JP2020040114A - Metal joining method - Google Patents

Abstract

To realize, without using a flux or special device, a solid phase joint that is less in void and highly reliable.SOLUTION: A joining method for a first metal member having a joint face containing Sn, Ni, or Cu or an alloy thereof and a second member, comprises: a step of measuring a change of electrodes 42, 43 in relation to a reference electrode with time under constant current, each electrode being composed of the Sn, Ni, or Cu or the alloy thereof and immersed in a formic acid solution 41; a step of keeping the joint face of the first metal member in contact with the formic acid solution or formic acid vapor for a predetermined contact time; and a step of abutting the joint face of the first metal member, kept in contact with the formic acid solution or the formic acid vapor, and the second metal member and applying heat and pressure to them. The predetermined contact time is determined based on the potential.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a method for joining metal members.

  The connection between an electronic component and an electronic circuit board for operating an electronic device is generally connected by solder. For example, in order to release heat from a semiconductor element such as a semiconductor device, the semiconductor element and a heat radiating plate for the semiconductor element may be joined by soldering. In general, such soldering is performed by arranging solder on the joint surface of a heat sink, arranging a semiconductor element on the solder, melting the solder by heating, and then cooling by cooling. The solder thus obtained is solidified, and the semiconductor element and the heat sink are joined by soldering. In the joining of a heat exchanger used for heating or cooling, for example, copper or aluminum is used, and diffusion joining, brazing, or soldering using an insert metal is performed.

  2. Description of the Related Art In recent years, electronic devices have been reduced in size and weight, have higher functions, and have higher frequencies corresponding to higher speeds. Along with this, the electronic circuit boards constituting the same have been required to be mounted at a higher density than ever before. For example, fine bonding for narrowing the pitch and downsizing of semiconductor elements is required. In the fine bonding, there is a concern that the reliability may be reduced due to voids, for example, the bonding strength and the insulation strength may be reduced. Further, in a heat exchanger, fine joining is required along with fine processing of a heat radiation path and the like.

  Heretofore, there has been known a method of joining metals at a low temperature for the purpose of improving component reliability and reducing thermal strain and in a solid phase for the purpose of improving positioning accuracy (for example, see Patent Document 1). . Further, there is a soldering method in which the pressure is reduced from the atmosphere during the melting of the solder to reduce voids (voids) in the solder joint (for example, see Patent Document 2).

JP 2011-200930 A JP-A-2007-915

In general, tin (Sn) is a metal material used for a solder base material, plating, insert metal, and the like, and is relatively inexpensive among metal materials and has a low melting point of 232 ° C. However, tin is easily oxidized, and the surface is oxidized in the air, and a tin oxide film such as SnO or SnO ₂ is formed on the entire surface. The tin oxide film formed on the solder or plating surface is a very stable compound. For example, the melting point of tin (II) oxide is 1080 ° C., and it is interposed at the time of joining metal members and hinders joining. Therefore, at the time of joining the metal members, a flux that reacts with the tin oxide film to dissolve and remove the tin oxide film is used. The flux is generally composed of a rosin (pine), which is a natural resin, an activator such as an organic acid, a hydrohalide, or an amino acid, and a solvent. In addition, members such as copper (Cu) and nickel (Ni) are used for bonding parts of recent semiconductor element forming parts, and various functional Ni platings such as Ni plating are often treated. As for Sn, the surface is easily oxidized similarly to Sn, and it is necessary to remove the surface oxide by some method. As a method for removing oxides, a flux, a reducing gas such as hydrogen, and formic acid are generally used.

When a flux is used, gases such as H ₂ O and H ₂ are generated during the volatilization and redox reaction of the solvent component, and a thermal decomposition gas of a metal salt such as CO ₂ is generated due to the activator. The gas left in the metal joint becomes a void, and the void in the metal joint causes a reduction in the fatigue life of the metal joint and a deterioration in thermal conductivity. In particular, in the case of fine bonding, the volume ratio per void alone becomes large with respect to the volume of the bonding portion, and there is a concern about a reduction in reliability such as a reduction in bonding strength. In particular, voids are likely to remain in the metal joints that have been die-bonded or in the solder joints of flat heat exchangers.

  If the bonding is performed under a reducing gas and under reduced pressure as in Patent Document 1, it is considered that the influence of the flux is small. In addition, the reducing gas is advantageous in that a gas that causes voids is not generated and good bonding is easily obtained, and a cleaning step after bonding is not required. However, in order to prevent gas leakage into the environment, a vacuum chamber that can be replaced under reduced pressure is required, and various vacuum evacuation systems and other abatement devices for combustion and the like are provided, which increases equipment costs. Further, when hydrogen gas is used, a temperature generally exceeding 250 ° C. is required to sufficiently reduce the oxide film on the surface of the member, and is not necessarily a versatile treatment method in consideration of the heat resistance temperature of the constituent members.

  In order to solve such a problem, the present invention controls the reduction condition of an oxide film formed on the surface of a metal member including Sn, Ni or Cu on a bonding surface, and can reduce voids in a metal bonding portion at low cost. An object is to provide a low-temperature and simple metal bonding method.

  According to one embodiment, the present invention is a method for joining a first metal member having a joint surface including Sn, Ni, or Cu, or an alloy thereof, and a second metal member, wherein the first metal member has a formic acid solution. Measuring the time-dependent change of the potential of the immersed electrode made of Sn, Ni, or Cu, or an alloy thereof with respect to the reference electrode under a constant current; and bonding the bonding surface of the first metal member to a formic acid solution. Or a step of contacting formic acid vapor for a predetermined contact time, a step of abutting the joining surface of the first metal member contacted with the formic acid solution or formic acid vapor and the second metal member, and heating and pressurizing. And the predetermined contact time is determined based on the potential.

  In the bonding method, it is preferable that the predetermined contact time is determined based on a time from the start of the potential measurement to a time point at which the potential inflection point is shown.

  In the bonding method, it is preferable that the first metal member is an alloy member of Sn and Au or an alloy plating member of Sn and Au.

  In the bonding method, it is preferable that the second metal member is Sn or a Sn-plated member.

  In the bonding method, it is preferable that the first metal member is a Ni or Ni plated member, and the second metal member is a Sn or Sn alloy member or a plated member thereof.

  In the bonding method, it is preferable that the first metal member is a Cu or Cu plated member, and the second metal member is a Sn or Sn alloy member or a plated member thereof.

  According to another aspect, the present invention relates to a joined body joined by the method according to any of the above.

  According to the present invention, the potential change in formic acid of Sn, Ni, or Cu, or an alloy thereof is measured, and the time for performing the reduction treatment of the oxide film of the metal member including these at the bonding surface is controlled, and the method is simple and easy. By performing high-precision preliminary reduction treatment on the metal member, it is possible to reduce voids after joining and improve joining quality. By using the present invention, for example, it is possible to improve the bonding quality and reliability of fine bonding accompanying fine pitching and downsizing of semiconductor elements and fine processing of a heat exchanger. Also, the present invention can improve the bonding quality in solid-phase bonding and liquid-phase bonding.

FIG. 1 is a conceptual cross-sectional view illustrating a test piece of a first metal member and a second metal member to be joined in a joining method according to an embodiment of the present invention. FIG. 2 is a conceptual diagram showing an example of an apparatus used for measuring a potential of a metal member in a bonding method according to an embodiment of the present invention. FIG. 3 is a conceptual diagram showing a typical example of a graph showing the result of measuring the potential of a tin or tin oxide electrode immersed in a formic acid solution with respect to a reference electrode over time under a constant current. FIG. 4 is a graph showing the results of measuring the potential of a tin oxide electrode immersed in a formic acid solution with respect to a reference electrode over time under a constant current in Example 1. FIG. 5 is a graph showing the results of measuring the potential of a tin electrode immersed in a formic acid solution with respect to a reference electrode over time under a constant current in Example 2. FIG. 6 shows a cross section of the joint of the joint test piece when the contact time with formic acid is 2.5 sec. FIG. 6 (a) is a scanning micrograph of the cross section of the joint, and FIG. 6 (b) is FIG. It is an enlarged photograph of (a). 7A and 7B show a cross section of the joint of the joint test piece when the contact time with formic acid is 30 sec. FIG. 7A is a scanning micrograph of the cross section of the joint, and FIG. 7B is FIG. It is an enlarged photograph of). FIG. 8 shows a cross section of the joint of the joint test piece when the contact time with formic acid is 300 sec. FIG. 8 (a) is a scanning micrograph of the cross section of the joint, and FIG. 8 (b) is FIG. It is an enlarged photograph of). FIG. 9 is a graph showing the normalized tensile strength of each of the joint test pieces having different contact times with formic acid.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the present invention is not limited by the embodiments described below.

According to one embodiment, the present invention relates to a method of joining a first metal member having a joint surface including Sn, Ni, or Cu, or an alloy thereof, and a second metal member. The joining method of the present invention includes the following steps.
(1) a step of measuring a temporal change under a constant current of a potential of an electrode made of Sn, Ni, Cu, or an alloy thereof and immersed in a formic acid solution with respect to a reference electrode; and (2) the first metal member. (3) contacting the bonding surface of the first metal member contacted with the formic acid solution or formic acid vapor with the formic acid solution or formic acid vapor for a predetermined contact time and the second metal member Abutting, heating and pressurizing step Here, the predetermined contact time is determined based on the potential.

  In the bonding method according to the present embodiment, the members to be bonded are at least two metal members, and at least one of the metal members has a bonding surface including Sn, Ni, Cu, or an alloy thereof. In this embodiment, the metal member is referred to as a first metal member. "Having a bonding surface containing Sn, Ni, or Cu, or an alloy thereof" means that at least a portion to be a bonding surface of the first metal member is Sn, Ni, or Cu or an alloy thereof, It means having a plating film made of Ni, Cu or an alloy thereof. When there is a plating film made of Sn, Ni, or Cu or an alloy thereof, the metal member to be plated is not particularly limited.

  Although Sn, Ni, or Cu, or an alloy thereof is not particularly limited, examples of an alloy containing Sn include SnAu (Sn 20% by mass, Au 80% by mass) and SnAu (Sn 10% by mass, Au 90% by mass). The alloy containing Ni includes SnNi, FeNi, SnAgCuNi, and TiNi, and the alloy containing Cu includes CuZn, FeZnCu, FeSnCu, NiSnCu, and SnAgCu. Hereinafter, in this specification, “Sn, Ni, or Cu, or an alloy thereof” is also referred to as a Sn alloy or the like. “Sn, Ni, or Cu, or an alloy thereof” usually exists in a state where its surface is oxidized to form an oxide at normal temperature and normal atmospheric pressure. Therefore, hereinafter, when referring to “Sn, Ni, or Cu, or an alloy thereof” of the bonding surface of the first metal member in the present invention, “Sn, Ni, or oxidized to form an oxide” Cu or its alloys ".

  The second metal member is not particularly limited, and may be the same as or different from the first metal member. Examples of specific combinations of the metal forming the bonding surface of the first metal member and the metal forming the bonding surface of the second metal member include AuSn and Sn, AuSn and Cu, AuSn and Ni, and AuSn. Examples include, but are not limited to, SnNi, AuSn and AuSn, AuSn and Pt, AuSn and Ag, AuSn and Ru, AuSn and Pd, and Sn and Sn.

  FIG. 1 is a cross-sectional view conceptually showing a state in which a first metal member to be joined and a second metal member are butted. In the illustrated embodiment, the first metal member 1 includes a Cu material 11, a Ni plating film 12 formed thereon, and an AuSn plating film 13 (Au 80%, Sn 20%) formed so as to cover the Ni plating film 12. It is a member. Therefore, the metal forming the joining surface of the first metal member is AuSn. On the other hand, the second metal member 2 serving as a mating member is a member in which a Ni plating film 22 is formed on a Cu material 21 and an Sn plating film 23 (Sn 100%) is further formed so as to cover the Ni plating film 22. . The composition and structure of the member shown in FIG. 1 are merely examples, and the present invention is not limited to the illustrated embodiment.

(1) Potential Measurement Step In the first step, a step of measuring a change with time of a potential of the electrode made of Sn, Ni, Cu, or an alloy thereof immersed in a formic acid solution under a constant current is performed. The potential of the electrode such as a Sn alloy is measured with respect to the reference electrode. This step is preferably performed using an alternative material such as a Sn alloy having the same composition, instead of the first metal member itself having a Sn alloy or the like on the joint surface. Therefore, for example, in the first metal member shown in FIG. 1, the metal forming the bonding surface is AuSn ((Au80%, Sn20%)), and the alternative material for measuring the potential in the first step has the same composition. In some cases, the substitute material may have not only the same composition but also substantially the same oxidation state.This step is a step in which the first metal member and formic acid are used in a subsequent step. It is performed to determine the contact time.

  FIG. 1 is a conceptual diagram illustrating an example of an apparatus used for the potential measurement in the first step. The potential measuring device 4 shown in FIG. 1 includes a working electrode 42 made of an alternative material such as a Sn alloy, a counter electrode 43, a reference electrode 44, and a constant current source / electrometer 45. Although not shown, a device for outputting a measurement result may be further included. The working electrode 42, the counter electrode 43, and the reference electrode 44 are immersed in the formic acid solution 41. An electrode having high stability such as a platinum electrode is used as the counter electrode 43, and an AgCl electrode can be used as the reference electrode. However, the electrode is not limited thereto, and is generally used as a counter electrode and a reference electrode in an electrochemical measurement. Electrodes can be used. As the formic acid solution 41, it is preferable to use a solution having the same conditions as the solution concentration and temperature used in the subsequent formic acid contact step.

  In the measurement, the value of the current I from the constant current source / electrometer 45 is kept constant, and the potential change of the working electrode 42 made of an alternative material such as a Sn alloy with respect to the reference electrode 44 is measured with time. FIG. 3 is a graph showing a typical example of a potential measurement result. When the horizontal axis is time and the vertical axis is potential, a time T at which the slope of the graph changes from negative to positive is obtained from the measurement result. In order to improve the accuracy of the calculation of the time T, it is desirable to observe the crystal morphology of the substitute material and to analyze the state of chemical bonding such as ESCA (Electron Spectroscopy for Chemical Analysis) together with the measurement of the potential.

  The mechanism of the above measurement will be described for the case where the measurement target is tin. When the alternative material used for the potential measurement is tin, the time T obtained in the above measurement can be regarded as the time when the oxide film on the tin surface is dissolved and removed. Considering the general reaction formula of tin oxide and formic acid, the reduction reaction of tin oxide (I) or tin oxide (II) proceeds, and when the oxide is removed, the slope of the potential change graph changes. is there. After time T, the reaction between the metal Sn and formic acid may proceed excessively, possibly generating an undesirable metal salt. Therefore, the time T can be set as an appropriate time for formic acid contact in the next step. Similarly, for Ni and Cu, the time T can be regarded as the time when the film of the surface oxide is dissolved and removed, and an appropriate time T for formic acid contact in the next step can be obtained in the same manner.

(2) Formic acid contacting step The second step is a step of bringing the joining surface of the first metal member into contact with a formic acid solution or formic acid vapor for a predetermined contact time. Here, the predetermined contact time is determined based on the time T measured using the substitute material in the first step. The predetermined contact time can be preferably time T seconds, but may be at least time (T-30) seconds and within the range of time (T + 90) seconds, and the time (T-1) ) Seconds or more, and more preferably within the range of about (T + 60) seconds. Conditions such as the concentration and temperature of formic acid used in the second step are preferably the same as the measurement conditions in the first step. This is for obtaining an appropriate reduced state of an oxide such as a Sn alloy with formic acid. Further, after the formic acid contacting step, it is more preferable to perform a step of removing formic acid residue on the joint surface of the first metal member. The step of removing formic acid residues includes, but is not limited to, pure water washing, drying after pure washing, and the like.

  The first step and the second step are performed when only one of the two members to be bonded corresponds to the first metal member having a bonding surface including Sn, Ni, or Cu, or an alloy thereof. Perform only one of them. When both of the two members to be joined correspond to the first metal member having the joining surface including Sn, Ni, or Cu, or an alloy thereof, and the composition of the joining surfaces is the same, The substitute material of the first step is common, and the second step may be performed only on one of the metal members. When both of the two members to be joined correspond to the first metal member having the joining surface including Sn, Ni, or Cu, or an alloy thereof, and the compositions of the joining surfaces are different, one of the two members is used. , The first step and the second step are sufficient, but the first step and the second step may be performed for both. In the case of using either one, the first step and the second step may be performed on a member that is more easily reduced.

(3) Heating / Pressurizing Step The third step is a step of abutting the joining surface of the first metal member brought into contact with the formic acid solution or formic acid vapor and the second metal member, and heating and pressing. In solid diffusion bonding, bonding can be performed at a temperature substantially equal to or lower than the melting point. Preferably, the bonding can be performed at a temperature substantially equal to or lower than the melting point and equal to or higher than 140 ° C. For example, since the melting point of Au20Sn (Au80%, Sn20%) is 284 ° C., it is desirable to apply a temperature around the melting point. The higher the pressure, the easier the joining becomes, but in this joining, it is desirable that the pressure be at least half the 0.2% proof stress of the metal. When the pressure exceeds 0.2% proof stress, plastic deformation of the material is possible, and diffusion becomes easy by plastic deformation. Since AuSn has a 0.2% proof stress of about 80 MPa or more, it is desirable that the bonding be performed at a pressure equal to or higher than that. However, for a material containing 10% or more of Sn, the 0.2% proof stress of Sn is about 35 to 40 MPa at room temperature, and the 0.2% proof stress at about 100 ° C. is 10 MPa, which is as high as 10 MPa. Since it can be easily plastically deformed, a pressing force of at least 10 MPa or more is required at a temperature equal to or higher than room temperature, depending on the heating temperature. The bonding atmosphere is preferably in an atmosphere of an inert gas such as nitrogen.

  According to the present embodiment, the conditions for reduction treatment of the metal member before joining can be controlled, and oxides on the joining surface can be appropriately removed, so that suitable joining with few occurrences of voids and the like is realized. be able to.

  Hereinafter, the present invention will be described in more detail with reference to examples of the present invention. However, the present invention is not limited to the scope of the following examples.

  As the first metal member and the second metal member to be joined, those having the composition and structure illustrated and illustrated in FIG. 1 were used. As a measuring device, a tin bar (100% tin) is immersed in a formic acid solution (25 ° C., formic acid concentration: 98% (JIS standard)) as a substitute, and platinum is used as a counter electrode. An electrode was used, and a silver chloride was used as a reference electrode.

  The potential of the tin round bar (100% tin) was measured as an alternative material subjected to forced oxidation, and an overview photograph of the surface of the tin round bar was taken (photograph not shown). The forced oxidation was performed in air at 200 ° C. for 30 minutes in order to emphasize tint on the tin surface. As shown in FIG. 4, at time (i) (60 sec) before reaching time T, the tint on the surface of the tin round bar is lighter than the tint after oxidation treatment (dark brown), and has no tint. (Silver). From this, it is confirmed that the tin oxide film on the surface of the tin round bar has been dissolved and removed at the time (i). At (ii) (600 sec) after the time T was reached, it was found that the tin particles had different gloss at each grain boundary and were corroded (oxidized).

  FIG. 5 shows the measurement results of the potential of the tin round bar without the oxidation treatment. From FIG. 5, it has been found that the time T at which the inflection point occurs is 30 sec in the potential change with time.

  FIGS. 6 to 8 show the results of a joining experiment using the test piece of FIG. 1 reflecting the measurement results of FIG. The bonding conditions were a maximum temperature of 300 ° C., a melting time of 5 minutes, and a pressure of 1 MPa, and a bonding experiment was performed in a nitrogen atmosphere. FIG. 6 is a cross section of the joint using the metal member immersed in formic acid for 2.5 sec (before reaching time T), FIG. 7 is a cross section of the joint using the metal member after 30 sec of formic acid immersion (time T), and FIG. The scanning micrograph of the cross section of the joining part using the metal member immersed in formic acid for 300 sec (after reaching time T) is shown. Table 1 shows the results of semi-quantitative analysis of the joints by SEM-EDX. The white portions in Table 1 are portions indicated by reference numeral 31 in FIGS. 6 to 8, and the gray portions are portions indicated by reference numeral 32 in FIGS. 6 to 8.

  As shown in FIG. 6B, in the metal member subjected to the formic acid treatment for 2.5 sec, the voids 33 are concentrated at the tin plating 2 side interface, which is assumed to be the interface between the tin plating 2 and the AuSn alloy plating 1 at the joint 3. Admitted. That is, it is inferred that the tin oxide film was not sufficiently reduced and the tin oxide film could not be completely dissolved and removed, and that the interdiffusion was hindered. FIG. 8B shows that in the formic acid treatment for 300 sec, more voids 33 were generated in the entire joint 3 than in the other joints. This is presumed to be due to excessive generation of decomposition products (water and gas) of the metal salt. It is presumed that the mutual diffusion is inhibited due to the void 33. On the other hand, from FIG. 7B, in the formic acid treatment for 30 sec, a gold-rich compound (white portion 31) was generated on the tin plating side, and the mutual diffusion became better than the other joints 3. Was admitted. This is presumed to be due to the fact that the amount of metal salt produced was smaller than the other joints 3 and the amount of decomposition products of the metal salt was smaller.

  A tensile strength test was performed on each of the joined test pieces shown in FIGS. The tensile strength test conditions were measured at a strain rate of 0.2% / sec until a fracture occurred. FIG. 9 shows the values normalized by setting the bonding strength of the test piece in the formic acid treatment for 30 seconds to 1. As shown in FIG. 9, the bonding strength is highest under the condition of formic acid treatment for 30 sec. This indicates that the bonding cross-sectional area is large, that is, the voids in the metal bonding are reduced, and the bonding quality is better than other bondings.

  INDUSTRIAL APPLICABILITY The joining method according to the present invention can be suitably used in fine processing for directly joining metal members without using a joining material such as solder.

1 first metal member 11 plating (AuSn)
12 Ni
13 Cu
2 Second metal member 21 Plating (Sn)
22 Ni
23 Cu
3 bonding layer 31 white part 32 gray part 33 void 4 potential measuring device 41 formic acid solution 42 working electrode 43 counter electrode 44 reference electrode 45 constant current source / electrometer

Claims (7)

A method of joining a first metal member having a joining surface including Sn, Ni, or Cu, or an alloy thereof, and a second metal member,
Immersed in a formic acid solution, the Sn, Ni, or Cu, or the potential of the electrode made of an alloy thereof with respect to the reference electrode, a step of measuring the change over time under a constant current,
Contacting the bonding surface of the first metal member with formic acid solution or formic acid vapor for a predetermined contact time;
Abutting the joining surface of the first metal member contacted with the formic acid solution or formic acid vapor and the second metal member, and heating and pressurizing;
The bonding method, wherein the predetermined contact time is determined based on the potential.   The method according to claim 1, wherein the predetermined contact time is determined based on a time from the start of the potential measurement to a time point indicating an inflection point of the potential.   The method according to claim 1, wherein the first metal member is an alloy member of Sn and Au or an alloy plating member of Sn and Au.   The method according to claim 3, wherein the second metal member is a Sn or a Sn-plated member.   The method according to claim 1, wherein the first metal member is a Ni or Ni plated member, and the second metal member is a Sn or Sn alloy member or a plated member thereof.   The method according to claim 1, wherein the first metal member is a Cu or Cu plated member, and the second metal member is a Sn or Sn alloy member or a plated member thereof.   A joined body joined by the method according to claim 1.