JP5849422B2 - Pb-free solder - Google Patents

Description

  The present invention relates to a Pb-free solder containing no Pb.

  The electronic circuit module component is a component in which a plurality of electronic components are mounted on a substrate with solder to have a single function. When such an electronic component is mounted on a substrate of an electronic device, the terminal electrode of the electronic circuit module component and the terminal electrode of the substrate of the electronic device are joined with solder. Conventionally, SnPb-based material solder has been used to join electronic components and electronic circuit module components. However, Pb-free solder has been used against the background of environmental issues, and Pb-free solder is used except for automobile-related and special cases. Has been.

  When an electronic circuit module component is mounted on a substrate using solder, reflow is required to melt the solder. During this reflow, the solder joining the electronic component in the electronic circuit module component and the substrate may melt and scatter or the solder may move. In order to avoid this, it is necessary to join the electronic component in the electronic circuit module component and the substrate using solder that does not melt at the reflow temperature when the electronic circuit module component is mounted on the substrate. For example, Patent Document 1 describes a powder solder material composed of 10 to 25% by mass of Ag, 5 to 10% by mass of Cu, and the balance of Sn and inevitable impurities.

Japanese Patent Laying-Open No. 2007-268568 (0013) JP-A-5-154687 JP 2005-319470 A

  Since the solder of patent document 1 is excellent in intensity | strength and heat resistance, while using the said solder, while the joint strength of terminal electrodes improves, the heat resistance of joining also improves. However, in recent electronic circuit module components, further strength and heat resistance are required for joining the terminal electrodes of the electronic components and the terminal electrodes of the substrate. Moreover, in joining by solder, in order to join terminal electrodes reliably and to improve the reliability of both joining, it is calculated | required that the wettability of the molten solder and a terminal electrode is good. The object of the present invention is to improve the strength and heat resistance after the Pb-free solder is solidified after melting, and to improve the wettability between the molten Pb-free solder and the terminal electrode.

  In order to solve the above-described problems and achieve the object, the present inventors have conducted extensive research on Pb-free solder. As a result, the first metal material containing at least Sn and the second metal containing at least a Ni-Fe alloy. It has been found that the bonding strength and heat resistance between the terminal electrodes are improved by combining the materials. However, it has been found that the Ni—Fe alloy is effective in improving the bonding strength and heat resistance, but reduces the wettability. As a result of studies on this point, the present inventors have found that the Ni—Fe alloy present in the molten solder is caused by a high diffusion rate into the first metal material. The present invention has been completed based on such findings.

  In order to solve the above-described problems and achieve the object, the present invention provides a method in which the surface of the first metal particle mainly composed of Sn and the core particle mainly composed of Ni—Fe alloy is Sn or Sn and alloy. And a second metal particle covered with at least one coating layer mainly composed of a metal for forming a Pb-free solder.

  When this Pb-free solder is first melted, the coating layer diffuses into the Sn phase to form an intermetallic compound. For this reason, while the Pb-free solder is melted, the diffusion of Ni into the Sn phase and the reaction of both are suppressed. As a result, in the Pb-free solder, a decrease in wettability during melting is suppressed. When such a Pb-free solder is used to join the terminal electrode of the electronic component and the terminal electrode of the circuit board, the molten Pb-free solder forms a fillet on the terminal electrode. The reliability of bonding can be improved.

  Moreover, when the terminal electrode of an electronic component and the terminal electrode of a circuit board are joined using such Pb-free solder, the structure of the joining metal obtained by melting after the Pb-free solder is melted is Ni—Fe. A first metal phase containing an alloy, a second metal phase surrounding the first metal phase mainly composed of Sn alloy, and a third metal phase mainly composed of Sn appear. And since a joining metal is strengthened when the different phase of a Ni-Fe alloy appears in Sn phase, and melting | fusing point rises, joining strength and heat resistance improve.

  As a desirable aspect of the present invention, the coating layer in contact with the surface of the core particles preferably has a thickness of 0.05 μm or more and 5 μm or less. By making the thickness of the coating layer provided on the surface of the core particles within the above range, the heat resistance of the joint metal obtained by curing the melted Pb-free solder while ensuring the wettability when the Pb-free solder is melted. Can be improved. In the case of a Sn single layer, the thickness is preferably 0.05 μm or more and 2 μm or less.

  As a desirable aspect of the present invention, it is preferable that the second metal particles have a plurality of the coating layers. When the second metal particles have a plurality of coating layers, different characteristics can be given to the respective coating layers, and the bonding strength, heat resistance, and wettability can be improved in a balanced manner.

  As a desirable embodiment of the present invention, the total thickness of the coating layers other than the innermost coating layer is preferably 0.05 μm or more and 5 μm or less. In this way, it is possible to improve the heat resistance of the joining metal obtained by curing the melted Pb-free solder while securing the wettability when the Pb-free solder is melted.

  As a desirable mode of the present invention, it is preferable that the innermost coating layer is mainly composed of Cu. By doing in this way, in the molten Pb-free solder, an alloy layer of Cu and Sn is formed on the surface of the core particle, and the reaction between Ni and Sn phase is suppressed. As a result, the obtained bonding metal has a structure in which a Ni—Fe alloy is dispersed in the Sn phase, so that the bonding strength and heat resistance are improved.

  As a desirable mode of the present invention, it is preferable that the outermost coating layer has Sn as a main component. Thus, the oxidation of the coating layer on the inner side of the coating layer can be suppressed by setting the coating layer mainly composed of Sn as the outermost layer.

  As a desirable mode of the present invention, it is preferable that the coating layer containing Sn as a main component does not contact the surface of the core particle. In this way, when the Pb-free solder is melted, the contact between the melted Sn and the core particles can be suppressed, so that a joining metal having a structure in which the Ni—Fe alloy is dispersed in the Sn phase can be obtained.

  As a desirable embodiment of the present invention, the average particle diameter of the core particles is preferably 10 μm or more and 30 μm or less. If the core particles in such a range are used, the bonding strength and heat resistance of the bonding metal can be improved.

  As a desirable mode of the present invention, it is preferable that the ratio of the Ni-Fe alloy is 10% by mass or more and 30% by mass or less with respect to the total mass of the Pb-free solder. By setting the addition ratio of the Ni—Fe alloy in the Pb-free solder within the above range, it is possible to improve the bonding strength and heat resistance while ensuring sufficient wettability of the Pb-free solder.

  The present invention can improve the strength and heat resistance after the Pb-free solder is solidified after being melted, and can improve the wettability between the melted Pb-free solder and the terminal electrode.

FIG. 1-1 is a cross-sectional view of an electronic circuit module component. FIG. 1-2 is an enlarged view showing a connection portion between the electronic component and the substrate. FIG. 2 is a side view showing a state in which the electronic circuit module component is attached to a substrate such as an electronic device. FIG. 3 is a conceptual diagram of Pb-free solder according to the present embodiment. FIG. 4 is an enlarged view of the second metal particles included in the Pb-free solder according to the present embodiment. FIG. 5 is an enlarged view showing another example of the second metal particles included in the Pb-free solder according to the present embodiment. FIG. 6 is a schematic view showing the structure in a state where the Pb-free solder according to the present embodiment is first melted and then hardened. FIG. 7 is a schematic diagram of a first metal phase and a second metal phase that the structure shown in FIG. 6 has. FIG. 8 is a conceptual diagram of Pb-free solder in which the first metal particles and the second metal particles having no coating layer are combined. FIG. 9A is a schematic diagram illustrating a reflow process. FIG. 9-2 is a schematic diagram illustrating a reflow process. FIG. 9C is a schematic diagram illustrating a reflow process. FIG. 10 is a schematic diagram illustrating an example of a change in temperature with respect to time during reflow. FIG. 11A is a schematic diagram illustrating a reflow process when the Pb-free solder according to the present embodiment is used. FIG. 11B is a schematic diagram illustrating a reflow process when the Pb-free solder according to the present embodiment is used. FIG. 11C is a schematic diagram illustrating a reflow process when the Pb-free solder according to the present embodiment is used. FIG. 12 is an explanatory diagram for evaluating the wettability of Pb-free solder. FIG. 13 is an explanatory diagram for evaluating the joint strength of the joint metal obtained by melting and hardening the Pb-free solder.

  DESCRIPTION OF EMBODIMENTS Hereinafter, modes (embodiments) for carrying out the present invention will be described in detail with reference to the drawings. The following embodiments do not limit the present invention. In addition, constituent elements disclosed in the following embodiments include those that can be easily assumed by those skilled in the art, those that are substantially the same, and those in a so-called equivalent range. Furthermore, the components of the following embodiments can be combined as appropriate.

  FIG. 1-1 is a cross-sectional view of an electronic circuit module component. FIG. 1-2 is an enlarged view showing a connection portion between the electronic component and the substrate. FIG. 2 is a side view showing a state in which the electronic circuit module component is attached to a substrate such as an electronic device. As shown in FIG. 1A, the electronic circuit module component 1 is an electronic component having a group of functions by mounting a plurality of electronic components 2 on a circuit board 3. The electronic component 2 may be mounted on the surface of the circuit board 3 or may be mounted inside the circuit board 3. In the present embodiment, the electronic component 2 included in the electronic circuit module component 1 includes, for example, a passive element such as a coil, a capacitor, or a resistor, but an active element such as a diode or a transistor, an IC (Integral Circuit), or the like can The component 2 may be mounted on the surface of the circuit board 3 or inside the circuit board 3. Moreover, the electronic component 2 is not limited to these.

  As illustrated in FIG. 1A, the electronic circuit module component 1 includes a circuit board 3 on which the electronic component 2 is mounted, an insulating resin 4 that covers the electronic component 2, and a shield layer 5 that covers the surface of the insulating resin 4. ,including. The electronic circuit module component 1 may not have the shield layer 5. As illustrated in FIG. 1B, the terminal electrode 2T of the electronic component 2 and the terminal electrode 3T of the circuit board 3 are joined by a joining metal 10. The bonding metal 10 is a metal that is hardened after the Pb-free solder according to the present embodiment is melted. With such a structure, the electronic component 2 is mounted on the circuit board 3. Thus, the joining metal 10 joins two members, the terminal electrode 2T of the electronic component 2 and the terminal electrode 3T of the circuit board 3.

  As shown in FIG. 1A, in the electronic circuit module component 1, the electronic component 2 mounted on the surface of the circuit board 3 is covered with an insulating resin 4. In the electronic circuit module component 1, the surface (referred to as component mounting surface) of the circuit board 3 on the side where the electronic component 2 is mounted is simultaneously covered with the insulating resin 4. Thus, the electronic circuit module component 1 covers the plurality of electronic components 2 and the component mounting surface with the insulating resin 4, thereby integrating the circuit board 3 and the plurality of electronic components 2 and ensuring the strength.

  The electronic circuit module component 1 has a shield layer 5 on the surface of an insulating resin 4 that covers the plurality of electronic components 2. In the present embodiment, the shield layer 5 is made of a conductive material (a material having conductivity, which is a metal in the present embodiment). In this embodiment, the shield layer 5 may be a single conductive material or a layer of a plurality of conductive materials. The shield layer 5 covers the surface of the insulating resin 4 to shield the electronic component 2 enclosed in the insulating resin 4 from high-frequency noise or electromagnetic waves from the outside of the electronic circuit module component 1, High frequency noise radiated from 2 is shielded. Thus, the shield layer 5 functions as an electromagnetic shield. In the present embodiment, the shield layer 5 covers the entire surface of the insulating resin 4. However, the shield layer 5 may be coated with the insulating resin 4 so as to exhibit a necessary function as an electromagnetic shield, and does not necessarily need to cover the entire surface of the insulating resin 4. Therefore, the shield layer 5 only needs to cover at least a part of the surface of the insulating resin 4.

The electronic circuit module component 1 is manufactured by the following procedure, for example.
(1) A solder paste containing Pb-free solder according to the present embodiment is printed on the terminal electrode of the circuit board 3.
(2) The electronic component 2 is mounted on the circuit board 3 using a mounting device (mounter).
(3) By putting the circuit board 3 on which the electronic component 2 is mounted into a reflow furnace and heating the solder paste, the Pb-free solder according to this embodiment included in the solder paste is melted and cured. Then, the cured Pb-free solder, that is, the bonding metal 10 bonds the terminal electrode 2T of the electronic component 2 and the terminal electrode 3T of the circuit board 3 together.
(4) The flux adhering to the surfaces of the electronic component 2 and the circuit board 3 is washed.
(5) Cover the electronic component 2 and the circuit board 3 with the insulating resin 4.

  The circuit board 3 of the electronic circuit module component 1 has terminal electrodes (module terminal electrodes) 7 on the side opposite to the component mounting surface. The module terminal electrode 7 is electrically connected to the terminal electrode 2T of the electronic component 2 included in the electronic circuit module component 1, and is a substrate (for example, a substrate of an electronic device) to which the electronic circuit module component 1 is attached as shown in FIG. The terminal electrode (device substrate terminal electrode) 9 and the solder 20 are connected to each other. With such a structure, the electronic circuit module component 1 exchanges electrical signals and power between the electronic component 2 and the device substrate 8.

  A device substrate 8 shown in FIG. 2 is a substrate on which the electronic circuit module component 1 is mounted. For example, the device substrate 8 is mounted on an electronic device (such as an in-vehicle electronic device or a portable electronic device). When mounting the electronic circuit module component 1 on the device substrate 8, for example, a solder paste containing solder 20 is printed on the device substrate terminal electrode 9, and the electronic circuit module component 1 is mounted on the device substrate 8 using a mounting device. Then, the device substrate 8 on which the electronic circuit module component 1 is mounted is placed in a reflow furnace and the solder paste is heated, so that the solder 20 of the solder paste is melted and then cured, thereby the module terminal electrode 7 and the device. The substrate terminal electrode 9 is joined. Thereafter, the flux adhering to the surfaces of the electronic circuit module component 1 and the device substrate 8 is washed.

  The melting temperature of Pb-free solder, which is widely used at present, is about 220 ° C., but the maximum temperature in reflow is about 240 ° C. to 260 ° C. The solder used when the electronic component 2 included in the electronic circuit module component 1 is mounted on the circuit board 3 is reflowed when the electronic circuit module component 1 is mounted on the device substrate 8 as described above. For this reason, solder (high temperature solder) that does not melt at the temperature in the reflow is used.

  Solder using Pb includes solder having a melting temperature of about 300 ° C. However, at present, there is no Pb-free solder having a melting temperature of 260 ° C. or higher and appropriate characteristics. Therefore, when Pb-free solder is used, the solder used for joining the electronic component 2 included in the electronic circuit module component 1 and the solder used when mounting the electronic circuit module component 1 on the device substrate 8 are melted. You must use one with a small temperature difference.

  When the solder used for joining the electronic component 2 included in the electronic circuit module component 1 is remelted during reflow, problems such as movement of the solder and solder flash (solder scattering) occur. As a result, there is a possibility of causing a short circuit or poor contact between the terminal electrode 2T of the electronic component 2 and the terminal electrode 3T of the circuit board 3. For this reason, the solder that joins the electronic component 2 of the electronic circuit module component 1 does not remelt during reflow when the electronic circuit module component 1 is mounted, or remelting does not cause solder movement or solder flash. It is desirable to use what is to the extent. There are conductive adhesives (Ag paste, etc.) as an alternative to solder with a high melting temperature, but there are problems such as low mechanical strength, high electrical resistance, and high cost. It is not. The Pb-free solder according to the present embodiment is used for joining the electronic component 2 included in the electronic circuit module component 1 and satisfies the above-described requirements.

  FIG. 3 is a conceptual diagram of Pb-free solder according to the present embodiment. FIG. 4 is an enlarged view of the second metal particles included in the Pb-free solder according to the present embodiment. The Pb-free solder 6 according to the present embodiment includes a first metal particle 6A mainly composed of Sn and a second metal particle 6B mainly composed of a Ni-Fe alloy before use (before first melting). ,including. The main component is the most abundant component among the components constituting the substance (the same applies hereinafter). In the present embodiment, the Pb-free solder 6 includes a flux PE in addition to the first metal particles 6A and the second metal particles 6B, and the first metal particles 6A and the second metal particles 6B are mixed with the flux PE, This is a solder paste in a dispersed state. The Pb-free solder 6 only needs to include at least the first metal particles 6A and the second metal particles 6B, and the flux PE is not always necessary.

  As shown in FIG. 4, the second metal particle 6B has at least one coating layer 6BS in which the surface of a particle (core particle) 6BC whose main component is a Ni—Fe alloy is a metal whose main component is Sn and an alloy. Covered with. Examples of the metal that forms an alloy with Sn contained in the coating layer 6BS include Cu, Ni, Au, Ag, Pd, and Bi. In this embodiment, Cu is used. The symbol Rc in FIG. 4 indicates the diameter of the core particle 6BC, and the symbol Rs indicates the thickness of the coating layer 6BS.

  FIG. 5 is an enlarged view showing another example of the second metal particles included in the Pb-free solder according to the present embodiment. As shown in FIG. 5, the second metal particle 6B may have a plurality (two in the present embodiment) of different types of coating layers 6BSa and 6BSb. The number of coating layers 6BSa and 6BSb is not limited to two, and may be three, four, or more. When the second metal particles 6B have a plurality of coating layers, different properties can be given to the respective coating layers, and the bonding strength, heat resistance, and wettability can be improved in a balanced manner. Symbols Rsa and Rsb in FIG. 5 indicate the thicknesses of the coating layers 6BSa and 6Bsb, respectively. Moreover, the symbol Rs in FIG. 5 indicates the total thickness of the coating layer of the second metal particle 6B. In the example shown in FIG. 5, the total thickness Rs is the sum (Rsa + Rsb) of the thickness Rsa of the coating layer 6BSa and the thickness Rsb of the coating layer 6BSb.

  Since the Pb-free solder 6 does not contain Pb, the first metal particles 6A mainly containing Sn also do not contain Pb. In the present embodiment, the second metal particles 6B are mainly composed of a Ni—Fe alloy, but may contain other components. For this reason, the second metal particle 6 </ b> B requires a Ni—Fe alloy and may contain at least one of Co (cobalt), Mo (molybdenum), Cu (copper), and Cr (chromium). Good.

  In the present embodiment, Pb-free solder with Sn as a base material is used as the first metal particles 6A. More specifically, Sn-Ag (silver) -based or Sn-Cu (copper) -based (Sn is 90 mass% or more) solder (Pb-free solder) is used as the first metal particles 6A. For example, as the first metal particles 6A, Sn-3.5% Ag (tin-silver eutectic solder, melting point 221 ° C.) or Sn-3% Ag-0.5% Cu (tin-silver-copper solder, melting point) 217-219 ° C.) or Sn-0.75% Cu (tin-copper eutectic solder, melting point 227 ° C.) can be used. In the present embodiment, the Sn-based Pb-free solder used for the first metal particles 6A has Sn of 90% by mass or more. In such a solder, the Sn phase occupies most of the structure after reflow. Therefore, when the solder is melted once and hardened and then reflowed a plurality of times, the Sn phase is remelted.

  In this embodiment, the structure after the Pb-free solder 6 is melted and hardened for the first time is a structure in which a Ni—Fe alloy is dispersed in the Sn phase. The joining metal 10 of the electronic circuit module component 1 shown in FIG. 1 has such a structure. The structure in which the Ni—Fe alloy is dispersed in the Sn phase has a higher strength than the structure in which Sn is predominant. For this reason, when the terminal electrode 2T of the electronic component 2 shown in FIG. 1 and the terminal electrode 3T of the circuit board 3 shown in FIG. In addition, the structure in which the Ni—Fe alloy is dispersed in the Sn phase has higher heat resistance than the structure in which Sn is predominant. For this reason, even when the joining metal 10 is heated by, for example, reflowing again, its own remelting is suppressed. Furthermore, since Ni contained in the bonding metal 10 can easily form an alloy with Sn, when the bonding metal 10 is heated during reflow, Ni diffuses into the Sn phase and forms an alloy with Sn. As a result, the bonded metal 10 after heating has a structure having a Ni—Sn phase, and has a higher melting point than before heating. For this reason, since the heat-resistant joining metal 10 improves further, even when it is heated by reflow etc. again, it can further suppress own remelting. Moreover, the phase (the 1st metal phase mentioned later) which has a Ni-Fe alloy as a main component is a structure | tissue harder than Sn phase. The structure of the bonding metal 10 in which structures having different hardnesses are mixed has a wedge effect due to a phase mainly composed of a harder phase Ni—Fe alloy, so that the bonding strength and heat resistance are increased. Further, by arranging a second metal phase (second metal phase described later) smaller than the phase around the phase mainly composed of the Ni—Fe alloy, the second metal phase has a structure with different hardness. In addition to being able to play the role of joining, it also has the effect of suppressing the propagation of cracks, and is effective in improving the joining strength and heat resistance.

  Since Ni has a high diffusion rate into the Sn phase, when Pb-free solder in which the first metal particles 6A and the metal particles of the Ni—Fe alloy are combined is used, when the Pb-free solder first melts, As a result of diffusion into the Sn phase and generation of an intermetallic compound, the viscosity of the molten Pb-free solder may increase and the wettability with respect to the terminal electrode may decrease. As a result, at the time of reflow, there is a possibility that problems such as a decrease in the self-alignment function of the electronic component 2 may occur.

  In the present embodiment, the Pb-free solder 6 shown in FIG. 3 is provided with a coating layer 6BS on the surface of Ni—Fe alloy core particles 6BC. Since the coating layer 6BS is a metal that forms an alloy with Sn, when the Pb-free solder 6 is first melted, the coating layer 6BS diffuses into the Sn phase to form an intermetallic compound. For this reason, while the Pb-free solder 6 is melting, the diffusion of Ni into the Sn phase and the reaction between the two are suppressed. As a result, the Pb-free solder 6 is prevented from being reduced in wettability during melting, so that problems such as a decrease in the self-alignment function of the electronic component 2 are suppressed during reflow. For this reason, the yield of the electronic circuit module component 1 is improved and the defect rate is reduced.

  In the present embodiment, the Pb-free solder 6 may include third metal particles in addition to the first metal particles 6A and the second metal particles 6B. In this case, like the second metal particle 6B, the third metal particle has at least the surface of the particle (core particle) whose main component is a Ni—Fe alloy as a main component, the metal that forms an alloy with Sn or Sn. It is covered with one coating layer. The average particle diameter of the core particles of the third metal particles is smaller than the average particle diameter of the core particles 6BC of the second metal particles 6B. By doing so, when the Pb-free solder 6 is melted, the core particles 6BC of the second metal particles 6B having a large average particle diameter are settled in the vicinity of the terminal electrodes of the circuit board, and the heat resistance and bonding of the bonding metal Improve strength.

  At the same time, the core particles of the third metal particles having a small average particle diameter are distributed to the terminal electrode side of the electronic circuit farther from the terminal electrode of the circuit board than the core particles 6BC of the second metal particles, and the heat resistance of the bonding metal 10 is increased. The wettability of the molten Pb-free solder 6 is improved and the self-alignment function of the electronic circuit is exhibited while maintaining the properties and the bonding strength. As a result, the bonding strength and heat resistance of the bonding metal 10 can be improved, and at the same time, the wettability of the Pb-free solder 6 can be improved and the self-alignment function of the electronic circuit can be exhibited. Next, the structure of the bonding metal 10 will be described.

  The electronic circuit module component 1 shown in FIG. 1 can be manufactured by an electronic circuit module component manufacturing method using Pb-free solder 6. First, the surface of the Pb-free solder 6, that is, the first metal particles 6A containing Sn as a main component and the core particles 6BC containing Ni-Fe alloy as a main component is at least mainly composed of a metal that forms an alloy with Sn. Pb-free solder 6 including second metal particles 6B covered with one coating layer 6BS is interposed between the terminal electrode 2T of the electronic component 2 and the terminal electrode 3T of the circuit board 3 on which the electronic component 2 is mounted. Provide. Next, the Pb-free solder 6 is melted. By such a procedure, the electronic circuit module component 1 is manufactured.

FIG. 6 is a schematic view showing the structure in a state where the Pb-free solder according to the present embodiment is first melted and then hardened. FIG. 7 is a schematic diagram of a first metal phase and a second metal phase that the structure shown in FIG. 6 has. Reference sign Ra in FIG. 7 is a representative dimension of the first metal phase 11, and reference sign Rb is a representative dimension of the second metal phase 12. The joining metal 10 shown in FIGS. 1-1 and 1-2 is obtained by hardening after the Pb-free solder 6 is first melted. When the cross section of the bonding metal 10 was analyzed by EPMA (Electron Probe Micro Analyzer), a structure as shown in FIG. 6 was observed. That is, the bonding metal 10 is interposed between the terminal electrode 2T of the electronic component 2 and the terminal electrode 3T of the circuit board 3 shown in FIG. 1, and the first metal phase 11 mainly composed of a Ni—Fe alloy, It has the 2nd metal phase 12 which has Sn alloy as a main component and surrounds the 1st metal phase 11, and the 3rd metal phase 13 which has Sn as a main component. As shown in FIG. 7, in the bonding metal 10, the representative dimension Ra of the first metal phase 11 is larger than the representative dimension Rb of the second metal phase. In the present embodiment, the bonding metal 10 includes the Ag—Sn phase 14 (Ag ₃ Sn) containing Ag and Sn, and the Fe phase 15, but may not necessarily include these. The Ag—Sn phase 14 appears in the structure of the bonding metal 10 when Pb-free solder containing Ag is used as the first metal particles 6 </ b> A of the Pb-free solder 6.

As described above, in the bonding metal 10, the first metal phase 11 including the Ni—Fe alloy appears in the structure, and the second metal phase 12 mainly composed of the Sn alloy surrounds the first metal phase 11. It is out. The second metal phase 12 is made of an alloy of Cu and Sn, more specifically, a plurality of metal particles including Cu ₆ Sn ₅ . Since the bonding metal 10 has such a structure, bonding strength and heat resistance are improved. In the structure of the bonding metal 10, the second metal phase 12 surrounds the first metal phase 11. In this structure, the metal (Cu in this embodiment) that forms an alloy with Sn contained in the coating layer 6BS of the second metal particles 6B of the Pb-free solder 6 reacts with the Sn of the molten Pb-free solder 6 to form Sn. It is thought that it was obtained as a result of the production of the compound. In the present embodiment, the second metal phase 12 includes Cu and Sn, but may further include Ni. That is, the second metal phase 12 may be an alloy of Cu, Sn, and Ni.

  The second metal phase 12 is 0.7 volume% or more and 10 volume% or less with respect to the entire volume of the bonding metal 10. The second metal phase 12 mainly appears around the first metal phase 11. If the ratio of the second metal phase 12 is within the above range, the entire area around the first metal phase 11 is the second metal phase 12. Can be enclosed. As a result, since the diffusion of Ni contained in the first metal phase 11 into the Sn phase can be suppressed during the melting of the Pb-free solder 6, a decrease in wettability can be suppressed. The Fe phase 15 is 0.2 volume% or more and 2 volume% or less with respect to the total volume in the bonding metal 10. The third metal phase 13 includes 90 mass% or more of Sn, has an elliptical cross section, and has an equivalent diameter of 10 μm to 20 μm. The equivalent diameter is 4 × A / C, where A is the area of the third metal phase 13 and C is the perimeter. A representative dimension of the third metal phase 13 is the equivalent diameter. The representative dimension of the third metal phase 13 is larger than the representative dimension of the second metal phase 12. Moreover, it is preferable that the 3rd metal phase 13 is 53 volume% or more and 65 volume% or less with respect to the whole volume of the joining metal 10. FIG. Within this range, the wettability of the molten Pb-free solder 6 and the bonding strength of the bonding metal 10 can be improved.

  The remaining part of the bonding metal 10 excluding the second metal phase 12, the third metal phase 13, and the Fe phase 15 becomes the first metal phase 11 and the Ag—Sn phase 14. The volume ratio (volume%) of the second metal phase 12, the third metal phase 13, etc. in the bonding metal 10 is determined from the cross-sectional image of the bonding metal 10. That is, the ratio of the area of the second metal phase 12, the third metal phase 13, etc. in the predetermined area of the cross section of the bonding metal 10 is obtained, and this is set as the respective volume ratio.

  Next, the influence of the representative dimension Ra of the first metal phase 11 and the representative dimension Rb of the second metal phase 12 on bonding strength and wettability will be described. Here, since the first metal phase 11 has a substantially circular cross-sectional shape, the diameter (equivalent diameter) of the first metal phase 11 is used as the representative dimension Ra. Since the second metal phase 12 is a plurality of metal particles surrounding the first metal phase 11, the representative dimension Rb is the diameter (equivalent diameter) of the particles when the cross-sectional shape of the particles can be identified as a substantially circular shape. Is used. In this case, it is preferable to use a value obtained by averaging the diameters of a plurality of (for example, 10) particles. When the cross-sectional shape of the particles of the second metal phase 12 cannot be identified as a substantially circular shape and the cross-sectional shape of the second metal phase 12 can only be recognized as one layer, the representative dimension Rb uses the thickness of the layer.

  The representative dimension Rb of the second metal phase 12 is preferably 0.2 μm or more and 5 μm or less. If Rb is within this range, the diffusion of Ni into the Sn phase of the molten Pb-free solder 6 is moderately suppressed, leaving the first metal phase 11 of the Ni—Fe alloy in the structure of the bonding metal 10. Therefore, the bonding strength of the bonding metal 10 can be improved. Next, the representative dimension Ra of the first metal phase 11 is preferably 3 μm or more and 25 μm or less. If Ra is within this range, the bonding strength of the bonding metal 10 can be improved. The representative dimension Ra of the first metal phase 11 is preferably larger than the representative dimension Rb of the second metal phase 12. In this way, the wettability of the molten Pb-free solder 6 and the bonding strength of the bonding metal 10 can be improved. In this case, Ra is more preferably 5 times or more of Rb. If it does in this way, the said wettability and the said joint strength can be improved more.

  Next, the first metal particles 6A and the second metal particles 6B included in the Pb-free solder 6 shown in FIG. 3 will be described in detail. The average particle diameter of the first metal particles 6A is not particularly specified, but in the present embodiment, the average particle diameter of the first metal particles 6A (D50, the same applies hereinafter) is about 30 μm, more specifically from 25 μm. The range is 36 μm. This is a size that can accommodate the printing of the solder paste when the chip-type electronic component has a size of 0603M (0.6 mm × 0.3 mm).

  When mounting a smaller chip-type electronic component or a larger chip-type electronic component, it is preferable to reduce or increase the average particle diameter of the first metal particles 6A with respect to the above range. For example, for a chip-type electronic component smaller than 0603M, the average particle diameter of the first metal particles 6A is in the range of 15 μm to 25 μm (average particle diameter of about 20 μm), or for a chip-type electronic component larger than 0603M. Thus, the average particle diameter of the first metal particles 6A can be in the range of 25 μm to 45 μm (average particle diameter of about 35 μm). Note that the surface area of the first metal particle 6A increases as the average particle diameter of the first metal particle 6A decreases. As a result, since the first metal particles 6A tend to be oxidized, it is preferable to increase the average particle diameter of the first metal particles 6A as much as possible within a range where the solder paste can be printed.

  Next, the effect of the coating layer 6BS of the second metal particles 6B included in the Pb-free solder 6 on the bonding strength, heat resistance, and wettability will be described. The covering layers 6BS and 6BSa in contact with the surface of the core particle 6BC shown in FIGS. 4 and 5 preferably have thicknesses Rs and Rsa of 0.05 μm or more and 2 μm or less. By setting the thicknesses Rs and Rsa of the coating layers 6BS and 6BSa provided on the surface of the core particle 6BC within the above range, the heat resistance of the bonding metal 10 is improved while ensuring the wettability when the Pb-free solder 6 is melted. That is, melting (remelting) in reflow can be suppressed.

  As shown in FIG. 5, when the second metal particle 6B has a plurality of coating layers 6BSa and 6BSb, all of the coating layers 6BSa other than the innermost coating layer 6BSa, that is, the coating layer 6BSa in contact with the surface of the core particle 6BC. The thickness (Rsb in the example shown in FIG. 5) is preferably 0.05 μm or more and 5 μm or less. In this way, it is possible to improve the heat resistance of the bonding metal 10 while ensuring the wettability when the Pb-free solder 6 is melted, that is, to suppress melting (remelting) in reflow.

Moreover, when the 2nd metal particle 6B has several coating layer 6BSa, 6BSb, it is preferable that innermost coating layer 6BSa has Cu as a main component. By doing so, the molten Pb-free solder 6 is formed with an alloy of Cu and Sn, more specifically, a layer of Cu ₆ Sn _{5 on} the surface of the core particle 6BC. Reaction is suppressed. As a result, the bonding metal 10 has a structure in which a Ni—Fe alloy is dispersed in the Sn phase, so that the bonding strength and heat resistance are improved. When the second metal particle 6B has a plurality of coating layers 6BSa and 6BSb, the outermost coating layer (the coating layer 6BSb in the example shown in FIG. 5) preferably contains Sn as a main component. In this way, when the Pb-free solder 6 is melted, the familiarity between the Sn from the first metal particles 6A and the second metal particles 6B is improved, so that the Ni—Fe alloy is dispersed in the Sn phase of the bonding metal 10. It becomes easy.

  Further, when the coating layer containing Sn as a main component is in contact with the core particle 6BC, when the Pb-free solder 6 is melted, the Sn from the first metal particle 6A and the coating layer 6BS of the second metal particle 6B are melted. As a result, the melted Sn and the core particle 6BC come into contact with each other. Since the rate at which Ni diffuses into Sn is high, when molten Sn and the core particle 6BC come into contact with each other, Ni in the core particle 6BC diffuses into the Sn and reacts. In some cases, a dispersed structure may not be formed. For this reason, it is preferable that the coating layer which has Sn as a main component does not contact | connect the surface of the core particle 6BC. By doing so, it is possible to obtain the bonding metal 10 having a structure in which the Ni—Fe alloy is dispersed in the Sn phase. Moreover, the oxidation of the coating layer inside the said coating layer can be suppressed by making the coating layer which has Sn as a main component into the outermost part. As a result, the Ni—Fe alloy can be uniformly dispersed in the Sn phase while ensuring wettability when the Pb-free solder 6 is melted, so that the heat resistance and bonding strength of the bonding metal 10 can be improved. it can.

  Next, the core particle 6BC of the second metal particle 6B will be described. As described above, the core particle 6BC has a Ni—Fe alloy as a main component. As the average particle size (D50, the same applies hereinafter) of the core particle 6BC increases, the melting point rises after the Pb-free solder 6 melts and hardens. For this reason, the occurrence of a problem in reflow is expected. Moreover, when the average particle diameter of the core particle 6BC becomes small, there exists a tendency for joining strength to fall. In the present embodiment, the average particle diameter of the core particles 6BC is preferably 10 μm or more and 30 μm or less. If the core particle 6BC in such a range is used, the bonding strength and heat resistance of the bonding metal 10 can be improved.

  Next, the influence of the ratio (addition ratio) of the Ni—Fe alloy to the total mass of the Pb-free solder 6 (the sum of the mass of the first metal particles 6A and the mass of the second metal particles 6B) will be described. When the Pb-free solder 6 is combined with the first metal particles 6A and the second metal particles 6B not having the coating layer 6BS, the bonding strength and heat resistance of the bonding metal 10 are reduced when the addition ratio of the Ni—Fe alloy is reduced. May not improve. For this reason, in this embodiment, the addition rate of a Ni-Fe alloy shall be 5 mass% or more, and joining strength and heat resistance are ensured.

  Moreover, the wettability at the time of melting falls, so that the addition ratio of a Ni-Fe alloy becomes large. And when the addition ratio of a Ni-Fe alloy exceeds 35 mass%, wettability will fall and there exists a possibility that the self-alignment effect of the electronic component 2 at the time of reflow may fall. This decrease in wettability will be briefly described.

  FIG. 8 is a conceptual diagram of Pb-free solder in which the first metal particles and the second metal particles having no coating layer are combined. FIGS. 9A to 9C are schematic diagrams illustrating a reflow process. FIG. 10 is a schematic diagram illustrating an example of a change in temperature with respect to time during reflow. The Pb-free solder 106 shown in FIG. 8 is a combination of the first metal particles 106A mainly composed of Sn and the second metal particles 106B mainly composed of a Ni—Fe alloy and mixed with flux PE. It is. As shown in FIG. 9A, a Pb-free solder 106 having first metal particles 106A and second metal particles 106B is provided between the terminal electrode 2T of the electronic component 2 and the terminal electrode 3T of the circuit board 3. Heat on. Then, as shown in FIG. 9-2, as the Pb-free solder 106 is melted and becomes a molten metal 106L as the heating proceeds, the terminal electrode of the electronic component 2 is cured as shown in FIG. 2T and the terminal electrode 3T of the circuit board 3 are joined.

  For example, as shown in FIG. 10, the temperature change at the time of reflow is such that the temperature (temperature in the reflow furnace) θ changes from θ1 to θ2, θ3, as time changes to t1, t2, t3, t4, and t5. It changes as θ4, θ5, and θ4. The temperature θ4 is a temperature at which the Pb-free solder starts to melt, and the temperature θ5 is the maximum temperature during reflow. When the temperature becomes θ4 at time t4, the Pb-free solder 106 is melted to become a molten metal 106L shown in FIG. 9-2. The molten metal 106L attempts to spread to the terminal electrode 2T of the electronic component 2. At this time, the surface of the second metal particles 106B in the Pb-free solder 106 is in contact with the molten Sn. Since the second metal particle 106B has a Ni—Fe alloy as a main component, when it contacts with molten Sn, Ni diffuses into Sn to form an alloy. When Ni begins to form an alloy with Sn, the viscosity of the molten metal 106L increases. Since the diffusion rate of Ni into Sn is fast, the viscosity of the molten metal 106L starts to increase before the molten metal 106L spreads to the terminal electrode 2T.

  As a result, the wetting spread of the molten metal 106L to the terminal electrode 2T is reduced (decreasing wettability). When the molten metal 106L is cured, the fillet F is not sufficiently formed by the bonding metal 110 with respect to the terminal electrode 2T of the electronic component 2 as shown in FIG. Further, since the gas H existing in the molten metal 106L is difficult to escape to the outside of the molten metal 106L, as shown in FIG. 9-3, a void Ha is formed in the bonding metal 110 obtained by curing the molten metal 106L. End up. Furthermore, when the wettability of the molten metal 106L decreases, the self-alignment function of the electronic component 2 decreases.

  FIGS. 11A to 11C are schematic diagrams illustrating a reflow process when the Pb-free solder according to the present embodiment is used. As shown in FIG. 11A, a Pb-free solder having first metal particles 6A mainly containing Sn and second metal particles 6B having a coating layer on the surface of core particles mainly containing Ni-Fe alloy. 6 is provided between the terminal electrode 2T of the electronic component 2 and the terminal electrode 3T of the circuit board 3, and then heated. Then, as shown in FIG. 11-2, as the Pb-free solder 6 is melted to become a molten metal 6L as the heating proceeds, the terminal electrode of the electronic component 2 is cured as shown in FIG. 2T and the terminal electrode 3T of the circuit board 3 are joined.

  The molten metal 6L shown in FIG. 11B tends to spread to the terminal electrode 2T of the electronic component 2. At this time, in the second metal particles 6B in the Pb-free solder 6, the coating layer 6BS mainly composed of a metal that forms an alloy with Sn is in contact with the molten Sn. For this reason, since the diffusion of Ni into Sn is suppressed in the second metal particles 6B, the increase in the viscosity of the molten metal 6L is suppressed, and the molten metal 6L spreads to the terminal electrode 2T. As described above, the Pb-free solder 6 is prevented from being deteriorated in wettability when melted by the coating layer 6BS of the second metal particle 6B. As a result, when the molten metal 6L is cured, a fillet F made of the bonding metal 10 is formed on the terminal electrode 2T of the electronic component 2 as shown in FIG. Further, since the gas H existing in the molten metal 6L escapes from the molten metal 6L as a result of suppressing the increase in the viscosity of the molten metal 6L, the molten metal 6L is obtained by curing as shown in FIG. Voids are hardly formed in the bonded metal 10 formed. Furthermore, since the deterioration of the wettability of the molten metal 6L is suppressed, the self-alignment function of the electronic component 2 is maintained.

  By these actions, when the electronic component 2 is mounted on the circuit board 3 using the Pb-free solder 6, the terminal electrode 2T of the electronic component 2 and the terminal electrode 3T of the circuit board 3 are securely bonded by the bonding metal 10. Therefore, reliability is improved. In addition, since the self-alignment function of the electronic component 2 is exhibited in the reflow process, the positional accuracy of the electronic component 2 mounted on the circuit board 3 is also improved.

  Thus, since the Pb-free solder 6 covers the surface of the core particle 6BC with the coating layer 6BS, even when the addition ratio of the Ni—Fe alloy is increased, a decrease in wettability during melting is suppressed. For this reason, in order to improve the joining strength and heat resistance of the joining metal 10, even if the addition ratio of the Ni—Fe alloy is increased to about 30% by mass, the Pb-free solder 6 is ensured to have sufficient wettability. Therefore, in the present embodiment, in the Pb-free solder 6, the addition ratio of the Ni—Fe alloy is set to 5% by mass or more and 30% by mass or less, and sufficient wettability of the Pb-free solder 6 is ensured, and the joint strength and heat resistance are increased. Improve sexiness.

  When the electronic component 2 is mounted on the circuit board 3 using the Pb-free solder 6, an appropriate self-alignment effect can be obtained as described above. For this reason, the electronic component 2 can be positioned. Further, the bonding metal 10 also improves the bonding strength and heat resistance. For this reason, even when the electronic circuit module component 1 manufactured using the Pb-free solder 6 is reflowed again, melting of the bonding metal 10 is suppressed. As a result, it is possible to reduce the possibility of solder flash and solder movement in the electronic circuit module component 1. Since the electronic circuit module component 1 using the Pb-free solder 6 can reduce the risk of a bonding failure between the terminal electrode 2T of the electronic component 2 and the terminal electrode 3T of the circuit board 3, etc., yield and reliability Will improve. As described above, the Pb-free solder 6 is suitable for mounting the electronic component 2 mounted on the electronic circuit module component 1.

  Further, since the melting point of the Pb-free solder 6 increases once it has been melted and cured, it is also effective for joining parts that require heat resistance. In this case, the temperature at which the Pb-free solder 6 is first melted is the same as that of Sn-based (Sn-based solder, for example, Sn-3.5% Ag solder) (about 220 ° C.). Therefore, the workability at the time of joining is equivalent to that when Sn-based solder is used.

  The Pb-free solder 6 was melted using the amount of oxygen contained in the core particle 6BC of the second metal particle 6B as a parameter. As a result, when the ratio of oxygen contained in the core particle 6BC exceeds 1.5% by mass, an oxide film is formed on the surface of the core particle 6BC and / or the surface of the coating layer 6BS, and when the Pb-free solder 6 is melted, The second metal particles 6B were separated from the Pb-free solder 6. For this reason, it is preferable that the oxygen contained in core particle 6BC shall be 1.5 mass% or less. In order to prevent the separation, it is preferable that the amount of oxygen is smaller. By setting the amount in this range, the first metal phase 11 of Ni—Fe is present in the bonding metal 10 to improve the bonding strength and heat resistance. it can.

  When the proportion of Fe in the second metal particles 6B is 8% by mass or more, an increase in the melting point of the Pb-free solder 6 cured after melting is observed. If the ratio is 5% by mass or more, the melting point of the Pb-free solder 6 cured after melting becomes higher than the initial melting temperature. On the other hand, if it exceeds 16% by mass, there is a risk of cost increase, segregation, and deterioration of fluidity, but the effect of increasing the melting temperature cannot be expected as the addition amount is increased. Therefore, the proportion of Fe in the second metal particles 6B is preferably 5% by mass to 16% by mass, and more preferably 8% by mass to 16% by mass.

  The powder of the core particle 6BC of the 2nd metal particle 6B is produced by metal powder manufacturing methods, such as a water atomizing method and a gas atomizing method, for example. Thereafter, a coating layer 6BS is formed on the surface of the core particle 6BC by plating or sputtering. When the surface of the second metal particle 6B is oxidized and combined with the first metal particle 6A to form Pb-free solder 6, in the state where the Pb-free solder 6 is melted, the second metal particle is affected by the oxide film. The 6B powder collects on the surface of the cured Pb-free solder 6. As a result, the reaction between the Sn phase from the first metal particle 6A and the Sn metal from the coating layer 6BS of the second metal particle 6B and the metal forming the alloy is not promoted, and the Ni-Fe alloy is dispersed in the Sn phase. There is a high possibility that the bonding metal 10 is not obtained.

  Therefore, when the second metal particle 6B is oxidized in the manufacturing process of the second metal particle 6B, for example, it is reduced in a hydrogen atmosphere and then combined with the first metal particle 6A. By doing so, the Pb-free solder 6 undergoes a reaction between the Sn phase from the first metal particles 6A and Sn from the coating layer 6BS of the second metal particles 6B and the metal forming the alloy during the first melting. Promoted. As a result, the bonding metal 10 having a structure in which the Ni—Fe alloy is dispersed in the Sn phase is obtained, so that the bonding strength and heat resistance of the bonding metal 10 are improved.

  After the core particle 6BC is produced, the coating layer 6BS is formed on the surface of the core particle 6BC by, for example, vacuum deposition, electroplating, electroless plating, barrel sputtering, or the like. When the coating layer 6BS is a multilayer, these methods may be combined. For example, the electroplating method is performed as follows.

  (Formation of Cu coating layer) Desalting treatment is performed on Ni-Fe core particles having a predetermined average particle diameter prior to plating. Next, in a copper sulfate plating bath containing an organic additive or the like, a Cu coating layer is formed by electroplating using an insoluble anode electrode and Ni-Fe core particles as a cathode. Ni-Fe core particles are accommodated in an insulator case together with an insulator stirring ball. The case is insoluble in the plating bath and has a structure (for example, a cage) that can replace the plating solution. The Ni—Fe core particles accommodated by vibrating the case are stirred, and electroplating is performed so that a coating layer is uniformly formed on the Ni—Fe core particles while performing air bubbling of the plating solution.

  (Formation of Sn coating layer) In order to form two coating layers of Cu and Sn, tin plating may be performed on Ni-Fe particles on which Cu coating is formed. In the electroplating method described above, an Sn coating layer is formed by using a tin plating bath instead of the copper sulfate plating bath.

(Evaluation example)
For the Pb-free solder 6 according to the present embodiment and the Pb-free solder according to the comparative example, the wettability of the molten Pb-free solder, the bonding strength and the heat resistance of the bonding metal obtained by curing after melting were evaluated.

  FIG. 12 is an explanatory diagram for evaluating the wettability of Pb-free solder. The wettability of the molten Pb-free solder was evaluated by the height (wetting height) hf of the fillet F when the terminal electrode 2T of the electronic component 2 and the terminal electrode 3T of the circuit board 3 were joined by Pb-free solder. . The electronic component 2 used in this evaluation has a dimension of 2012M. When the wettability of the molten Pb-free solder is high, the Pb-free solder tends to spread to the terminal electrode 2T of the electronic component 2. For this reason, it can be determined that the wettability of the Pb-free solder is higher as the wetting height hf of the fillet F is larger.

  The wetting height hf is the distance from the surface of the terminal electrode 3T of the circuit board 3 to the boundary (wetting end portion) between the terminal electrode 2T of the electronic component 2 and the fillet F. When it is difficult to measure the distance from the terminal electrode 3T of the circuit board 3 to the wet end, the distance tt of the terminal electrode 3T is measured by measuring the distance from the surface 3P of the circuit board 3 to the wet end of the fillet F. May be subtracted to obtain the wetting height hf. As for wettability, the wetting height hf is 70% or more of the height hc of the terminal electrode 2T of the electronic component 2, 50% or more but less than 70% ○, 30% or more but less than 50% △, less than 30% X.

  FIG. 13 is an explanatory diagram for evaluating the joint strength of the joint metal obtained by melting and hardening the Pb-free solder. The strength of the bonding metal was evaluated by a shear (shear) test. In the shear (shear) test, a shear stress was applied to the Pb-free solder cured after melting. In the shear test, as shown in FIG. 13, Pb-free solder and electronic component 2 (size 0603M) are put on the electrode 71 of the substrate 70 and melted, and the surface of the electrode 71 is joined to the electronic component 2. A piece (cured Pb-free solder) 72 was targeted. A substrate 70 having a test piece 72 is attached to a table 73 of a test apparatus. In this state, the table 73 moves toward the share tool 74. At this time, since the table 73 moves in a direction orthogonal to the longitudinal direction of the electronic component 2, the shear tool 74 collides with the electronic component 2 from a direction orthogonal to the longitudinal direction of the electronic component 2. And the state of a fracture | rupture when the test piece 72 was destroyed with the shear tool 74 was observed, and joint strength was evaluated by this. A high speed bond tester (Dage, Dage-4000HS) was used for the shear test. The test speed V was 0.1 mm / sec. The bonding strength was judged at the location where the strength test was broken. In FIG. 13, when the substrate 70 or the electronic component 2 is broken without being broken in the cured test piece 72, it is assumed that the strength of the test piece 72 is sufficiently high, and broken at the interface between the electrode 71 and the substrate 70. In this case, the strength of the test piece 72 is assumed to be high, Δ when the test piece 72 is broken near the interface with the terminal electrode 2T of the electronic component 2, and the strength of the test piece 72 is sufficient when the test piece 72 is broken at the solder structure of the joint. It was set as x because it was not high.

  The heat resistance was evaluated by heating the joining metal obtained by curing the first molten Pb-free solder. The heat resistance was evaluated based on the endothermic amount when the bonding metal was heated at the reflow temperature (240 ° C. to 260 ° C.). The endothermic amount was measured using a heat flux differential scanning calorimeter (Shimadzu Corporation DSC-50). If the absolute value of the endothermic amount is 0 J / g, the bonding metal does not melt. If there is an endothermic amount, the joining metal will melt, but if the absolute value of the endothermic amount is in the range of 25 J / g to 35 J / g, even if the joining metal is melted, the electrons will be re-flowed during reflow. There will be no cracks or solder flash or other defects in the insulating resin of the circuit component module. For this reason, when the absolute value of the endothermic amount is less than 25 J / g, the joining metal does not melt (◯), and when the absolute value of the endothermic amount is 25 J / g or more and 35 J / g or less. When the melting of the joining metal was small (Δ) and the absolute value of the endothermic amount was larger than 35 J / g, it was determined that the joining metal was melted (x).

  First, the evaluation of the representative dimension Ra of the first metal phase 11 and the representative dimension Rb of the second metal phase 12 of the bonding metal 10 shown in FIG. 7 will be described. The evaluation results are shown in Tables 1 and 2. When the wettability is Δ or less, or when at least one of the wettability and the bonding strength is x, the comprehensive evaluation is x. When the wettability and the bonding strength were ○, the overall evaluation was ○. When the wettability was ◎ or the bonding strength was ◯ or more, the overall evaluation was ◎. The overall evaluation allowed a value of ○ or higher.

  Table 1 shows the representative dimension Ra of the first metal phase 11, the representative dimension Rb of the second metal phase 12, the representative dimension ratio (Ra / Rb), and the ratio (volume%) of the third metal phase 13 in the structure of the bonding metal 10. It is a result when changing. The results of Table 1 show that in the Pb-free solder 6 including the first metal particles 6A and the second metal particles 6B shown in FIG. 3, the components and ratios of the first metal particles 6A and the second metal particles 6B and the second metal particles It was obtained by changing the material and dimensions of the 6B coating layer 6BS. Table 2 shows the composition and average particle diameter (D50) of the first metal particles 6A of the Pb-free solder 6 in Table 1, and Table 3 shows the composition and average of the second metal particles 6B of the Pb-free solder 6 in Table 1. The particle diameter (D50) is shown. Table 4 shows the coating layer 6BS of the Pb-free solder 6 in Table 1. The numbers in Table 2, the numbers in Table 3, and the numbers in Table 4 correspond to the numbers of the first metal particles, the numbers of the second metal particles, and the numbers of the coating process in Table 1, respectively. In the present embodiment, the overall evaluation is based on “◯” and “OK” or more is acceptable.

  From the results of Table 1, when Rb ≦ Ra and 1 ≦ representative dimension ratio (Ra / Rb) ≦ 75 (sample Nos. 1-4, 6-9, 11), the heat resistance of the bonding metal 10 It was also found that the joint strength and the wettability of the molten Pb-free solder 6 were excellent. Although not shown in Table 1, when the representative dimension Rb of the second metal phase 12 is 2 μm and the representative dimension Ra of the first metal phase 11 is 1 μm to 2 μm, the wettability is ◎, but the bonding strength Was x, and the overall evaluation was x.

  If the representative dimension of the second metal phase 12 is 0.2 μm or more and 5 μm or less (Sample Nos. 1-4, 6-9, 11), the heat resistance and bonding strength of the bonding metal 10 and the molten Pb-free solder 6 It was found that the wettability was excellent. If the representative dimensions of the first metal phase 11 are 3 μm or more and 25 μm or less (Sample Nos. 1-4, 6-9, 11), the heat resistance and bonding strength of the bonding metal 10 and the molten Pb-free solder 6 It was found to be excellent in wettability. Sample No. 5, the representative dimension of the first metal phase 11 is in the above range, but it is considered that the overall evaluation was lowered because the proportion of the third metal phase 13 in the bonding metal 10 was low.

  When the coating layer 6BS is not present on the second metal particles 6B of the Pb-free solder 6 (Sample Nos. 14, 15, and 16), at least one of the first metal phase 11 and the second metal phase 12 is present in the bonding metal 10 do not do. In this case, at least one of heat resistance, wettability, and bonding strength decreased, and the overall evaluation was x. The bonding strength and heat resistance of these samples are decreased because the second metal having a representative dimension smaller than that of the first metal phase 11 surrounding the first metal phase 11 and the first metal phase 11 in the structure of the bonding metal 10. This is probably because phase 12 does not appear.

  The Pb-free solder 6 has higher heat resistance, wettability, and bonding strength when the second metal particles 6B have the coating layer 6BS than when the second metal particles 6B do not have the coating layer 6BS. (Sample Nos. 1 to 13). In the range of the evaluation results shown in Table 1, it is sufficient that at least one coating layer 6BS exists. Further, the outermost layer of the coating layer 6S may be Sn or Cu (for example, Sample Nos. 3, 4, 7, 8, and 9). In the range of the evaluation results shown in Table 1, it was found that when the total thickness of the coating layer 5BS is 0.5 μm or more and 6 μm or less, the heat resistance, wettability, and bonding strength are increased.

  Next, the influence given by the proportion of the third metal phase 13 in the bonding metal 10 shown in FIG. 6 will be described. From the result of Table 1, when the 3rd metal phase 13 is 52 volume% or more and less than 70 volume% with respect to the whole volume of the joining metal 10, (sample No. 1-4, 6-9, 11), It was found that the bonding strength and heat resistance of the bonding metal 10 and the wettability of the Pb-free solder 6 were excellent. Sample No. 14 is the range in which the third metal phase 13 is described above, but since the second metal phase 12 does not exist, it is considered that the heat resistance, the bonding strength, and the wettability were inferior. Sample No. 13 is 1 ≦ representative dimension ratio (Ra / Rb) ≦ 75, but the ratio of the first metal phase 11 in the bonding metal 10 is decreased because the ratio of the second metal particles 6B of the Pb-free solder 6 is low. This is considered to be the reason why the overall evaluation was lowered. Next, evaluation of the Pb-free solder 6 shown in FIG. 3 will be described.

  The composition and dimensions of the core particles 6BC of the Pb-free solder 6 shown in FIG. did. The evaluation results are shown in Table 5. Details of the composition and dimensions of the core particle 6BC are shown in Table 6. Core particles A, C, and D in Table 6 correspond to core particles A, C, and D in Table 5. The diameter (particle diameter) of the core particle 6BC is an average particle diameter (D50), and was measured before the coating layer 6BS was provided. The average particle diameter (D50) of the core particle 6BC was measured using a laser diffraction particle size distribution measuring device (SALD-2200, manufactured by Shimadzu Corporation). When the heat resistance was ○ and the wettability was ◎, the overall evaluation was ◎. When the heat resistance was ○ and the wettability was ○, the overall evaluation was ○. When the heat resistance was less than Δ and the wettability was more than ○, the overall evaluation was Δ. When the heat resistance was Δ or less and the wettability was Δ or less, the overall evaluation was x. The overall evaluation allowed Δ or more. Sample No. 50 is a thing in which a 2nd metal particle does not have a coating layer.

  Sample No. From the comprehensive evaluation of 29, 30, 31, etc., the thickness of the coating layer 6BS in contact with the surface of the core particle 6BC, that is, the first layer is preferably 0.05 μm or more and 2 μm or less. By setting the thickness of the coating layer provided on the surface of the core particle 6BC within the above range, heat resistance can be improved while ensuring wettability when the Pb-free solder 6 is melted. Sample No. From the comprehensive evaluation of 18 to 21 and 37 to 40, when the second metal particle 6B has a plurality of coating layers, the total thickness of the coating layers other than the innermost coating layer, that is, the outer side of the second layer is 0. It is preferable that it is 05 μm or more and 5 μm or less. In this way, it is possible to improve heat resistance while ensuring wettability when the Pb-free solder 6 is melted.

  Sample No. From the comprehensive evaluation of 17 to 21, 37 to 40, and 43, when the second metal particle 6B has a plurality of coating layers, the innermost coating layer (first layer) preferably contains Cu as a main component. By doing in this way, wettability and heat resistance improve. Sample No. From the comprehensive evaluation of 18 to 20, 23, 24 to 27, and 37 to 40, when the second metal particle 6B has a plurality of coating layers, the outermost coating layer (second layer) is mainly composed of Sn. It is preferable. In this way, when the Pb-free solder 6 is melted, the familiarity between the Sn from the first metal particles 6A and the second metal particles 6B is improved, so that the Ni—Fe alloy is dispersed in the Sn phase of the bonding metal 10. It becomes easy. Sample No. 18, 19, 23, 26 and sample no. 34 and 35, the overall evaluation is higher in the case where the coating layer containing Sn as a main component does not contact the surface of the core particle 6BC (sample Nos. 18, 19, 23, and 26). For this reason, it is preferable that the coating layer containing Sn as a main component does not come into contact with the surface of the core particle 6BC. In this way, since the joining metal 10 having a structure in which the Ni—Fe alloy is dispersed in the Sn phase can be obtained, the heat resistance of the joining metal 10 is improved while ensuring the wettability of the Pb-free solder 6. be able to.

  Sample No. From the results of 45, 18-21, etc., the average particle diameter of the core particles 6BC is preferably 10 μm or more and 30 μm or less. If the core particle 6BC in such a range is used, the bonding strength and heat resistance of the bonding metal 10 can be improved. Sample No. From the results of 44, 38, 18-21, etc., the ratio of the Ni—Fe alloy is preferably 10% by mass or more and 30% by mass or less with respect to the total mass of the Pb-free solder 6. Within such a range, the bonding strength and heat resistance of the bonding metal 10 can be improved, and the wettability of the Pb-free solder 6 can be secured.

  As described above, the Pb-free solder according to the present invention is useful for improving the bonding strength and heat resistance of the bonding metal obtained after the Pb-free solder is melted for the first time and then cured.

DESCRIPTION OF SYMBOLS 1 Electronic circuit module component 2 Electronic component 2T, 3T Terminal electrode 3 Circuit board 3P Surface 4 Insulation resin 5 Shield layer 6BC Core particle | grains 6,106 Pb free solder 6A, 106A 1st metal particle 6B, 106B 2nd metal particle 6BS, 6BSa , 6BSb coating layer 6L, 106L Molten metal 7 Module terminal electrode 8 Device substrate 9 Device substrate terminal electrode 10 Bonding metal 11 First metal phase 12 Second metal layer 13 Third metal phase 14 Ag—Sn phase 15 Fe phase 70 Substrate 71 Electrode 72 Test piece 73 Table 74 Share tool

Claims (6)

A Sn-Ag-based alloy or a Sn-Cu-based alloy, wherein the first metal particles contain 90 mass% or more of Sn,
As the Ni—Fe alloy, the surface of the core particle made of Ni-10 mass% Fe alloy or Ni-20 mass% Fe alloy is at least one coating layer mainly composed of Sn or a metal that forms an intermetallic compound with Sn. Covered second metal particles;
Including
Of the coating layer, the thickness of the layer in contact with the surface of the core particle is 0.5 μm or more and 2 μm or less, and when the coating layer is a plurality of layers, the total thickness of the layers other than the layer in contact with the surface of the core particle is A Pb-free solder characterized by having a thickness of 0.05 μm or more and 5 μm or less.   The Pb-free solder according to claim 1, wherein the innermost coating layer contains Cu as a main component.   The Pb-free solder according to claim 1, wherein the outermost coating layer is mainly composed of Sn.   The Pb-free solder according to claim 1, wherein the coating layer containing Sn as a main component does not contact the surface of the core particle.   5. The Pb-free solder according to claim 1, wherein an average particle diameter of the core particles is 10 μm or more and 30 μm or less.   The Pb free solder whose ratio of the said Ni-Fe alloy is 10 to 30 mass% with respect to the total mass of the Pb free solder of any one of Claim 1 to 5.

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