JP5656144B2 - Method of joining metal members - Google Patents

Description

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

As information technology advances, the importance of bonding technology that organically connects semiconductor elements and makes them systematic is increasing. For example, it is preferable to apply a low dielectric constant insulating material (low-k material) to the interlayer film for high speed and high performance of the element. However, the low dielectric constant material is mechanically fragile, so it is strong. There is a need for bonding.
In addition, as the miniaturization of connecting parts progresses due to the development of thin and high-layer wiring boards and the pursuit of miniaturization and weight reduction, it is required to minimize the load during joining. The development of low-stress bonding technology that does not add is accelerated.

On the other hand, in recent years, flip chip bonding, in which chip electrodes and substrate electrodes are directly connected to each other, has attracted attention as a bonding method that realizes high-density mounting such as reduction of mounting area, suppression of mounting height, and shortening of transmission paths. Yes.
As shown in FIG. 16, the flip-chip bonding is performed by directly connecting the chip electrode 51 provided on the lower surface of the electronic component 54 such as a logic IC and the substrate electrode 53 on the mounting substrate 52 facing each other. At this time, for example, the chip electrode 51 and the substrate electrode 53 are connected via a connection portion 55 such as a solder bump.

Examples of the flip chip bonding method include thermocompression bonding, ultrasonic bonding, resin bonding, and solder bonding.
For example, in the large-area bonding between various electrode substrates of chip components and heat buffer plates, a thermocompression bonding method in which pressure is applied and heated in a vacuum or in an inert atmosphere is employed.
In this thermocompression bonding, as shown in FIG. 17A, metal bumps 65 and the like are formed and arranged between the electrodes of the mounting substrate 62 and the electrodes of an electronic component 64 such as a logic IC mounted on the mounting substrate 62.
Then, for example, bonding by solid phase diffusion is performed by applying pressure in the direction indicated by arrow A3 while applying heat of about 420 ° C. This method is characterized by high bonding strength.

  In addition, ultrasonic bonding is becoming a mainstream in the recent electronics mounting field as one of the low-temperature connection methods for chip electrodes and substrate electrodes. 17B, the mounting substrate 72 and the electronic component 74 such as a logic IC are brought into contact with each other through a metal bump 75. As shown in FIG. Then, bonding by solid phase diffusion is performed by applying ultrasonic vibration indicated by arrows A4 and A5 while pressing the electronic component 74 in the vertical direction.

  The joining surfaces are rubbed by ultrasonic vibration, the oxide film and the like are removed to expose a clean metal surface, and are joined in a solid state by plastic deformation due to pressurization. This ultrasonic bonding is advantageous in that bonding in a very short time such as about 4 seconds is possible.

In addition, as shown in FIG. 17C, for example, a film-type anisotropic resin conductive film 85 is sandwiched between the substrate 82 and the bumps of the electronic component 84, and the resin bonding is performed by heat-pressing the substrate using conductive particles contained in the resin. 82 and the electronic component 84 are electrically connected.
In particular, an anisotropic resin can obtain conductivity in the vertical direction and insulation in the horizontal direction with respect to the bonding surface, so that the mounting substrate 82 and the electronic component 84 are connected without short-circuiting adjacent bumps. be able to.
In many cases, such resin bonding is performed at a temperature of about 260 ° C., and the bonding can be performed in a relatively short time of about several tens of seconds.

  In addition, as shown in FIG. 17D, solder bonding that has been conventionally used is a liquid phase diffusion by placing and heating a solder bump 95 having a melting point lower than that of the base material between the electrodes of the mounting substrate 92 and the electronic component 94. Join by. In solder bonding, heating at about 180 to 240 ° C. is generally performed, and various chip components can be mounted simultaneously.

On the other hand, Patent Document 1 below discloses that an oxide film on a metal surface is removed by an oxide film removing liquid when solid-phase bonding is performed between metals each made of copper. In this method, the surface of the metal to be joined is brought into contact with an oxide film removing solution, and the surface is brought into contact with the oxide film removing solution, and is pressurized and heated, whereby a temperature of 300 ° C. or less and 4 kg / mm ² It is said that it is possible to join at a surface pressure of.

JP 2006-334652 A

  However, since the above-described resin bonding requires a high temperature of, for example, about 260 ° C., the load applied to the electronic component and the substrate is large. In addition, there is a problem in that the bonding strength is inferior because of adhesion by a resin, and various chip components cannot be simultaneously mounted on the substrate.

Also, since solder bonding also requires a high bonding temperature, the mounted electronic components may be damaged.
In particular, as shown in FIG. 18, when the electronic component 97 is to be joined by the solder bump 98 formed on the substrate 96, the solder bump 98 is likely to spread in the directions of arrows A6 and A7 due to the liquid phase joining. . For this reason, the adjacent solder bumps 98 come into contact with each other at the location shown in the region T1, causing a short circuit.
The possibility of such a short-circuit has not been completely eliminated due to its high temperature and high load even in solid phase bonding such as thermocompression bonding and ultrasonic vibration.

There is also a reflow connection method using a solder paste, but this also similarly causes a short circuit between adjacent electrodes when the molten solder flows out from the electrode region. Such solder wetting and spreading is a major factor that hinders the narrow pitching of very small electronic component wiring.
Furthermore, since the thermal expansion coefficients of the electronic component and the substrate are different, the joint portion connected by the reflowed solder is subjected to shear stress and strain, and there is a risk of reducing the connection reliability.

Further, even in the case of solid-phase diffusion bonding that enables bonding at a relatively low temperature, thermocompression may require a high bonding temperature of 420 ° C., and distortion due to bending of the chip or the substrate is likely to occur. Furthermore, in the conventional construction method, it is necessary to apply a high pressure for a long time, for example, about 5 minutes, and the damage to the chip parts is also great.
In addition, although ultrasonic bonding is used as one of low-temperature connection methods, positioning accuracy is poor, and the bonding interface is locally hot (several hundred degrees C). In addition, since the substrate needs to be vapor-phase cleaned, there is a cost, and there is a concern that the element is damaged by ultrasonic vibration.

  As another method using the solid phase diffusion phenomenon, for example, there is a bonding surface activated bonding. In this method, an oxide layer, an adsorption layer, and the like on the bonding surface are removed by a physical method such as ion bombardment. However, in this bonding method, it is necessary to perform the process from surface activation treatment to bonding in a vacuum, which is not only applicable, but also increases the cost of the apparatus and the cost.

  This invention is made | formed in view of the said subject, and it aims at providing the method of joining metals in low temperature and a solid-phase state by an inexpensive method.

In order to solve the above problems, a metal member joining method according to the present invention includes a step of boiling the surfaces of metal members containing at least one tin in a formic acid or citric acid solution, and the surface of the boiled metal member. Bringing them together, heating and pressurizing.

According to the method for joining metal members of the present invention, the oxide on the surface of the metal member can be effectively removed. Furthermore, the surface of the metal member from which the oxide has been removed can be replaced with a formic acid compound in which the metal and formic acid have reacted or a citric acid compound in which the metal and citric acid have reacted. Thereby, it can also suppress that the metal surface from which the oxide was removed again oxidizes.
Since the formic acid compound and the citrate compound formed on the surface of the metal member are aggregated at the bonding interface at the time of bonding, the actual bonding area where the metal member to be bonded directly contacts can be increased.
In particular, the formic acid compound and the citric acid compound usually aggregate at a lower temperature than the metal oxide formed on the metal surface. For this reason, it becomes possible to ensure a large true junction area at a lower temperature than in the past.
Further, since bonding is performed by solid phase diffusion, it is possible to eliminate the risk of short-circuiting due to contact between adjacent bumps as in the prior art.

  According to the present invention, a larger true bonding area can be ensured at a lower temperature than in the past. For this reason, joining of a metal member can be performed at temperature lower than before.

It is a flowchart which shows the procedure of the joining method of the metal by this invention. A is a front view showing a state in which metal members subjected to surface modification face each other in a metal bonding method according to the present invention, and B is a method of bringing together metal members subjected to surface modification together, heating and It is a schematic diagram which shows a mode that a pressure is applied. It is explanatory drawing which shows the joining strength at the time of joining a tin block and a copper block by this invention and the conventional method. It is an enlarged view which shows the torn surface of the metal block joined by this invention. A, C, and E are copper side fracture surfaces. B, D, and F are tin side fracture surfaces. A, D, and G are the enlarged photograph figures which image | photographed the copper block side fracture surface with the electron microscope. B, E, and H are enlarged photograph diagrams in which the distribution of copper on the copper block side fracture surface is analyzed. C, F, and I are enlarged photograph diagrams in which the distribution of tin on the copper block side fracture surface is analyzed. It is an enlarged view which shows the torn surface of the metal block joined by the conventional method. A, C, and E are copper side fracture surfaces, and B, D, and F are tin side fracture surfaces. A, D, and G are enlarged photograph diagrams obtained by photographing the copper block side fracture surface of FIG. 6 with an electron microscope, and B, E, and H are enlarged photograph diagrams that analyze the distribution of copper on the copper block side fracture surface. is there. C, F, and I are enlarged photograph diagrams in which the distribution of tin on the copper block side fracture surface is analyzed. . It is explanatory drawing which shows the joining strength at the time of joining a tin block and a nickel block by this invention and the conventional method. It is an enlarged photograph figure which shows the fracture surface of the metal block joined by this invention, A, C, E, and G show a nickel side fracture surface, B, D, F, and H show a tin side fracture surface. It is an enlarged photograph figure. It is an enlarged photograph figure which shows the fracture surface of the metal block joined by the conventional method, A, C, E, and G show a nickel side fracture surface, B, D, F, and H show a tin side fracture surface It is an enlarged photograph figure shown. It is explanatory drawing which shows the joining strength of the tin block and copper block at the time of performing surface modification of a joining surface with a citric acid. It is explanatory drawing which shows the joint strength of a tin block and a nickel block at the time of performing surface modification of a joint surface with a citric acid. A is a schematic view showing a chip laminated in multiple layers according to the present invention, and B is a schematic perspective view showing an IC chip surface-mounted on a mounting substrate according to the present invention. It is a schematic front view which shows the electronic component mounted on the heat dissipation copper plate by this invention. It is a schematic perspective view which shows a mode that an electronic component is packaged by this invention. It is a schematic diagram which shows the mode of flip chip joining. A is a schematic diagram illustrating bonding by thermocompression bonding, B is a schematic diagram illustrating ultrasonic bonding, C is a schematic diagram illustrating resin bonding, and D is a schematic diagram illustrating solder bonding. It is a schematic diagram which shows a mode that an adjacent solder bump contacts and short-circuits.

  Hereinafter, although the example of the form for implementing this invention is demonstrated, this invention is not limited to the following examples.

1. Embodiment FIG. 1 is a flowchart showing a procedure of a metal joining method according to this embodiment (hereinafter referred to as “this embodiment”).
In the metal joining method of this example, first, a metal member to be joined is boiled in a formic acid or citric acid solution (step S1). Alternatively, instead of boiling in formic acid or citric acid solution, the surface of the metal member to be bonded to formic acid vapor or vapor containing citric acid may be exposed. Note that at least one metal member to be joined at this time contains tin.

  Thereby, the oxide film on the surface of the metal member is removed, and in the metal member containing tin, the tin oxide is replaced with the formic acid compound and the citric acid compound (step S2). By replacing the metal surface with a formic acid compound or a citric acid compound, the surface of the metal member can be prevented from being oxidized again.

  Next, the metal member is taken out from the formic acid, citric acid solution or formic acid or citric acid vapor, and dried (step S3). After drying, it is preferable to immediately move to the next step so that an oxide film is not formed on the surface of the metal member.

  Next, the surfaces of metal members such as metal terminals boiled in formic acid or exposed to formic acid vapor are brought together, heated and pressurized (step S4). Thereby, metal members are joined by solid phase diffusion.

That is, in the joining method according to this example, as shown in FIG. 2A, for example, the metal electrode 3 on the substrate 2 and the electrode 1 containing tin provided on the electronic component 4 are previously surface-modified with formic acid.
2B, the electrode 1 of the electronic component 4 and the electrode 3 of the substrate 2 are brought together, and pressure is applied in the direction of the arrow A1 while applying the heat indicated by the arrow A2, so that the electrodes 1 and 3 Can be joined by solid phase diffusion.

  This method can be classified as a thermocompression bonding method, but bonding at a very low temperature is possible even when compared with the conventional thermocompression bonding method. In the method according to this example, the metal surface is modified by removing a chemical adsorption layer such as an oxide film that inhibits bonding or replacing it with a formic acid compound. For this reason, it becomes possible to perform bonding at a temperature and load lower than those of the prior art.

2. EXAMPLES Examples of the method for joining metal members according to the present invention will be described below.
[1] Example 1
A rectangular parallelepiped tin block whose surface is smoothed by electrolytic polishing and a copper block whose surface is mechanically polished up to 4000 using emery paper are immersed in boiling formic acid (98%) and boiled for 10 minutes. went.
Next, the tin block and the copper block were taken out from the formic acid, the surfaces of each other were brought together, and the tin block and the copper block were joined by applying a pressure of 7 MPa for 30 minutes at a joining temperature of 130 ° C.
[2] Example 2
The joining was performed in the same manner as in Example 1 except that the joining temperature of the copper block and the tin block was 140 ° C.
[3] Example 3
The joining was performed in the same manner as in Example 1 except that the joining temperature of the copper block and the tin block was 150 ° C.
[4] Example 4
The joining was performed in the same manner as in Example 1 except that the metal block to be joined was a tin block and a nickel block.
[5] Example 5
The joining was performed in the same manner as in Example 4 except that the joining temperature of the tin block and the nickel block was 120 ° C.
[6] Example 6
The joining was performed in the same manner as in Example 4 except that the joining temperature of the tin block and the nickel block was 140 ° C.
[7] Example 7
The joining was performed in the same manner as in Example 4 except that the joining temperature of the tin block and the nickel block was 150 ° C.
[8] Example 8
A solution prepared by adding 20 g of citric acid powder to 100 ml of distilled water was boiled at about 100 ° C. And the same tin block and copper block as the above-mentioned Example 1 were immersed in this boiling solution, and it boiled for 5 minutes.
After boiling, the tin block and the copper block were washed with distilled water for 10 seconds, and then distilled water remaining on the joint surface was removed by air blow and dried.
Then, the tin block and the copper block were taken out and bonded to each other, and bonding was performed at a bonding temperature of 120 ° C. to 140 ° C. in 10 ° C. increments. In addition, the pressure at the time of joining applied the pressure of 7 MPa for 30 minutes.
[9] Example 9
The metal blocks to be joined were tin blocks and nickel blocks, and joining was carried out in the same manner as in Example 8 except that joining was carried out at temperatures of 140 ° C. to 170 ° C. in increments of 10 ° C., respectively.
[10] Comparative Example 1
A rectangular parallelepiped tin block and a copper block whose surfaces were smoothed by electropolishing were put together, and a pressure of 7 MPa was applied for 30 minutes at a joining temperature of 160 ° C. to join the tin block and the copper block.
[11] Comparative example 2
The joining was performed in the same manner as Comparative Example 1 except that the joining temperature of the tin block and the copper block was 170 ° C.
[12] Comparative Example 3
The joining was performed in the same manner as in Comparative Example 1 except that the joining temperature of the tin block and the copper block was 180 ° C.
[13] Comparative example 4
The joining was performed in the same manner as in Comparative Example 1 except that the joining temperature of the tin block and the copper block was 190 ° C.
[14] Comparative Example 5
Joining was performed in the same manner as in Comparative Example 1 except that the metal block to be joined was a tin block and a nickel block, and the joining temperature was 180 ° C.
[15] Comparative Example 6
The joining was performed in the same manner as in Comparative Example 1 except that the joining temperature of the tin block and the nickel block was 190 ° C.
[16] Comparative example 7
The joining was performed in the same manner as in Comparative Example 1 except that the joining temperature of the tin block and the nickel block was 200 ° C.
[17] Comparative example 8
The joining was performed in the same manner as in Comparative Example 1 except that the joining temperature of the tin block and the nickel block was 210 ° C.
[18] Comparative Example 9
The joining was performed in the same manner as Comparative Example 1 except that the joining temperature of the tin block and the nickel block was 220 ° C.

3. Evaluation Results FIG. 3 shows the relationship between bonding strength and bonding temperature by applying the metal joints of Examples 1 to 3 and Comparative Examples 1 to 4 in which tin and copper were bonded to a tensile tester.
The longitudinal direction was the longitudinal direction of the joint surface, and a tensile test was performed in this direction. An Instron type testing machine was used for the tensile test.
The horizontal axis is the bonding temperature, and the vertical axis is the joint strength ratio. The joint strength ratio is the strength ratio of the joint interface with respect to the case where the strength of the tin block itself is 100.
Therefore, when the tensile strength when a metal block joined with tin and copper is broken by a tensile test is σ _j and the tensile strength when a tin block is broken by a tensile test is σ _B , the joint strength ratio Can be represented by (σ _j / σ _B ) × 100.

  Symbol a in FIG. 3 is a joint strength ratio in Examples 1 to 3 in which the tin block and the copper block were surface-modified with formic acid according to the present invention, and symbol b was joined without modification of the surface of the tin block and the copper block. It is a joint strength ratio in Comparative Examples 1-4.

When the surface modification was not performed, the joint strength was zero because the tin block and the copper block were not joined at all when the joining temperature was 160 ° C. And it can confirm that the tin block and the copper block were joined in joining temperature 170 degreeC.
However, bonding is insufficient at this temperature, and peeling from the bonding interface occurs when the tensile load is increased, so the joint strength ratio is as low as about 70.

On the other hand, when the joining temperature is 190 ° C., the joint strength ratio is 100. This is because the bonding strength between the tin block and the copper block exceeds or equals the strength of the tin block itself, so that the fracture occurred in the tin block. Therefore, the tensile strength σ _j when the metal block joined with tin and copper is broken by the tensile test is equal to the tensile strength σ _B when the tin block is broken by the tensile test, and the joint strength The ratio is 100. Therefore, 190 ° C., which is the temperature at this time, can be regarded as a temperature at which tin and copper are completely joined.

On the other hand, when a tin block and a copper block that have been surface-treated with formic acid are joined, a joint strength ratio of a value close to 100 can already be obtained at a joining temperature of 140 ° C.
When the joining temperature is 150 ° C., the joint strength ratio is 100, and it can be seen that high joint strength is obtained at a lower temperature than when the surface treatment is not performed.
That is, by performing the surface treatment with formic acid, the bonding temperature can be significantly reduced from 190 ° C. to 150 ° C. by 40 ° C.

Here, FIG. 4 is an enlarged photograph of the fractured surfaces in Examples 1 to 3.
4A is a fracture surface on the copper block side of Example 1 at a bonding temperature of 130 ° C., and FIG. 4B is a fracture surface on the tin block side.
4C is a fracture surface on the copper block side of Example 2 at a bonding temperature of 140 ° C., and FIG. 4D is a fracture surface on the tin block side.
4E is a fracture surface on the copper block side of Example 3 at a bonding temperature of 150 ° C., and FIG. 4F is a fracture surface on the tin block side.

4A and 4B, when the bonding temperature is 130 ° C. (Example 1), since the bonding is insufficient, the brittle fracture occurs from the bonding interface, and the surfaces of the copper block and the tin block are almost exposed as they are. doing.
In addition, when the bonding temperature is slightly increased to 140 ° C. (Example 2), as shown in FIG. 4C, interface fracture occurs brittlely in the lower region of the fracture surface. On the other hand, in the upper region, the tin block partially left on the copper side is ductilely broken, and the base material fracture and the interface fracture are mixed.
Therefore, in FIG. 4D, which is a fracture surface on the tin block side, interface fracture occurs in the lower region, and the upper region is in a state where only the fragments remaining on the copper block side are missing.

On the other hand, when the bonding temperature is 150 ° C. (Example 3), as shown in FIGS. 4E and 4F, tin remains on the entire bonding interface on the copper side, and after the cross section shrinks in the tin block, the ductility occurs. It has been confirmed that it is broken.
This also shows that the bonding of tin and copper can be performed firmly at a bonding temperature of 150 ° C.

FIG. 5 shows the distribution of copper or tin contained in the fracture surface on the copper block side, using an energy dispersive X-ray analyzer (EDX).
FIG. 5A is an enlarged photograph of a copper block side fractured surface observed at a junction temperature of 130 ° C. (Example 1) with a scanning electron microscope (SEM). FIG. 5B shows the copper distribution in this sectional region as EDX. It was analyzed by. FIG. 5C is an analysis of tin distribution in the same cross-sectional area by EDX.

  FIG. 5D is an enlarged photograph of a copper block side fractured surface observed at a junction temperature of 140 ° C. (Example 2) with a scanning electron microscope (SEM), and FIG. 5E is a distribution of copper in this sectional region. Was analyzed by EDX. FIG. 5F is an analysis of tin distribution in the same cross-sectional area by EDX.

  FIG. 5G is an enlarged photograph of the copper block side fractured surface observed at a junction temperature of 150 ° C. (Example 3) with a scanning electron microscope (SEM). FIG. 5H shows the copper distribution in this sectional region. Was analyzed by EDX. FIG. 5I is an analysis of tin distribution in the same cross-sectional region by EDX.

  In FIGS. 5B, 5E, and 5H in which copper is detected, the darker and lighter regions in the figure contain more copper. Further, in FIGS. 5C, 5F, and 5 where tin is detected, tin is contained more in the lighter and darker regions in the figure.

  In FIG. 5B in which the copper is detected at the fracture surface at a bonding temperature of 130 ° C. (Example 1), the overall density is light and copper is detected on one side. Further, in FIG. 5C in which the tin is detected, the entire surface is dark and light, and tin is hardly detected. From this, it can be seen that when the bonding temperature is 130 ° C., fracture occurs at the bonding interface between the copper block and the tin block.

On the other hand, in FIG. 5E in which copper was detected at the fracture surface at a bonding temperature of 140 ° C. (Example 2), the concentration was generally low, and the measured fracture surface on the copper block side was almost copper. In the upper right in the figure, a dark and light area is locally observed. Moreover, when FIG. 5F which detected the tin is seen, the shading is thin in the same area | region, and it turns out that tin remains here.
Therefore, it can be seen that when the bonding temperature is 140 ° C., interfacial fracture between the copper block and the tin block and base metal fracture of the tin block are mixed.

Further, in FIG. 5H in which copper was detected at a fracture surface with a bonding temperature of 150 ° C. (Example 3), the overall dark and light spots increased, and in FIG. It can be confirmed that tin is attached to the entire fractured surface on the copper side.
This also shows that when the joining temperature is 150 ° C., the base material fracture of the tin block occurs.

On the other hand, the photograph figure which observed macroscopically the fracture surface of Comparative Examples 2-4 which did not perform the surface modification with formic acid is shown in FIG.
6A is a fracture surface on the copper block side when the bonding temperature is 170 ° C., and FIG. 6B is a fracture surface on the tin block side at this time.
6C is a fracture surface on the copper block side when the bonding temperature is 180 ° C., and FIG. 6D is a fracture surface on the tin block side at this time.
6E is a fracture surface on the copper block side when the bonding temperature is 190 ° C., and FIG. 6F is a fracture surface on the tin block side at this time.

6A and 6B in the case where the joining temperature is 170 ° C. (Comparative Example 2), almost no deformation is observed on the fracture surface, and brittle interface fracture occurs. For this reason, as shown in FIG. 3, the joint strength ratio is as low as about 70.
On the other hand, in FIG. 6C and FIG. 6D when the joining temperature is 180 ° C. (Comparative Example 3), it can be confirmed that the joining strength is increased and the ductile fracture is caused with the cross-sectional contraction.
Also, in FIGS. 6E and 6F when the bonding temperature is 190 ° C. (Comparative Example 4), the ductile fracture with the cross-sectional shrinkage is similarly performed.

FIG. 7 shows the distribution of copper and tin by measuring the fracture surface on the copper block side of Comparative Examples 2 to 4 using an energy dispersive X-ray analyzer (EDX).
FIG. 7A is an enlarged photograph obtained by observing a fracture surface on the copper block side at a bonding temperature of 170 ° C. (Comparative Example 2) with an electron microscope (SEM). FIG. 7B shows the case where copper in the same region is detected by EDX, and FIG. 7C shows the case where tin is detected.

Moreover, FIG. 7D is the enlarged photograph figure which observed the fracture surface by the side of the copper block in joining temperature 180 degreeC (comparative example 3) with the electron microscope (SEM). FIG. 7E shows copper detected in the same region by EDX, and FIG. 7F shows tin detected.
Moreover, FIG. 7G is the enlarged photograph figure which observed the fracture surface by the side of the copper block in joining temperature 190 degreeC (comparative example 4) with the electron microscope (SEM). FIG. 7H shows the detection of copper in the same region by EDX, and FIG. 7I shows the detection of tin.

  7B, E, and H, the lighter the tint, the more copper is distributed on the surface. In FIGS. 7C, F, and I, the lighter the tint, the more tin is distributed on the surface.

  In FIG. 7B, the density is thin over almost the entire surface, and copper is detected. On the other hand, in FIG. 7C, the entire surface is thick and tin is not detected. That is, it can be seen that when the bonding temperature is 170 ° C. (Comparative Example 2), the fracture surface on the copper block side is formed only of copper, and interface fracture occurs.

  When FIG. 7E and F which is a case where joining temperature is 180 degreeC (comparative example 3) are seen, the thin and light location is scattered in FIG. 7F. This is because tin adheres to the copper block side fracture surface, and the interface fracture and the base material fracture of the tin block are mixed. Therefore, in FIGS. 6C and 6D, the joint strength ratio remains at a value of about 80 (FIG. 3) while showing ductile fracture accompanied by cross-sectional shrinkage.

  When the bonding temperature is 190 ° C. (Comparative Example 4), the entire surface is dark in FIG. 7H, and the entire surface is light in FIG. 7I. That is, since tin adheres to the entire fracture surface on the copper block side, it can be seen that the base material fracture of the tin block occurs at this joining temperature.

In general, it is believed that tin is immediately covered by an oxide film when exposed to the atmosphere. The tin oxide film and the tin matrix phase react in the following general formulas (1) to (3) by boiling in formic acid.
Sn + 2HCOOH → Sn (HCOO) ₂ + H ₂ ↑ (1)
SnO + 2HCOOH → Sn (HCOO) ₂ + H ₂ O ↑ (2)
SnO ₂ + 2HCOOH → Sn (HCOO) ₂ + H ₂ ↑ + O ₂ ↑ (3)

  Therefore, the tin block is immersed in formic acid and boiled, whereby the oxide film on the surface of the tin block is removed and replaced with highly active tin formate. This substitution can also be performed by exposure to formic acid vapor.

When surface modification with formic acid is not performed, if bonding is performed at a low temperature, inclusions such as oxide films remain as they are at the bonding interface, so that the bonding strength is reduced and fracture from the bonding interface occurs.
On the other hand, when the bonding temperature is increased, the oxide film such as tin oxide formed on the tin surface is aggregated, and the aggregated particles are coarsened and the distribution density is lowered as the bonding temperature is increased. That is, at the time of joining at high temperature, inclusions such as oxides intervening between tin and copper agglomerate to increase the actual joining area where tin and copper are in direct contact with each other. It is done.

  On the other hand, when tin oxide is substituted with tin formate as in this embodiment, tin formate aggregates and coarsens at a lower temperature than tin oxide, and the real junction area increases. For this reason, it is possible to realize bonding at a temperature of 150 ° C., which is significantly lower than the conventional temperature.

  Moreover, since it can be performed by a simple method of being immersed in formic acid and boiled or exposed to formic acid vapor, it does not require expensive equipment as in the conventional surface activation method, and the cost can be reduced. it can. Furthermore, since the bonding temperature can be greatly lowered, if the present invention is applied to mounting of electronic components, short circuit between bumps and thermal damage to surrounding elements can be suppressed.

Next, FIG. 8 shows the relationship between the bonding temperature and the joint strength ratio in Examples 4 to 7 and Comparative Examples 5 to 9 where tin and nickel are bonded.
Symbol c is the joint strength ratio in Examples 4-7, and symbol d is the joint strength ratio in Comparative Examples 5-9.

In Comparative Examples 5 to 9 of the symbol d, the joint strength ratio is as low as about 30 when the joining temperature is 180 ° C. to 200 ° C., and starts increasing at 210 ° C. And when joining temperature is 220 degreeC, joint strength ratio will be set to 100, and the base material fracture | rupture has occurred from the tin block.
On the other hand, in the case of Examples 4 to 7 in which surface modification with formic acid was performed, a joint strength ratio of 80 or higher was already obtained at a joining temperature of 130 ° C., and the joint strength ratio became 100 at a temperature as low as 150 ° C. .
Thus, even when tin and nickel are joined, the joining temperature at which the base metal breaks is reduced from 220 ° C. to 150 ° C. by 70 ° C. By performing surface modification with formic acid, the joining temperature is greatly increased. Can be lowered.

The photograph figure which observed the tin block side fracture surface in these Examples 4-7 and the nickel block side fracture surface macroscopically is shown in FIG.
FIG. 9A is a fracture surface on the nickel block side at a bonding temperature of 120 ° C. (Example 4), and FIG. 9B is a fracture surface on the tin block side.
FIG. 9C is a fracture surface on the nickel block side at a bonding temperature of 130 ° C. (Example 5), and FIG. 9D is a fracture surface on the tin block side.
FIG. 9E is a fracture surface on the nickel block side at a bonding temperature of 140 ° C. (Example 6), and FIG. 9F is a fracture surface on the tin block side.
9G is a fracture surface on the nickel block side at a bonding temperature of 150 ° C. (Example 7), and FIG. 9H is a fracture surface on the tin block side.

  As shown in FIG. 8, when the joining temperature was 120 ° C., the joint strength ratio was zero, and the joint was not joined at all. For this reason, as shown in FIGS. 9A and 9B, no change occurs on the joint surface.

  Also, in FIGS. 9C and 9D where the joining temperature is 130 ° C., the fracture surface is hardly deformed, and it can be seen that interface fracture occurs at the joining interface. For this reason, as shown in FIG. 8, the joint strength ratio has a value of about 80.

  In FIGS. 9E and F, which are cases where the joining temperature is 140 ° C., the ductile fracture surface with the shrinkage of the cross section is shown with the base material being fractured by stretching the tin block. However, in the fracture surface on the nickel side in FIG. 9E, the nickel block surface is exposed in the lower right region in the figure, and here, interface fracture occurs. For this reason, although the joint strength ratio shows a high value, it does not reach 100.

  In addition, when the bonding temperature is 150 ° C., as shown in FIGS. 9G and 9H, the tin remains on the entire bonding surface on the nickel side, and the duct is fractured ductilely with cross-sectional shrinkage. Complete base material breakage of

On the other hand, the photograph figure which image | photographed the fracture surface of the comparative examples 5 and 7-9 which did not perform formic acid processing is shown in FIG.
FIG. 10A is a fracture surface on the nickel block side at a bonding temperature of 180 ° C. (Comparative Example 5), and FIG. 10B is a fracture surface on the tin side.
10C is a fracture surface on the nickel block side at a bonding temperature of 200 ° C. (Comparative Example 7), and FIG. 10D is a fracture surface on the tin side.
10E is a fracture surface on the nickel block side at a bonding temperature of 210 ° C. (Comparative Example 8), and FIG. 10F is a fracture surface on the tin block side.
FIG. 10G is a fracture surface on the nickel block side at a bonding temperature of 220 ° C. (Comparative Example 9), and FIG. 10H is a fracture surface on the tin block side.

When the joining temperature is 180 ° C. to 210 ° C., as shown in FIGS. 10A to F, no major deformation occurs on the fracture surface, and interface fracture occurs at the joining surface. For this reason, as shown in FIG. 8, the value of the joint strength ratio is low.
On the other hand, when the bonding temperature is 220 ° C., the nickel block surface is exposed in the left region of the fracture surface shown in FIG. 10G. Further, in the right region, a part of the tin block that has been fractured in the base material remains, and the interface fracture and the base material fracture are mixed.

Although the interface fracture has occurred, the joint strength ratio becomes 100 as shown in FIG. 8 because the tin block is strongly restrained by the plastic deformation of the nickel block.
That is, as a result of the plastic deformation of the joining surface during the joining and the extrusion of the joining surface as burrs, the nickel block restrains the tin block. In addition, since the amount of plastic deformation increases as the joining temperature increases, it is considered that such high tensile strength was exhibited.

As described above, even when nickel and tin are joined, the joining surfaces can be treated with formic acid so that the joining can be performed at a temperature significantly lower than that of the prior art.
This is because, as in the case of joining copper and tin, tin formate substituted from tin oxide by formic acid aggregates and coarsens at a low joining temperature and decreases the distribution density, so that the true joining area can be increased. Because.
Moreover, this effect | action is acquired with respect to the metal by which the tin oxide is formed in the surface. Therefore, if at least one of the metals contains tin, the bonding temperature can be effectively lowered by performing surface modification with formic acid.

Next, the result of Example 8 which modified the surface of the tin block and the copper block with citric acid is shown in FIG.
The horizontal axis is the bonding temperature, and the vertical axis is the joint strength ratio. Symbol e is the result of Example 8 in which modification with citric acid was performed, and symbol f is the result of Comparative Examples 1 to 4 that were joined without modification.

  As shown by the symbol f, in the case where bonding is performed without modification, the base material of tin is broken at a bonding temperature of 190 ° C. On the other hand, in the symbol “e” subjected to the surface modification with citric acid, the joint strength ratio becomes 100 at the joining temperature of 140 ° C., and it can be seen that the base material fracture of the tin block occurs. That is, when surface modification with citric acid is performed, bonding can be performed at a temperature lower by 50 ° C. than in the past.

In this way, the joining temperature can be significantly lowered by immersing the metal joining surface in the boiled citric acid solution. By immersing the metal in a boiling citric acid solution, the metal surface oxide is removed and replaced with a citric acid compound. Since this citric acid compound also aggregates and coarsens at a lower temperature than tin oxide and increases the actual bonding area, it is considered that bonding at a lower temperature than before is possible.
The surface can also be modified by exposing the metal bonding surface to steam containing citric acid.

FIG. 12 shows the results of Example 9 in which the tin block and the nickel block were subjected to surface modification with citric acid.
The horizontal axis is the bonding temperature, and the vertical axis is the tensile strength when breakage occurs.
Symbol g is the result of Example 9 when surface modification was performed with citric acid, and symbol h is the result of Comparative Examples 5 to 8 where bonding was performed without modification.

It can be seen that when bonding is performed without surface modification with citric acid, a bonding strength slightly exceeding 9 MPa is obtained at a bonding temperature of 210 ° C. as indicated by symbol h. On the other hand, in symbol g, which is a case where the modification with citric acid is performed, a bonding strength of about 9 MPa has already been obtained at a bonding temperature of 170 ° C., and an equivalent strength is obtained at a temperature lower by about 40 ° C. than conventional. Is obtained.
Thus, even when tin and nickel are joined, it is possible to obtain the same strength at a lower temperature than before by performing surface modification with citric acid.

As described above, according to the present invention, it is possible to significantly reduce the metal bonding temperature by performing surface modification with formic acid or citric acid.
Therefore, for example, as shown in FIG. 13A, the present invention can be effectively applied in the case where chips 14, 15, 16, and 17 such as logic ICs are stacked on the mounting substrate 12. For example, at least one of the substrate-side electrode 13 and the chip-side electrode 11 is formed of a material containing tin, and bonding is performed by the method according to the present invention. As a result, since bonding at a low temperature and a low load becomes possible, even when chips are stacked in multiple layers, they can be mounted without damaging each chip.
Of course, as shown in FIG. 13B, the present invention can also be suitably used in normal surface mounting in which the IC chip 24 and the like are connected to the mounting substrate 22 by the electrode 25.

  Moreover, you may apply to the planar body which connects not only the above-mentioned dot connection of the point connection but a plane, and the connection which connects by a line. For example, as shown in FIG. 14, when an electronic component 34 such as a power module is mounted on the heat radiating copper plate 32, for example, the joint portion 35 is formed of a metal containing tin. And by surface-modifying by this invention and joining to the thermal radiation copper plate 32, it can mount firmly, reducing the thermal load to an electronic component.

Further, as shown in FIG. 15, the electronic component 47 such as an integrated circuit may be sealed with a lower container 42 and an upper lid 44, and this package may be applied to a substrate or the like (not shown) with an external electrode 46. .
In this case, for example, the joint 45 between the lower container 42 and the upper lid 44 is formed of a metal containing tin, and after surface modification with, for example, formic acid or citric acid, the upper lid 44 is put on and joined. Thereby, low-temperature and strong bonding is possible, and packaging with reduced heat load applied to the electronic component 47 can be performed.

  In the above, embodiment of the metal joining method by the present invention, an example, and an evaluation result were explained. The present invention is not limited to the above-described embodiment, and it is needless to say that the present invention includes various conceivable forms without departing from the gist of the present invention described in the claims.

  51... Chip electrode, 2, 12, 22, 52, 62, 72, 92... Mounting board, 3... Metal electrode, 11, 13, 25. ... Chip, 24 ... IC chip, 32 ... Heat dissipating copper plate, 35, 45 ... Joint portion, 42 ... Lower container, 44 ... Upper lid, 46 ... External electrode, 53 ... Substrate electrodes, 4,34,47,54,64,74,84,94,97 ... Electronic components, 55 ... Connection parts, 65,75 ... Metal bumps, 82, 96・ Board, 85... Anisotropic resin conductive film, 95, 98... Solder bump

Claims (3)

Boiling the surfaces of metal members containing tin in at least one in a formic acid or citric acid solution;
Bringing together the surfaces of the boiled metal members, heating and pressurizing;
A method for joining metal members.   The method for joining metal members according to claim 1, further comprising a step of cleaning the surface of the metal member after boiling the metal member.   The method for joining metal members according to claim 1, further comprising a step of drying the surface of the metal member.