper foil sheet located on the surface of the ceramic sheet 100, an organic solderability film is provided on a side of the copper foil sheet away from the ceramic sheet 100, the solder layer is located between the organic solderability film and the object to be welded, and the solder layer contains flux.

[0026] As an example, the copper-clad ceramic substrate 1 is a double-sided copper-clad ceramic substrate, that is, the copper foil includes a first copper foil 101 and a second copper foil 102 , and the first copper foil 101 and the second copper foil 102 are separately arranged on two opposite sides of the ceramic sheet 100 .

[0027] As an example, the organic solderability film includes a first organic solderability film 200 and a second organic solderability film 201, wherein the first organic solderability film 200 is located on a side of the first copper foil 101 away from the ceramic sheet 100, and the second organic solderability film 201 is located on a side of the second copper foil 102 away from the ceramic sheet 100; the organic solderability film includes a rosin-based organic solderability film, an active resin-based organic solderability film or an imidazole-based organic solderability film, which is selected according to demand, wherein the thickness of the organic solderability film ranges from 0.2 to 0.5 microns.

[0028] As an example, the solder layer includes a first solder layer 300 and a second solder layer 301 , wherein the first solder layer 300 is located on a side of the first organic solderability film 200 away from the ceramic sheet 100 , and the second solder layer 301 is located on a side of the second organic solderability film 201 away from the ceramic sheet 100 .

[0029] As an example, the object to be soldered includes a chip 400 and a heat sink 401, the chip 400 is located on the side of the first solder layer 300 away from the ceramic sheet 100, the heat sink 401 is located on the side of the second solder layer 301 away from the ceramic sheet 100, and the heat sink 401 is made of copper; that is, the copper-clad ceramic substrate 1 in this embodiment adopts a double-sided copper-clad ceramic substrate, one side of the copper-clad ceramic substrate 1 is used for soldering with the chip 400, and the other side of the copper-clad ceramic substrate 1 is used for soldering with the heat sink 401; in other examples, the copper-clad ceramic substrate 1 can also adopt a single-sided copper-clad ceramic substrate, and only one side is soldered with the object to be soldered, which is selected according to needs.

[0030] As an example, the first copper foil 101 is a patterned copper surface, and the first copper foil 101 is divided into several areas. The first surface of the chip 400 is subsequently welded to the first copper foil 101 in a preset area by reflow soldering, and the second surface of the chip 400 is connected to the first copper foil 101 in other areas by wire bonding by an ultrasonic wire bonding method; the second copper foil 102 is a whole copper surface, which is used for subsequent welding with the heat sink 401 by reflow soldering.

[0031] As an example, the copper-clad ceramic substrate 1 is immersed in a chemical solvent for film formation and then washed and dried to form the first organic solderability film 200 and the second organic solderability film 201 on two opposite sides of the copper-clad ceramic substrate 1; wherein the first organic solderability film 200 and the second organic solderability film 201 have a resistant temperature of above 300°C, the furnace temperature of the reflow soldering is about 250°C, and the first organic solderability film 200 and the second organic solderability film 201 will not decompose at high temperature during the welding process.

[0032] As an example, the first solder layer 300 and the second solder layer 301 are formed by a stencil opening printing method.

[0033] Next, see Figure 3 , executing step S2: placing the welding assembly in a reflow oven to weld the object to be welded and the copper-clad ceramic substrate 1, wherein during the welding process, the flux removes the organic solderability film between the copper foil and the solder layer, and retains the organic solderability film in the non-welding area to protect the copper foil in the non-welding area from being oxidized during the welding process.

[0034] As an example, the first solder layer 300 and the second solder layer 301 include solder paste layers, wherein the first solder layer 300 and the second solder layer 301 contain flux. The inventor of the present application has found through a series of research and verification that solder absorbs heat when it melts. When the number of the soldering assemblies in the same carrier is large and the spacing is small, the soldering assemblies aggregate, resulting in an increase in temperature, which increases the oxidation reaction rate of the copper foil. In the present application, since the copper surface of the copper-clad ceramic substrate 1 is provided with the first organic solder protection film 200 and the second organic solder protection film 201, the first organic solder protection film 200 covers the surface of the first copper foil 101, and the second organic solder protection film 201 covers the surface of the second copper foil 102, so that oxygen does not contact the copper surface of the copper-clad ceramic substrate 1, that is, in the present application, the organic solder protection film is provided on the copper surface of the copper-clad ceramic substrate 1 to resist the oxidation effect of oxygen on the copper surface of the copper-clad ceramic substrate 1, thereby preventing the copper-clad ceramic substrate 1 from being oxidized during the reflow soldering process.

[0035] As an example, during the welding process, the first organic solder protection film 200 located in the welding area is removed by the flux in the first solder layer 300, which will not affect the welding of the first copper foil 101 and the first solder layer 300; the second organic solder protection film 201 located in the welding area is removed by the flux in the second solder layer 301, which will not affect the welding of the second copper foil 102 and the second solder layer 301; and in the non-welding area, the organic solder protection film will not be removed to protect the copper foil from oxidation during the welding process.

[0036] As an example, the welding assembly is transferred by a carrier, and the reflow oven is divided into a preheating zone, a high-temperature welding zone and a cooling zone. The carrier is transferred by a chain, thereby driving the welding assembly to be transferred; wherein, after a single carrier is transferred to the high-temperature welding zone, a solid cover is buckled down and fitted with a rubber ring to form a sealed chamber, and then vacuum high-temperature welding is performed. After welding is completed, it is transferred to the cooling zone for cooling, and another carrier enters the high-temperature welding zone for welding, and so on.

[0037] As an example, as described above, the aggregation of the soldering assemblies in a single carrier causes the temperature to rise, which increases the oxidation reaction rate of the copper foil. In this embodiment, on the basis of providing the organic solder protection film on the copper surface of the copper-clad ceramic substrate 1, the number of the soldering assemblies placed in a single carrier is reduced to reduce the degree of temperature rise caused by the aggregation of the soldering assemblies, and at the same time, the degree to which the single soldering assembly is protected by nitrogen is increased to further prevent the copper surface of the copper-clad ceramic substrate 1 from being oxidized.

[0038] As an example, reducing the number of the welding assemblies placed in the carrier is done on the basis of a reference number, wherein the reference number is the number of the welding assemblies that can be placed on the carrier under a standard state. For example, under a standard state, one carrier can hold eight welding assemblies. In this embodiment, one carrier can be provided to hold six welding assemblies.

[0039] As an example, when the sealed chamber of the high-temperature welding area is evacuated, nitrogen needs to be introduced to reduce the residual oxygen content in the sealed chamber. In the present application, since the copper surface of the copper-clad ceramic substrate 1 is provided with the organic solderability film, it can resist the oxidation reaction between oxygen and the copper surface of the copper-clad ceramic substrate 1, that is, the amount of nitrogen introduced into the sealed chamber can be reduced. The increased cost of the organic solderability film is far lower than the benefit of reduced nitrogen consumption, thereby reducing the welding cost of the power module.

[0040] As an example, the power module reflow soldering method of this embodiment does not need to lower the reflow oven temperature to ensure that cold soldering does not occur.

[0041] As an example, since the number of the welding assemblies in a single carrier is reduced, the spacing between the carriers can be reduced and the chain speed can be increased to compensate for the reduction in hourly output caused by the reduction in the number of welding assemblies in a single carrier; the first solder layer 300 and the second solder layer 301 in the welding assembly will absorb heat during the melting process. When the number of the welding assemblies in the carrier is reduced, the spacing between the welding assemblies increases, so the spacing between the carriers can be reduced and the chain speed can be increased without reducing the welding quality.

[0042] As an example, after reflow soldering is completed, a cleaning process is used to remove the residual organic solderability film and residual flux, and then the second side of the chip 400 is connected to the first copper foil 101 by ultrasonic wire bonding. Since the first copper foil 101 is protected by the first organic solderability film 200 and is not oxidized, it will not affect the wire bonding strength between the chip 400 and the first copper foil 101.

[0043] As an example, taking the EconoPack 3 package as an example, the first organic solderability film 200 and the second organic solderability film 201 are both imidazole organic solderability films, the measured peak temperature of the reflow oven is 245°C, the number of the welding assemblies placed in a single carrier is reduced from the benchmark 8 to 6, the carrier spacing is changed from 70cm to 40cm, the chain speed is changed from 0.8m / min to 1.0m / min, the residual oxygen content is increased from 200±200ppm to 600±200ppm, and the copper surface of the copper-clad ceramic substrate 1 is not oxidized after being taken out of the furnace, the single welding void area of ​​the chip 400 is less than 0.25%, and the total void rate is less than 0.5%, the single welding void area of ​​the heat sink 401 is less than 1%, and the total void rate is less than 3%, and the shear force of 15mil aluminum wire is greater than 2000gf, meeting the product qualification requirements.

[0044] This embodiment also provides a power module, and the power module is obtained by welding using the above-mentioned power module reflow welding method.

[0045] In summary, in the power module reflow soldering method and power module of the present invention, the copper surface of the copper-clad ceramic substrate is provided with an organic solder-protective film, which can resist the oxidation reaction between oxygen and the copper surface of the copper-clad ceramic substrate, and prevent the copper-clad ceramic substrate from being oxidized during the reflow soldering process; in addition, on the basis of the reference number, by reducing the number of welding assemblies in the carrier and increasing the distance between the welding assemblies, the temperature rise caused by the aggregation of the welding assemblies is reduced, and the degree of nitrogen protection of the single welding assembly is also increased, further preventing the copper-clad ceramic substrate from being oxidized during the reflow soldering process. Therefore, the present invention effectively overcomes various shortcomings in the prior art and has a high industrial utilization value.

[0046] 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 familiar with 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 a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. A power module reflow soldering method, characterized in that: The following steps are involved: A welding assembly is provided, comprising a copper-clad ceramic substrate, a solder layer, and an object to be welded, wherein the copper-clad ceramic substrate comprises a ceramic sheet and a copper foil located on a surface of the ceramic sheet, an organic solderability film is provided on a side of the copper foil away from the ceramic sheet, the solder layer is located between the organic solderability film and the object to be welded, and the solder layer contains flux; The soldering assembly is placed in a reflow oven to solder the object to be soldered and the copper-clad ceramic substrate, wherein during the soldering process, the flux removes the organic solderability film between the copper foil and the solder layer, and retains the organic solderability film in the non-soldering area to protect the copper foil in the non-soldering area from oxidation during the soldering process.

2. The power module reflow soldering method according to claim 1, wherein: The soldering assembly is placed on a carrier, and the soldering assembly is transferred to the reflow oven via the carrier.

3. The power module reflow soldering method according to claim 2, wherein: The distance between the welding assemblies is increased by reducing the number of the welding assemblies in the carrier based on the reference number.

4. The power module reflow soldering method according to claim 1, wherein: The organic solderability film includes a rosin-based organic solderability film, an active resin-based organic solderability film, or an imidazole-based organic solderability film.

5. The power module reflow soldering method according to claim 1, wherein: The thickness of the organic solderability film is in the range of 0.2 to 0.5 microns.

6. The power module reflow soldering method according to claim 1, wherein: The object to be soldered includes one or more of a chip and a heat sink.

7. The power module reflow soldering method according to claim 1, wherein: The copper-clad ceramic substrate adopts a double-sided copper-clad ceramic substrate, the copper foil includes a first copper foil and a second copper foil, and the first copper foil and the second copper foil are separated and arranged on two opposite sides of the ceramic sheet; the organic solder protection film includes a first organic solder protection film and a second organic solder protection film, the first organic solder protection film is located on the side of the first copper foil away from the ceramic sheet, and the second organic solder protection film is located on the side of the second copper foil away from the ceramic sheet; the solder layer includes a first solder layer and a second solder layer, the first solder layer is located on the side of the first organic solder protection film away from the ceramic sheet, and the second solder layer is located on the side of the second organic solder protection film away from the ceramic sheet; the object to be soldered includes a chip and a heat sink, the chip is located on the side of the first solder layer away from the ceramic sheet, and the heat sink is located on the side of the second solder layer away from the ceramic sheet.

8. The power module reflow soldering method according to claim 1, wherein: The heat sink is made of copper.

9. The power module reflow soldering method according to claim 1, wherein: The solder layer includes a solder paste layer.

10. A power module, characterized in that: The power module is obtained by soldering by the power module reflow soldering method according to any one of claims 1 to 9.

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