JP7233680B2 - Low temperature bonding method and bonded product - Google Patents

Description

TECHNICAL FIELD The present invention relates to a low-temperature bonding method for various members and a bonded body.

Solders containing lead have conventionally been used as high-temperature solders used at high service temperatures. However, since lead is toxic, there is a marked trend toward lead-free solder. Since there is no other good substitute material for high-temperature solder, lead solder is still used, but from the viewpoint of environmental problems, there is a strong demand for a joint material that does not use lead.

On the other hand, as a substitute material for high-temperature solder, a bonding material using metal nanoparticles mainly composed of noble metals such as silver and gold has been actively developed (for example, Japanese Patent Application Laid-Open No. 2012-046779). However, not only are metal nanoparticles expensive, but organic substances used as metal nanoparticle dispersants and solvents remain in the bonding layer obtained by sintering the metal nanoparticles. In addition, the bonding layer obtained by sintering metal nanoparticles has a large proportion of crystal grain boundaries, which causes a decrease in thermal conductivity and electrical conductivity. In addition, when metal nanoparticles are used for bonding, there is a problem that volume change during bonding increases due to evaporation of organic substances.

In addition, when silver nanoparticles are used, although they are excellent in terms of bonding strength, etc., in addition to the problem of high cost, silver is ionized and reprecipitated outside the circuit when used under high humidity. It is regarded as a problem that a phenomenon called migration, which short-circuits between electrodes, is very likely to occur.

On the other hand, the inventors are studying a bonding method using copper particles, which can be manufactured at a relatively low cost and has excellent migration resistance. For example, Japanese Patent Application Laid-Open No. 2017-74598 describes a bonding method using copper particles, wherein the average particle size of the copper particles is 1 to 50 μm, and the first step of oxidizing the copper particles to obtain surface oxide copper particles. and a second step of interposing surface copper oxide particles between two members to be joined, followed by heating in a reducing atmosphere.

JP 2012-046779 A JP 2017-74598 A

The joint obtained by the bonding method using the copper particles described in Patent Document 2 has excellent migration resistance compared to the joint obtained with silver nanoparticles, and the cost required for bonding is also reduced. can be done.

However, with a bonding agent using particles, the ratio of dispersant and solvent is high. For example, when the area to be bonded has a large area of several square millimeters or more, "organic substances tend to remain even after heating", " There is an unavoidable problem that the volume change is large due to the evaporation of organic matter. In addition, the use of fine particles deteriorates storage stability and handleability, and it is difficult to say that the price is sufficiently reduced as a general-purpose bonding agent.

In view of the above circumstances, an object of the present invention is to enable bonding of various members at low temperature without using a dispersant or solvent, and to obtain a bonded portion having high strength and excellent migration resistance. , to provide a low-cost and simple joining method for replacing high-lead solder. Another object of the present invention is to provide a bonded body having a dense and homogeneous bonding layer and having high strength and excellent migration resistance.

As a result of intensive research on low-temperature bonding methods to achieve the above object, the inventors of the present invention have found that the above object can be achieved by reducing a copper sheet oxidized under appropriate conditions while interposing it between the surfaces to be joined. The present inventors have found that it is extremely effective in doing so, and have arrived at the present invention.

That is, the present invention
a first step of interposing a copper sheet having an oxidized surface between one member to be joined and the other member to be joined to form a joint interface;
a second step of heating the interface to be joined in a reducing atmosphere;
A low-temperature bonding method characterized by:

The copper oxide formed on the surface of the copper sheet is reduced in the second step to produce copper having a microscopic shape, and at the same time, the copper is sintered to form one member to be joined and the other member to be joined through the copper sheet. A joining member can be joined. Since copper sheets can be manufactured at a lower cost than general copper particles, the cost of joining can be greatly reduced.

In addition, the use of a copper sheet completely eliminates the need for an organic coating layer to ensure the dispersibility of the metal nanoparticles, which was essential in conventional bonding compositions using metal nanoparticles. Residual organic constituents in the bonding layer and the volume change of the bonding layer during the bonding process can be prevented.

In addition, the structure of the bonding layer formed by sintering the metal particles largely depends on the metal particles. For example, when the particle size of the metal particles is small, the proportion of grain boundaries increases. On the other hand, the structure of the copper sheet can be changed to any desired state by heat treatment or the like of the copper sheet used, and it is also possible to reduce the ratio of grain boundaries. As a result, a joint having excellent thermal conductivity and electrical conductivity can be obtained.

Here, the copper sheet may be oxidized after being placed at the interface to be joined. For example, oxidation and reduction can proceed as a series of processes by performing oxidation treatment using a heating apparatus capable of controlling the atmosphere used in the second step. In this case, the object can be easily achieved by changing the atmosphere of the heating device. By separating the oxidation treatment from the bonding process, the state of oxidation of the surface of the copper sheet can be controlled more precisely. It is preferred to place the sheet at the interface to be joined.

In the low temperature bonding method of the present invention, the surface of the copper sheet has unevenness with an average roughness (Ra) of 1 μm or less due to copper oxide, and in the second step, the copper oxide is reduced by the heating to copper It is preferable that particles are generated and the copper particles are sintered to join the one member to be joined and the copper sheet, and the other member to be joined and the copper sheet.

The surface of the copper sheet has irregularities with an average roughness (Ra) of 1 μm or less due to copper oxide, so that the reduction in the second step produces fine-shaped copper particles having sufficient low-temperature sinterability. can be done. As a result, a bonding temperature and a bonding strength equivalent to those of a bonding method using metal nanoparticles can be achieved. The copper particles are produced by reduction of copper oxide formed on the surface of the copper sheet, and do not need to be completely particulate.

In the low-temperature bonding method of the present invention, it is preferable that in the second step, the reducing atmosphere is a formic acid atmosphere, and the heating temperature and heating time are 150 to 350° C. and 1 to 60 minutes, respectively.

In the low-temperature bonding method of the present invention, it is necessary to reduce copper oxide on the surface of the copper sheet in the second step. Here, as long as the copper oxide is reduced to form fine copper particles, the reducing atmosphere, treatment temperature, and the like are not particularly limited, and as the reducing atmosphere, a formic acid atmosphere, an atmosphere containing hydrogen, or the like can be used. However, the reducing atmosphere is preferably a formic acid atmosphere, and more preferably the surface copper oxide is held in a formic acid atmosphere at 150 to 350° C. for 1 to 60 minutes. By holding the copper oxide in a formic acid atmosphere at 150 to 350° C. for 1 to 60 minutes, the copper oxide coating the surface of the copper sheet can be efficiently and easily reduced, and the copper oxide is coated with fine copper particles. A polished copper sheet can be obtained. In addition, safety can be ensured by using a formic acid atmosphere instead of an atmosphere containing hydrogen.

Further, in the low-temperature bonding method of the present invention, it is preferable that the atmosphere for the oxidation is air, and the oxidation temperature and oxidation time are 150 to 350° C. and 5 to 40 minutes, respectively. In the atmosphere, copper oxide can be formed on the surface of the copper sheet by heating at 150 ° C. or higher, and by heating at 350 ° C. or lower, the formation of a dense and thick copper oxide layer reduces the low temperature of copper after reduction. A decrease in sinterability can be suppressed. In addition, by setting the oxidation time to 5 minutes or more, copper oxide can be uniformly formed on the surface of the copper sheet, and by setting it to 40 minutes or less, the copper oxide is coarsened, resulting in low-temperature sintering of copper. It is possible to suppress the decline in sexuality.

Further, in the low-temperature bonding method of the present invention, it is preferable that the one material to be bonded and/or the other material to be bonded is copper or a copper alloy. The type of material to be joined is not particularly limited as long as the effect of the present invention is not impaired, and conventionally known various members can be joined. Affinity with the fine copper particles generated on the surface of the surface is increased, and a more reliable bonded body can be obtained.

Further, in the low-temperature bonding method of the present invention, it is preferable that a pressure of 1 to 50 MPa is applied substantially perpendicularly to the interface to be bonded in the second step. In the conventional bonding method using metal nanoparticles, since the bonding layer is formed by the metal nanoparticle sintered body, a firing process accompanied by pressure is required to obtain a sufficient density. application has greatly restricted the application range of the bonding method. On the other hand, in the low-temperature bonding method of the present invention, in addition to the fact that the copper sheet occupies most of the bonding layer, the generated copper nanoparticles are quickly sintered, so it is possible to apply a pressure of 1 MPa or more. Industrially applicable bonding strength can be obtained. Also, by setting the pressure to 50 MPa or less, it is possible to expand the applicable joints.

Furthermore, in the low-temperature bonding method of the present invention, it is preferable to apply the pressure after the reduction in the second step has progressed. Pressurizing the interface to be joined promotes the joining of copper particles, but inhibits the supply of gas for reducing copper oxide. In particular, there is a risk that the reduction at the center of the joint will be delayed. On the other hand, in the initial stage of the second step, no pressure is applied to promote uniform reduction of copper oxide, and then pressure is applied to obtain a homogeneous joint.

In addition, the present invention
A joined body in which one member to be joined and the other member to be joined are joined,
Having a copper sheet at the joint interface between the one member to be joined and the other member to be joined,
The copper sheet and the one member to be joined are metallurgically joined,
that the copper sheet and the other member to be joined are metallurgically joined;
Also provided is a conjugate characterized by:

In the bonded body of the present invention, most of the bonding layer is a dense copper sheet, which is completely different from the porous bonding layer obtained with a bonding composition such as a metal nanoparticle paste. In addition, since organic components such as dispersants and solvents do not exist, defects caused by volatile components do not exist at the bonding interface between the copper sheet and one member to be bonded and between the copper sheet and the other member to be bonded. Furthermore, the joint interface is firmly formed by metallurgical bonding between the copper particles and the member to be joined, and has extremely high joint strength and reliability as compared with a joined body using an adhesive or the like.

Further, the structure of the copper sheet can be changed to any desired state by heat treatment of the copper sheet to be used, etc., and it is also possible to reduce the proportion of crystal grain boundaries. As a result, it is possible to obtain a joint having excellent thermal conductivity and electrical conductivity. In addition, since it is a dense copper bulk body, it has excellent migration resistance as compared with a bonding layer formed of silver nanoparticles or the like.

In the joined body of the present invention, it is preferable that the copper sheet has a thickness of 10 to 1000 μm. By setting the thickness of the copper sheet to 10 μm or more, in addition to forming a homogeneous bonding interface, it is advantageous in terms of ease of handling and cost. Moreover, by setting the thickness of the copper sheet to 1000 μm or less, it is possible to prevent the ratio of the copper sheet from becoming too high at the joint interface, and to reduce the influence of the copper sheet on the mechanical properties of the joint.

Furthermore, in the joined body of the present invention, it is preferable that the one member to be joined and/or the other member to be joined is copper or a copper alloy. The type of material to be joined is not particularly limited as long as the effect of the present invention is not impaired, and conventionally known various members can be joined. Affinity with the fine copper particles generated on the surface of the surface is increased, and a more reliable bonded body can be obtained.

According to the present invention, it is possible to join various members at low temperature without using a dispersant or solvent, and it is possible to obtain a joint with high strength and excellent migration resistance, low cost and simple high lead. It is possible to provide an alternative joining method to solder. In addition, according to the present invention, it is possible to provide a bonded body having a dense and homogeneous bonding layer and having high strength and excellent migration resistance.

It is process drawing of the low-temperature joining method of this invention. It is a schematic diagram of the low-temperature bonding method of the present invention. 1 is a schematic cross-sectional view of a joined body of the present invention; FIG. 4 is a SEM photograph of the surface of the pure copper sheet before oxidation treatment in Example 1. FIG. 4 is a SEM photograph of the surface of the pure copper sheet after oxidation treatment in Example 1. FIG. It is the XRD pattern from the surface of the pure copper sheet before and after the oxidation treatment in the example. 4 is an appearance photograph showing the state of a bonded test piece in an example. 4 is a graph showing the shear strength of bonded bodies obtained in Examples. 1 is a SEM photograph of a cross section of a joined body obtained in Example 1. FIG. 1 is a SEM photograph of the surface of a pure copper sheet after oxidation and reduction treatment under the treatment conditions of Example 1. FIG. 1 is a SEM photograph of a fracture surface of a joined body obtained in Example 1. FIG. 4 is a SEM photograph of the surface of the pure copper sheet after oxidation and reduction treatment under the treatment conditions of Example 2. FIG. 4 is a SEM photograph of the surface of a pure copper sheet after oxidation treatment under the treatment conditions of Example 3. FIG. 4 is a SEM photograph of the surface of the pure copper sheet after oxidation and reduction treatment under the treatment conditions of Example 3. FIG. 4 is a SEM photograph of a fracture surface of the joined body obtained in Example 3. FIG.

A preferred embodiment of the low-temperature bonding method and bonded body of the present invention will be described in detail below. It should be noted that the following description merely shows one embodiment of the present invention, and the present invention is not limited by these, and redundant description may be omitted.

≪Low temperature bonding method≫
FIG. 1 is a process chart of the low-temperature bonding method of the present invention. The low-temperature bonding method of the present invention achieves bonding by utilizing the reduction of copper oxide formed on the surface of the copper sheet and the low-temperature sinterability of the copper particles generated by the reduction, and the copper sheet is interposed. It includes a first step (S01) of forming an interface to be joined, and a second step (S02) of reducing copper oxide and joining.

(1) First step (S01: formation of interface to be bonded)
FIG. 2 shows a schematic diagram of the low-temperature bonding method of the present invention. In the first step (S01), a copper sheet 6 is interposed between one member 2 to be joined and the other member 4 to be joined to form a joint interface. Since the copper oxide layer 8 exists on the surface of the copper sheet 6, the copper oxide layer 8 and the one member to be joined 2 and the copper oxide layer 8 and the other member to be joined 4 are brought into contact at the interface to be joined. Become. As the copper sheet 6, a metal sheet containing copper as a main component can be used, and a sheet made of pure copper or various copper alloys can be used. Alternatively, by using materials having similar electrical characteristics, etc., a better joint can be obtained.

Here, the copper sheet 6 may be oxidized after the copper sheet 6 is placed on the interface to be joined. For example, oxidation and reduction can proceed as a series of processes by performing oxidation treatment using a heating device capable of controlling the atmosphere used in the second step (S02). In this case, the object can be easily achieved by changing the atmosphere of the heating device. By separating the oxidation treatment from the bonding process, the oxidation state of the surface of the copper sheet 6 can be controlled more precisely. Therefore, when the stability and reliability of bonding are emphasized, it is preferable to arrange a copper sheet 6 whose surface has been oxidized in advance at the interface to be bonded.

As for the oxidation of the copper sheet 6, it is preferable to set the atmosphere to the air, the oxidation temperature and the oxidation time to 150 to 350° C. and 5 to 40 minutes, respectively. In the air, copper oxide can be formed on the surface of the copper sheet 6 by heating at 150° C. or higher, and by heating at 350° C. or lower, a dense and thick copper oxide layer is formed, thereby reducing copper after reduction. It is possible to suppress the deterioration of low-temperature sinterability due to coarsening. Further, by setting the oxidation time to 5 minutes or more, copper oxide can be uniformly formed on the surface of the copper sheet 6, and by setting the oxidation time to 40 minutes or less, the copper obtained by coarsening the copper oxide can be baked at a low temperature. It is possible to suppress the deterioration of the binding property. The more preferred oxidation temperature is 175-325°C, and the most preferred oxidation temperature is 200-300°C. Further, for example, when the oxidation temperature is 300° C., the oxidation time is more preferably 10 to 30 minutes, and the most preferable oxidation time is 15 to 25 minutes.

Due to the formation of the copper oxide layer 8, the surface of the copper sheet 6 preferably has irregularities with an average roughness (Ra) of 1 μm or less. Micro-shaped copper particles having sufficient low-temperature sinterability by reduction in the second step (S02) because the surface of the copper sheet 6 has unevenness with an average roughness (Ra) of 1 μm or less due to copper oxide. can be generated. As a result, a bonding temperature and a bonding strength equivalent to those of a bonding method using metal nanoparticles can be achieved. The copper particles are generated by reduction of the copper oxide layer 8 formed on the surface of the copper sheet 6, and do not need to be completely particulate. good.

Here, the average roughness (Ra) of the copper sheet 6 is more preferably 700 nm or less, and the most preferable average roughness (Ra) is 500 nm or less. By making the copper oxide on the surface of the copper sheet 6 have a fine uneven structure, the copper particles generated in the second step (S02) can be made finer, and more excellent low-temperature sinterability can be exhibited. . Incidentally, the average roughness (Ra) may be measured with a surface roughness meter, or simply obtained from an electron microscope photograph or the like.

The members to be joined (2, 4) that can be used in the present embodiment are not particularly limited as long as joining is achieved by heating and sintering copper particles. It is preferable that the material is heat-resistant to the extent that it does not.

Materials constituting the members to be joined (2, 4) include, for example, polyamide (PA), polyimide (PI), polyamideimide (PAI), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate ( PEN) and other polyesters, polycarbonate (PC), polyethersulfone (PES), vinyl resins, fluororesins, liquid crystal polymers, ceramics, glass, metals, and the like. 2, 4) are preferably used. The members to be joined (2, 4) made of metal are preferable because they have excellent heat resistance and excellent affinity with copper particles. Examples of the metal members to be joined (2, 4) include aluminum and aluminum alloys, iron and iron alloys, titanium and titanium alloys, stainless steel, copper and copper alloys. - It is preferable to use copper and copper alloys because of their excellent thermal conductivity and ductility.

Also, the members to be joined (2, 4) may be in various shapes such as plate-like or strip-like, and may be rigid or flexible. The thickness of the base material can also be selected appropriately. A member having a surface layer formed thereon or a member subjected to surface treatment such as hydrophilic treatment may be used for the purpose of improving adhesiveness or adhesion or for other purposes.

(2) Second step (S02: reduction and bonding of copper oxide)
In the second step (S02), the interface to be joined formed in the first step (S01) is heated in a reducing atmosphere to reduce the copper oxide layer 8 on the surface of the copper sheet 6 to generate fine copper particles. Together with this, it is a step of performing bonding using low-temperature sintering of the copper particles.

The surface of the copper sheet 6 has unevenness with an average roughness (Ra) of 1 μm or less due to copper oxide, so that the reduction in the second step produces fine-shaped copper particles having sufficient low-temperature sinterability. can be done. As a result, a bonding temperature and a bonding strength equivalent to those of a bonding method using metal nanoparticles can be achieved. The particle size of the copper particles is preferably 300 nm or less, more preferably 200 nm or less, and most preferably 100 nm or less. The copper particles are produced by reduction of copper oxide formed on the surface of the copper sheet 6, and do not need to be completely particulate. .

As long as fine copper particles are formed by reducing copper oxide, the reducing atmosphere, treatment temperature, and the like are not particularly limited. is preferably in a formic acid atmosphere, and more preferably, the copper oxide is held in a formic acid atmosphere at 150 to 350° C. for 1 to 60 minutes. By holding the copper oxide in a formic acid atmosphere at 150 to 350° C. for 1 to 60 minutes, the copper oxide layer 8 covering the surface of the copper sheet 6 can be efficiently and simply reduced, and fine copper A copper sheet 6 coated with particles can be obtained. In addition, by using a formic acid atmosphere as the reducing atmosphere, it is possible to ensure the safety of the work and to easily achieve a reduction rate suitable for producing fine copper particles.

More specifically, by setting the holding temperature in the formic acid atmosphere to 150° C. or higher, the reduction of the copper oxide layer 8 can be promoted, and by setting the holding temperature to 350° C. or lower, the copper oxide layer 8 is formed by reduction of the copper oxide layer 8. Formation of voids in the bonding layer due to partial and rapid grain growth of fine copper particles can be suppressed. Furthermore, by setting the holding temperature to 150° C. or higher, the sintering process of the copper particles can proceed, and by setting the holding temperature to 350° C. or lower, it can be suitably used for joining various electronic devices.

Moreover, in the second step (S02), it is preferable to apply a pressure of 1 to 50 MPa substantially perpendicularly to the interface to be joined. In the conventional bonding method using metal nanoparticles, since the bonding layer is formed by the metal nanoparticle sintered body, a firing process accompanied by pressure is required to obtain a sufficient density. application has greatly restricted the application range of the bonding method. On the other hand, in the low-temperature bonding method of the present invention, in addition to the fact that the copper sheet occupies most of the bonding layer, the generated copper nanoparticles are quickly sintered, so it is possible to apply a pressure of 1 MPa or more. Industrially applicable bonding strength can be obtained. Also, by setting the pressure to 50 MPa or less, it is possible to expand the applicable joints. Here, the applied pressure is more preferably 10 to 40 MPa, and the most preferred applied pressure is 20 to 30 MPa.

When pressurization is applied, it is preferably applied after the reduction of the copper oxide layer 8 has progressed. Pressurizing the interface to be joined promotes the joining of copper particles, but inhibits the supply of gas for reducing copper oxide. In particular, there is a risk that the reduction at the center of the joint will be delayed. On the other hand, in the initial stage of the second step (S02), no pressure is applied to promote uniform reduction of the copper oxide layer 8, and then pressure is applied to obtain a homogeneous joint.

The method for raising the temperature of the interface to be bonded is not particularly limited, and for example, baking is performed so that the temperature of the interface to be bonded is, for example, 150 to 350° C., using a conventionally known oven or the like capable of controlling the atmosphere. can be joined by Here, since volatile components such as organic components basically do not exist at the interface to be bonded, it is possible to prevent the bonding layer from becoming porous.

In the present embodiment, the members to be joined (2, 4) may be surface-treated in order to further improve the adhesion between the members to be joined (2, 4) and the joining layer. Examples of surface treatment methods include dry treatments such as corona treatment, plasma treatment, UV treatment, and electron beam treatment, and methods in which a primer layer or a conductive paste-receiving layer is provided in advance on the surface.

≪Joints≫
FIG. 3 is a schematic cross-sectional view of a joined body of the present invention. The joined body 10 is formed by joining one member 2 to be joined and the other member 4 to be joined through a copper sheet 6. More specifically, the copper sheet 6 and the member 2 to be joined are metallurgically joined. , the copper sheet 6 and the member to be joined 4 are metallurgically joined. Here, "metallurgically joined" means that the members (2, 4) and the copper sheet 6 to be joined are not joined using an adhesive layer such as an adhesive or adhesive tape or joined by mechanical fastening. It means that they are directly connected.

In the joined body 10, most of the joining layer 12 is a dense copper sheet 6, which is completely different from a porous joining layer obtained with a joining agent such as a metal nanoparticle paste. In addition, since organic components such as dispersants and solvents do not exist, defects caused by volatile components do not exist at the bonding interfaces between the copper sheet 6 and one member to be joined 2 and between the copper sheet 6 and the other member to be joined 4. .

The structure of the copper sheet 6 can be arbitrarily set by heat treatment or the like of the copper sheet 6 to be used, and it is also possible to reduce the proportion of crystal grain boundaries. As a result, it is possible to obtain a joint having excellent thermal conductivity and electrical conductivity. In addition, it has excellent migration resistance as compared with a bonding layer formed of silver nanoparticles or the like.

The thickness of the copper sheet 6 is preferably 10-1000 μm. By setting the thickness of the copper sheet 6 to 10 μm or more, in addition to forming a homogeneous bonding interface, it is advantageous in terms of ease of handling and cost. In addition, by setting the thickness of the copper sheet 6 to 1000 μm or less, it is possible to suppress the ratio of the copper sheet 6 at the joint interface from becoming too high, and to reduce the influence of the copper sheet 6 on the mechanical properties of the joint. can be done.

The structure of the copper sheet 6 can be arbitrarily set by heat treatment of the copper sheet to be used, and it is also possible to reduce the proportion of crystal grain boundaries. As a result, the joint 10 having excellent thermal conductivity and electrical conductivity can be obtained.

One member to be joined 2 and/or the other member to be joined 4 is preferably copper or a copper alloy. The types of the materials to be joined (2, 4) are not particularly limited as long as the effects of the present invention are not impaired, and various conventionally known members can be used. By using a copper alloy, affinity with fine copper particles generated on the surface of the copper sheet 6 increases, and the bonded body 10 with higher reliability can be obtained.

The joined body 10 can be easily obtained by the above-described low-temperature joining method of the present invention.

Although representative embodiments of the present invention have been described above, the present invention is not limited to these, and various design changes are possible, and all such design changes are included in the technical scope of the present invention. be

The low-temperature bonding method and the bonded product of the present invention will be further described in the following examples, but the present invention is not limited to these examples.

<<Example 1>>
A pure copper sheet having a thickness of 100 μm was oxidized in the air at 300° C. for 20 minutes to form a copper oxide layer on the surface. SEM photographs of the surface of the pure copper sheet before and after the oxidation treatment are shown in FIGS. 4 and 5, respectively. Before oxidation, the surface is extremely smooth, but after oxidation, the copper oxide film forms extremely fine irregularities with an average roughness (Ra) of 1 μm or less. For SEM observation, an ultra-high resolution analytical scanning electron microscope SU-70 manufactured by Hitachi High-Technologies Corporation was used.

FIG. 6 shows the XRD patterns from the surface of the pure copper sheet before and after the oxidation treatment. Although only peaks due to copper (Cu) are detected from the pure copper sheet before oxidation treatment, it can be confirmed that copper oxide (Cu ₂ O) is generated on the surface of the pure copper sheet after oxidation treatment. For the XRD measurement, Ultima IV manufactured by Rigaku Co., Ltd. was used, and Cu-Kα rays (λ=1.5405 Å) were used at an acceleration voltage of 40 kK.

Oxygen-free copper was bonded to each other using an oxidized copper sheet. FIG. 7 shows the bonding test piece made of oxygen-free copper used in the bonding test and the state of the copper sheet placed between the bonding test pieces. Large-diameter and small-diameter cylindrical pure copper members are in contact with each other through an oxidized pure copper sheet to form an interface to be joined (first step). The joint surface of each test piece was finished by lathe processing so that Rmax = 3.2S, and after ultrasonic cleaning in acetone and pickling in hydrochloric acid, it was washed with water and dried before being subjected to the test. provided.

Next, in the state shown in FIG. 7, the small-diameter oxygen-free copper member/pure copper sheet/large-diameter oxygen-free copper member are held in a formic acid atmosphere at 300° C. for 40 minutes to reduce the copper oxide layer on the surface of the pure copper sheet. Microscopic copper particles were generated and sintering of the copper particles was allowed to proceed to obtain the working joined body 1 (second step). While the high temperature was maintained, a pressure of 20 MPa was applied to the interface to be joined in a substantially vertical direction, and the test piece was air-cooled immediately after the test.

The bonded bodies obtained by the bonding test were subjected to a shear test using a bond tester to determine bonding strength. The obtained shear strength is shown in FIG. A joint strength tester (STR-1000) manufactured by Lesca Co., Ltd. was used as the shear tester, and the test was performed under the conditions of a shear rate of 1 mm/min and a shear height of 0.2 mm. The shear strength of the bonded body is about 31.6 MPa, which is sufficiently high for practical use.

FIG. 9 shows a SEM photograph of the cross section of the joined body. Since most of the joint portion is a pure copper sheet, it is extremely dense, and no significant defect is observed in the joint interface between the pure copper sheet and the material to be joined (oxygen-free copper member).

In order to confirm the change in the copper oxide layer due to the reduction treatment, the pure copper sheet subjected to the oxidation treatment was subjected to the reduction treatment under the above conditions. A SEM photograph of the surface of the pure copper sheet after the reduction treatment is shown in FIG. The surface is covered with fused and sintered copper particles, and it is considered that the fine copper particles generated by the reduction treatment are fused and sintered.

FIG. 11 shows a SEM photograph of the fracture surface of the joined body after the shear test. After the shear test, the fracture surface of the bonded body is in the same state as the surface of the pure copper sheet shown in Fig. 10. In the bonding process, the minute copper particles generated by the reduction treatment are fused and sintered to join. seems to be achieved.

<<Example 2>>
A working joined body 2 was obtained in the same manner as in Example 1, except that the reduction treatment time was set to 10 minutes.

The shear strength obtained in the same manner as in Example 1 is shown in FIG. The shear strength of the bonded body is about 26.7 MPa, which is sufficiently high for practical use.

In order to confirm the change of the copper oxide layer due to the reduction treatment, the pure copper sheet subjected to the oxidation treatment was subjected to the reduction treatment for 10 minutes. FIG. 12 shows a SEM photograph of the surface of the pure copper sheet after the reduction treatment. Compared to the reduction time of 40 minutes, the surface roughness is poor, and this shape is considered to be one of the reasons why the shear strength is lower than in the case of the reduction time of 40 minutes.

<<Example 3>>
A joint body 3 was obtained in the same manner as in Example 1, except that the oxidation treatment time was 40 minutes and the reduction treatment time was 20 minutes.

The shear strength obtained in the same manner as in Example 1 is shown in FIG. The shear strength of the joined body is about 32.2 MPa, which is sufficiently high for practical use.

FIG. 6 shows the XRD pattern from the surface of the pure copper sheet after oxidation treatment. It can be confirmed that copper oxides (Cu ₂ O and CuO) are generated on the surface of the pure copper sheet after the oxidation treatment.

In order to confirm the change in the copper oxide layer due to the reduction treatment, the pure copper sheet subjected to the oxidation treatment was subjected to a reduction treatment for 20 minutes. SEM photographs of the surface of the pure copper sheet before and after the reduction treatment are shown in FIGS. 13 and 14, respectively. The surface before the reduction treatment is coated with copper oxide of 100 nm to 300 nm, and the surface after the reduction treatment is covered with copper particles slightly coarser than the copper oxide.

FIG. 15 shows a SEM photograph of the fracture surface of the joined body after the shear test. The fracture surface of the joined body after the shear test is in the same state as the surface of the pure copper sheet shown in FIG.

<<Example 4>>
A working joined body 4 was obtained in the same manner as in Example 3, except that a gold plating layer was formed on the surface of the joining test piece. That is, the joint body 4 is joined by "gold/pure copper sheet/gold".

When the shear strength of the joined body was measured in the same manner as in Example 1, it was found to be 24.6 MPa, which is sufficiently high for practical use. From the results, it was confirmed that the low-temperature bonding method of the present invention is also effective for bonding metal materials other than copper.

2 ... one member to be joined,
4 ... the other member to be joined,
6... copper sheet,
8... copper oxide layer,
10 ... conjugate,
12... Joining layer.

Claims (4)

a first step of interposing a copper sheet having an oxidized surface between one member to be joined and the other member to be joined to form a joint interface;
a second step of heating the interface to be joined in a reducing atmosphere;
The surface of the copper sheet has unevenness with an average roughness (Ra) of 1 μm or less due to copper oxide,
In the second step, the copper oxide is reduced by the heating to generate copper particles,
By sintering the copper particles, the one member to be joined and the copper sheet, and the other member to be joined and the copper sheet are joined,
In the second step,
The reducing atmosphere is a formic acid atmosphere,
The heating temperature and heating time are 150 to 350 ° C. and 1 to 60 minutes, respectively,
The oxidizing atmosphere is atmospheric air,
The oxidation temperature and oxidation time are 150 to 350° C. and 5 to 40 minutes, respectively;
A low-temperature bonding method characterized by: The one material to be joined and/or the other material to be joined is copper or a copper alloy;
The low-temperature bonding method according to claim 1, characterized by: applying a pressure of 1 to 50 MPa substantially perpendicularly to the interface to be bonded in the second step;
The low-temperature bonding method according to claim 1 or 2, characterized by: Applying the pressurization after the reduction in the second step has progressed;
The low-temperature bonding method according to claim 3 , characterized by:

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