Junction structure

Abstract

To provide a junction structure with high breaking strength.SOLUTION: A junction structure 100 includes an IMC layer 20 composed of an intermetallic compound between a first layer 21 made of a first metal containing Sn and a second layer 22 made of a second metal forming the intermetallic compound with Sn. Here, metals are generally ductile and soft because of metallic bonding, while intermetallic compounds are hard and brittle materials. Therefore, the average thickness of the IMC layer 20 made of an intermetallic compound is set to 0.1 μm or more and 0.5 μm or less. By providing such a thin IMC layer 20, the junction structure 100 can be made difficult to bend due to the intermetallic compound, and can be made difficult to break due to the metal sandwiching the intermetallic compound.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a joint structure. [Background technology]

[0002] In recent years, advances in electronics have led to the development of technologies for mounting electronic components on substrates. For example, in the past, when assembling fine electronic components, gold was used for the terminals of the electronic components, and Sn was plated or thin-film-formed on the opposing wiring board, and the components were joined by soldering or diffusion bonding. When joining an electronic component and a wiring board using Au and Sn plating, there was a tendency for an intermetallic compound of Au and Sn to form at the joining interface due to a eutectic reaction. For example, Patent Document 1 specifies that a layer containing an AuSn alloy has a thickness within a predetermined range. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2017-216308 Summary of the Invention [Problem to be solved by the invention]

[0004] Here, since the intermetallic compound is hard, even if stress acts on the joint structure, the joint structure is not easily bent, but on the other hand, since it is brittle, there is a problem that the breaking strength is reduced.

[0005] An object of the present invention is to provide a joint structure with high breaking strength. [Means for solving the problem]

[0006] The joint structure according to the present invention is a joint structure for joining an electronic component and a wiring board, and comprises: a first layer provided on one side of the electronic component and the wiring board and made of a first metal containing Sn; a second layer provided on the other side of the electronic component and the wiring board and made of a second metal that forms an intermetallic compound with Sn; and a third layer provided at the joint interface between the first layer and the second layer and made of an intermetallic compound of the first metal and the second metal, wherein the average thickness of the third layer is 0.1 μm or more and 0.5 μm or less.

[0007] The joint structure according to the present invention includes a first layer made of a first metal containing Sn, a second layer made of a second metal that forms an intermetallic compound with Sn, and a third layer made of an intermetallic compound between them. Here, metals are generally soft and ductile due to their metallic bonding, while intermetallic compounds are hard and brittle. Therefore, the average thickness of the third layer made of the intermetallic compound is set to 0.1 μm or more and 0.5 μm or less. By providing such a thin third layer, the intermetallic compound makes the joint structure less likely to bend, and the metals sandwiching the intermetallic compound make it less likely to break. As a result, a joint structure with high fracture strength and high reliability can be obtained.

[0008] The second metal is any one of Au, Cu, Ni, Ag, and Pd, or an alloy of at least two of these metals. In this case, the second layer is likely to form an intermetallic compound with Sn.

[0009] The second metal may be a metal containing at least Au. By sandwiching a thin third layer between a second layer of Au, which is soft and has a particularly low Young's modulus among metals, and a first layer of Sn, the fracture strength is increased.

[0010] The third layer may contain AuSn4. Even if the intermetallic compound is AuSn4, which has low hardness and is prone to cracking among AuSn intermetallic compounds, sandwiching it between a softer metal makes the third layer less likely to bend or crack, and further increases the fracture strength.

[0011] The electronic component may be an LED. This prevents defects due to breakage during the many processes that follow when the wiring board with the LED attached is assembled into a display or the like. [Effects of the Invention]

[0012] According to the present invention, a joint structure with high breaking strength can be provided. [Brief explanation of the drawings]

[0013] [Figure 1] 1 is a schematic cross-sectional view showing a mounting substrate including a joint structure according to an embodiment of the present invention. [Figure 2] 1 is a schematic cross-sectional view showing a wiring board to which a joining structure according to an embodiment of the present invention is applied; [Figure 3] FIG. 1 is a diagram showing an example of an SEM image. [Figure 4] 10A and 10B are diagrams for explaining a method of joining an electronic component to a wiring board. [Figure 5] 1 is a table showing measurement results of Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0014] A joint structure 100 according to an embodiment of the present invention will be described with reference to Figures 1 and 2. Figure 1 is a schematic cross-sectional view showing a mounting substrate 1 equipped with the joint structure 100 according to an embodiment of the present invention. Figure 2 is a schematic cross-sectional view showing a wiring substrate 3 to which the joint structure 100 according to an embodiment of the present invention is applied.

[0015] 1, the mounting board 1 includes an electronic component 2 and a wiring board 3. The mounting board 1 is constructed by mounting the electronic component 2 on the wiring board 3 with a bonding material 4 interposed therebetween.

[0016] The electronic component 2 includes a main body 6 and a pair of terminals 7. The main body 6 is a member that functions as the electronic component 2. The terminals 7 are metal parts formed on the main surface of the main body 6. The electronic component 2 is configured, for example, by a micro LED. The micro LED is a component that emits light in response to input from the wiring board 3.

[0017] The wiring board 3 includes a base material 8, a wall 9, and a pair of terminals 10. The base material 8 is a flat main body of the wiring board 3. The wall 9 is a member formed of an insulator on the upper surface of the base material 8. The wall 9 is made of a resin material such as epoxy resin, acrylic resin, phenolic resin, melamine resin, urea resin, or alkyd resin. Epoxy resin or acrylic resin is particularly preferred. The terminal 10 is a metal portion formed on the main surface of the base material 8. The terminal 10 is made of a material such as Ni, Cu, Ti, Cr, Al, Mo, Pt, or Au, or an alloy selected from at least two of these metals. A conductive film 12 is formed on the upper surface of the terminal 10. The conductive film 12 may be made of a film of Ti, Cu, Ni, Al, Mo, Cr, or Ag, or a film containing metal particles mixed with a binder.

[0018] The bonding material 4 is a member that bonds the terminals 7 of the electronic component 2 and the terminals 10 of the wiring board 3. The bonding material 4 functions as solder. Before assembly, the wiring board 3 includes a bonding material 4A that is disposed on the upper surface of the conductive film 12. During assembly, the terminals 10, the conductive film 12, the bonding material 4, and the terminals 7 are stacked together, and then solder bonding is performed. Therefore, an IMC layer 20 of an intermetallic compound (IMC) formed by reaction between the metals of the terminals 10, the conductive film 12, the bonding material 4, and the terminals 7 is formed at the connection interface between the bonding material 4 and the terminals 7.

[0019] A recess 11 is formed in the wall 9. The recess 11 is formed by a through-hole that penetrates the wall 9. As a result, the top surface of the base material 8 is exposed at the bottom of the recess 11. The recess 11 is rectangular when viewed in the thickness direction of the wiring board 3. The terminals 7, 10, conductive film 12, and bonding material 4 are arranged in the recess 11 formed in the wall 9, and are thereby surrounded by the wall 9. Small gaps are formed between the terminals 7, 10, conductive film 12, and bonding material 4 and the four inner surfaces of the recess 11 (i.e., the inner surfaces of the wall 9).

[0020] Within the recess 11, a component 50 is disposed between the wall 9 and the electronic component 2 and bonding material 4. This supports the electronic component 2 with the component 50, making it less likely to peel off from the wiring board 3. Furthermore, the force applied to the electronic component 2, bonding material 4, and terminals 7 and 10 is alleviated, improving reliability. Examples of materials that can be used for the component 50 include epoxy resin, acrylic resin, phenolic resin, melamine resin, urea resin, alkyd resin, and mixtures thereof, as well as mixtures of any of the above resin materials with SiOx, ceramics, etc. Particularly preferably, epoxy resin or acrylic resin is used as the material for the component 50.

[0021] The junction structure 100 according to this embodiment includes a terminal 10, a conductive film 12, a bonding material 4, an IMC layer 20, and a terminal 7, which are layered in this order from the top surface of a substrate 8. The junction structure 100 includes a first layer 21 that is provided on one side of the electronic component 2 and the wiring board 3 and is made of a first metal containing Sn. The junction structure 100 also includes a second layer 22 that is provided on the other side of the electronic component 2 and the wiring board 3 and is made of a second metal that forms an intermetallic compound with Sn.

[0022] In this embodiment, the terminal 7 of the electronic component 2 corresponds to the second layer 22, and the bonding material 4 on the wiring board 3 side corresponds to the first layer 21. Therefore, the IMC layer 20 is provided at the bonding interface between the terminal 7 and the bonding material 4, and is composed of an intermetallic compound of a first metal and a second metal.

[0023] The first metal of the bonding material 4 may contain Sn or may be made of an alloy containing Sn. The first metal may contain, in addition to Sn, an element that lowers the melting point of Sn. An example of an element that lowers the melting point of Sn is Bi.

[0024] The second metal of the terminal 10 is any one of Au, Cu, Ni, Ag, and Pd, or an alloy of at least two of these metals. The second metal may be a metal containing at least Au. In this case, the IMC layer 20 contains AuSn4.

[0025] The average thickness of the IMC layer 20 is preferably 0.1 μm or more, and more preferably 0.2 μm or more. Because the surface of the Sn-containing bonding material 4 is rough, by making the IMC layer 20 at least this size, the bonding between the bonding material 4 and the terminal 7 is ensured, and electrical conductivity is ensured. The average thickness of the IMC layer 20 is preferably 0.5 μm or less, and more preferably 0.4 μm or less. By making the brittle IMC layer 20 at or below this size, bonding reliability can be improved.

[0026] A method for measuring the average thickness of the IMC layer 20 described above will be described. First, a portion near the center of the resulting junction structure 100 is cut perpendicular to the wiring substrate 3, and the phases of each layer are identified based on the element ratios measured by SEM-EDS. The average thickness of the IMC layer 20 is measured from the SEM image. Specifically, multiple points (e.g., five points) are taken at equal intervals on the interface between the second layer 22 and the IMC layer 20, and the shortest distance from each point to the interface between the first layer 21 and the IMC layer 20 is measured. The average of these multiple (five) distances is defined as the average thickness of the IMC layer 20. The average thickness based on the shortest distances between the multiple points should be 0.1 μm or more and 0.5 μm or less.

[0027] An example of an SEM image is shown in Figure 3. The interface between the first layer 21 and the IMC layer 20 is indicated by "F1." The interface between the second layer 22 and the IMC layer 20 is indicated by "F2." A number of points are taken at equal intervals from the interface F2.

[0028] The average thickness of the IMC layer 20 may be measured by determining the area of ​​the IMC layer 20 by image analysis and dividing the area by the length of the interface F2 to calculate the average thickness. The average thickness determined by this measurement method should be 0.1 μm or more and 0.5 μm or less.

[0029] Next, a method for bonding an electronic component 2 to a wiring board 3 will be described with reference to FIG. 4( a). First, as shown in FIG. 4( a), the terminals 7 of the electronic component 2 are placed on the bonding material 4A of the wiring board 3. If the bonding material 4A is heated for a long time (several minutes) at a temperature above the melting point of Sn, the entire structure will have a eutectic structure, making it difficult to form a thin IMC layer 20 in the bonding structure 100. Therefore, the bonding material 4A is heated for a short time and then rapidly cooled when it reaches a temperature at which the first metal of the Sn-containing bonding material 4A melts. For example, rapid heating and cooling may be performed by applying pulsed electromagnetic waves so that only the Sn-containing bonding material 4A melts instantaneously. As shown in FIG. 4( a), a cooling plate 30 is brought into contact with the wiring board 3 having the Sn-containing bonding material 4A, and a heating plate 31 is brought into contact with the electronic component 2 having the terminals 7 containing the second metal that forms an intermetallic compound with Sn. Then, temperature control may be performed so that only the contact portions between the terminals 7 and the bonding material 4A melt, forming the IMC layer 20 (see FIG. 4( b)). The bonding method is not particularly limited, and bonding may be performed using light energy.

[0030] Next, the functions and effects of the joint structure 100 according to this embodiment will be described.

[0031] The joint structure 100 according to this embodiment includes an IMC layer 20 made of an intermetallic compound between a first layer 21 made of a first metal containing Sn and a second layer 22 made of a second metal that forms an intermetallic compound with Sn. Here, metals are generally soft materials with ductility due to the metallurgical bonding. On the other hand, intermetallic compounds are hard and brittle materials. Therefore, the average thickness of the IMC layer 20 made of an intermetallic compound is set to 0.1 μm or more and 0.5 μm or less. By providing such a thin IMC layer 20, the intermetallic compound makes the joint structure 100 less likely to bend, and the metal sandwiching the intermetallic compound makes it less likely to break. As a result, a joint structure 100 with high fracture strength and high reliability can be obtained.

[0032] The second metal is any one of Au, Cu, Ni, Ag, and Pd, or an alloy selected from at least two of these metals. In this case, the second layer 22 is likely to form an intermetallic compound with Sn.

[0033] The second metal may be a metal containing at least Au. By sandwiching a thin third layer between the second layer 22 made of Au, which is soft and has a particularly low Young's modulus among metals, and the first layer 21 made of Sn, the fracture strength is further increased.

[0034] The IMC layer 20 may contain AuSn4. Even if the intermetallic compound is AuSn4, which has low hardness and is prone to cracking among AuSn intermetallic compounds, sandwiching the IMC layer 20 between softer metals makes the joining structure 100 less likely to bend or crack, and further increases the fracture strength.

[0035] The electronic component 2 may be an LED. This prevents defects due to breakage during the subsequent assembly of the wiring board with the LED mounted thereon into a display or the like through many steps.

[0036] The present invention is not limited to the above-described embodiments.

[0037] In the above-described embodiment, the layer on the wiring board 3 side is the first layer 21, and the layer on the electronic component 2 side is the second layer 22. Alternatively, the layer on the wiring board 3 side may be the second layer 22, and the layer on the electronic component 2 side may be the second layer 22.

[0038] Furthermore, the arrangement, size, and number of layers of the joining structure are not particularly limited, and may be changed as appropriate within the scope of the present invention.

[0039] [Example] Examples 1 to 3 and Comparative Examples 1 and 2 will be described with reference to FIG. 5 . However, the present invention is not limited to these examples. First, a method for manufacturing a mounting substrate 1 according to the examples and comparative examples will be described. An LED was prepared as the electronic component 2, and an Au terminal 7 was formed on the LED. An electrodeposited Ni conductive film 12 was formed on a Cu terminal 10 on the substrate side, and then a Sn bonding material 4A was formed on the conductive film 12. With the Au terminal 7 of the electronic component 2 and the Sn bonding material 4A of the wiring substrate 3 in contact with each other, a cooling plate 30 was placed in contact with the wiring substrate 3 side to maintain a constant temperature of 50°C, while a heating plate 31 at 300°C to 310°C was placed in contact with the electronic component 2 side for 3 minutes to control the thickness of the IMC layer 20, thereby obtaining a mounting substrate 1. Examples 1 to 3 and Comparative Examples 1 and 2 were manufactured under the same conditions except for the average thickness of the IMC layer 20. The average thickness of the IMC layer 20 was measured by the aforementioned method of identifying the phase of each layer from the element ratio obtained by SEM-EDS measurement and measuring the average thickness of the IMC layer 20 from the SEM image. The average thickness of the IMC layer 20 is shown in Figure 5. Next, for the mounting substrates 1 of Examples 1 to 3 and Comparative Examples 1 and 2, the breaking strength of the LED bonding portions was measured using a bond tester. The measurement results are shown in Figure 5.

[0040] In Comparative Example 1, the absence of the IMC layer 20 presumably led to poor bonding and susceptibility to fracture. In Comparative Example 2, the brittle IMC layer 20 was thick, presumably leading to susceptibility to fracture. In Examples 2 and 3, the thin IMC layer 20 resulted in high fracture strength. In Example 1, the thin IMC layer 20 resulted in higher fracture strength than in Comparative Example 1, but compared to Examples 2 and 3, the Au terminal 7 and the Sn bonding material 4A were not sufficiently bonded, resulting in lower fracture strength. [Explanation of symbols]

[0041] 2...electronic component, 3...wiring board, 20...IMC layer (third layer), 21...first layer, 22...second layer, 100...bonding structure.

Claims

1. A joining structure in which an electronic component and a wiring board are joined, a first layer provided on one side of the electronic component and the wiring board and made of a first metal containing Sn; a second layer provided on the other side of the electronic component and the wiring board and made of a second metal that forms an intermetallic compound with Sn; a third layer provided at a bonding interface between the first layer and the second layer and composed of an intermetallic compound of the first metal and the second metal; A junction structure, wherein the average thickness of the third layer is 0.1 μm or more and 0.5 μm or less.

2. The joint structure according to claim 1 , wherein the second metal is any one of Au, Cu, Ni, Ag, and Pd, or an alloy selected from at least two of these metals.

3. The junction structure according to claim 2 , wherein the second metal is a metal containing at least Au.

4. The third layer is AuSn ４ The joining structure according to any one of claims 1 to 3, comprising:

5. The joining structure according to any one of claims 1 to 4, wherein the electronic component is an LED.

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