Bonding structure

Abstract

In this bonding structure, a bonding layer includes an intermediate layer between IMC layers, and the intermediate layer is not composed of the intermetallic compound, but is composed of Sn. Consequently, due to the intermediate layer, the total thickness of the IMC layers is thinner than if the bonding layer were composed solely of an IMC layer. The bonding strength increases in accordance with the total thickness of the fragile IMC layers being thinner, and thus high bonding strength in the bonding layer overall is achieved in this bonding structure.

Description

Joint structure

[0001] This disclosure relates to a bonding structure.

[0002] In recent years, with the advancement of electronics, the technology for mounting electronic components onto wiring boards has been developed accordingly. For example, when mounting a miniature electronic component onto a wiring board, the terminals on the electronic component side are made of Au (gold), and the terminals on the wiring board side are made of Sn (sin) using plating or thin film deposition, and the two terminals are joined by soldering or diffusion bonding. When the Au terminal on the electronic component side and the Sn terminal on the wiring board side are joined, an intermetallic compound layer composed of an intermetallic compound of Au and Sn (Sn-Au intermetallic compound) may be formed at the joint interface by a eutectic reaction.

[0003] Japanese Patent Publication No. 2003-286531

[0004] The intermetallic compounds mentioned above are relatively brittle, and it is conceivable that the intermetallic compound layer formed at the bonding interface may reduce the strength of the bonding interface (i.e., the bonding strength).

[0005] One aspect of this disclosure aims to provide a joint structure that improves joint strength.

[0006] A bonding structure relating to one aspect of this disclosure comprises a pair of terminal layers, each mainly composed of Sn and bonded to each other, and a bonding layer that includes at least a part of a sandwich structure, in which an intermediate layer, mainly composed of Sn and made of a material different from the intermetallic compound, is interposed between a pair of intermetallic compound layers, each composed of an intermetallic compound containing Sn, which is provided at the bonding interface between the pair of terminal layers.

[0007] In the above-described bonding structure, the bonding layer provided at the bonding interface between a pair of terminal layers includes at least a portion of a sandwich structure in which an intermediate layer is interposed between a pair of intermetallic compound layers. The intermediate layer is mainly composed of Sn and is made of a different material from the intermetallic compound that constitutes the intermetallic compound layer. In other words, in the sandwich structure portion of the bonding layer, the thickness of the intermetallic compound layer is reduced compared to when the bonding layer is composed only of intermetallic compound layers, due to the intermediate layer interposed between the pair of intermetallic compound layers. As a result, the overall strength of the bonding layer is improved in the above-described bonding structure.

[0008] According to various aspects of this disclosure, a joint structure is provided in which the joint strength is improved.

[0009] This is a schematic cross-sectional view showing a joint structure according to one embodiment. This is a schematic cross-sectional view showing the configuration of the first terminal portion shown in Figure 1 before joining. This is a schematic cross-sectional view showing the configuration of the second terminal portion shown in Figure 1 before joining. This is a schematic cross-sectional view showing the state of the joint interface between the first terminal portion and the second terminal portion. This is a schematic cross-sectional view showing a joint interface of a different form than that shown in Figure 4. This is a schematic cross-sectional view showing a joint interface of a different form than that shown in Figure 4. This is a schematic cross-sectional view showing a joint interface of a different form than that shown in Figure 4.

[0010] Embodiments of this disclosure will be described in detail below with reference to the attached drawings. In the description of the drawings, the same or equivalent elements will be denoted by the same reference numeral, and redundant descriptions will be omitted.

[0011] First, a joint structure 100 according to one embodiment will be described with reference to Figure 1.

[0012] In the bonded structure 100, a first terminal portion 20 provided on the first substrate 10A and a second terminal portion 30 provided on the second substrate 10B are joined together and electrically connected to each other. In this embodiment, the first substrate 10A is an insulating wiring board, and may be, for example, a glass substrate, a silicon substrate, or an organic substrate. In this embodiment, the second substrate 10B is an electronic component mounted on the wiring board of the first substrate 10A.

[0013] The first terminal portion 20, before bonding, is composed of a plurality of conductive layers 21 to 24 laminated on the flat substrate surface 10a of the first substrate 10A, as shown in Figure 2. In this embodiment, the first terminal portion 20 is composed of four conductive layers, which are laminated in order from the substrate surface 10a: a base layer 21, a diffusion suppression layer 22, a terminal layer 23, and a coating layer 24. The base layer 21 is a metal or alloy layer directly laminated on the substrate surface 10a, and in this embodiment is composed of Cu. The thickness of the base layer 21 is 0.1 to 30 μm (1 μm as an example). The diffusion suppression layer 22 is a metal or alloy layer laminated on the base layer 21, and is composed of a material mainly composed of Ni or Pd. In this embodiment, the diffusion suppression layer 22 is composed of Ni. The thickness of the diffusion suppression layer 22 is 1 to 3 μm (2 μm as an example). The terminal layer 23 is a metal or alloy layer laminated on the diffusion suppression layer 22, and in this embodiment, it is composed of Sn. The thickness of the terminal layer 23 is 2 to 10 μm (7 μm as an example). The coating layer 24 is a metal or alloy layer covering the surface of the terminal layer 23, and in this embodiment, it is composed of Ag. The thickness of the coating layer 24 is 0.01 to 0.2 μm (0.05 μm as an example). The diffusion suppression layer 22 and the terminal layer 23 can be formed by electroless plating. In this case, compared to when they are formed by electrolytic plating, components in the plating solution are less likely to be mixed into the diffusion suppression layer 22 and the terminal layer 23 as impurities, and high crystallinity can be achieved.

[0014] The second terminal portion 30, before joining, is configured to include a terminal layer 33 formed on an electrode pad 31 provided on the flat substrate surface 10a of the second substrate 10B, as shown in Figure 3.

[0015] The electrode pad 31 is in the form of a film and has a thickness of, for example, 0.1 to 30 μm (1 μm as an example). The electrode pad 31 can be made of a metal film or an alloy film, and can be made of, for example, Cu, Cu alloy, Al, Al alloy, Au, or Au alloy. The electrode pad 31 according to this embodiment is made of Cu. The electrode pad 31 according to this embodiment has a circular outer shape with a diameter of 2 μm or more when viewed in plan (i.e., when viewed from a direction perpendicular to the surface 10a of the base material 10). The outer shape of the electrode pad 31 may be an elliptical or polygonal shape with a width of 2 μm or more when viewed in plan.

[0016] On the surface 10a of the substrate 10, the electrode pad 31 is surrounded by a frame 35. The frame 35 is insulating and can be made of a resist material containing, for example, an insulating resin. The frame 35 has an opening 35a through which the surface of the electrode pad 31 is exposed. In this embodiment, the opening 35a of the frame 35 is a perfect circle with a diameter of about 1 μm or more in plan view. The diameter of the opening 35a of the frame 35 is designed to be smaller than the diameter of the electrode pad 31, so that only a part of the surface of the electrode pad 31 is exposed through the opening 35a, rather than the entire surface. The frame 35 may cover the entire outer edge of the electrode pad 31 or it may cover it partially. The opening 35a of the frame 35 may be an elliptical or polygonal shape with a width of about 1 μm or more in plan view. If the opening 35a is a polygon in plan view, the average value of the distances between substantially opposite sides in the polygon can be considered as the diameter of the opening 35a.

[0017] The terminal layer 33 integrally covers the electrode pad 31 and the frame 35. The terminal layer 33 has a substantially dome shape, and its upper surface is curved in a convex shape. In this embodiment, the terminal layer 33 has a circular shape with a diameter of 15 μm or less (8 μm as an example) in a plan view. The terminal layer 33 may also have an elliptical or polygonal shape in a plan view. If the terminal layer 33 is polygonal in a plan view, the average value of the distances between substantially opposite sides in the polygon can be considered as the diameter of the terminal layer 33. Since the diameter of the terminal layer 33 is larger than the diameter of the opening 35a of the frame 35, the terminal layer 33 covers the upper surface 35b in the peripheral region of the opening 35a. The terminal layer 33 may cover the entire peripheral region of the opening 35a or it may cover it partially. By designing the terminal layer 33 to have a diameter of 15 μm or less, the entire second terminal portion 30 can be miniaturized, thereby enabling a narrower pitch and higher density arrangement by bringing adjacent second terminal portions 30 closer together on the second substrate 10B.

[0018] The terminal layer 33 has a thickness of, for example, 2 μm or more (8 μm as an example). By making the terminal layer 33 bulky with a thickness of 2 μm or more, sufficient clearance can be ensured during bonding. The terminal layer 33 can be formed by electroless plating, in which case, compared to when it is formed by electrolytic plating, components in the plating solution are less likely to be mixed into the terminal layer 33 as impurities, and high crystallinity can be achieved.

[0019] The terminal layer 33 is covered by a coating layer 34. Specifically, the coating layer 34 covers the surface of the terminal layer 33 (i.e., the convex curved surface). The coating layer 34 is made of a conductive material (metal or alloy), and may be made of a material mainly composed of Ag, Au, or Pd. In this embodiment, the coating layer 34 is made of Ag. The coating layer 34 has a thickness of, for example, 0.01 μm or more and 0.5 μm or less (0.1 μm as an example). The coating layer 34 may be made of a single layer, or it may be made of multiple layers with different materials and compositions. The coating layer 34 can also function as an antioxidant film that suppresses oxidation of the surface of the terminal layer 33.

[0020] In this embodiment, an underbump layer 32 (hereinafter also referred to as the UBM layer) is interposed between the electrode pad 31 and the terminal layer 33. The UBM layer 32 covers the entire area of ​​the electrode pad 31 exposed from the opening 35a within the opening 35a of the frame 35. In this embodiment, the UBM layer 32 directly covers the surface of the electrode pad 31. The UBM layer 32 is in the form of a film and has a thickness of, for example, 1 μm or more (1.5 μm as an example). The thickness of the UBM layer 32 can be 3 μm or less. The UBM layer 32 is composed of a material mainly composed of Ni or Pd. In this embodiment, the UBM layer 32 is composed of pure Ni. The UBM layer 32 may contain trace amounts of P or B. The UBM layer 32 can be formed by electroless plating. In this case, compared to when it is formed by electrolytic plating, components in the plating solution are less likely to be mixed into the UBM layer 32 as impurities, and high crystallinity can be achieved.

[0021] The first terminal portion 20 and the second terminal portion 30 are joined by solid-phase diffusion bonding while facing each other. In this embodiment, solid-phase diffusion bonding is performed, for example, by pressing the first terminal portion 20 and the second terminal portion 30 together at a temperature below the melting point in a vacuum or an inert gas atmosphere. At this time, the terminal layers 23 and 33 of the first terminal portion 20 and the second terminal portion 30 do not substantially melt, and their shapes are substantially maintained before and after bonding. As a result of the bonding, a bonding layer 40 is formed at the bonding interface between the first terminal portion 20 and the second terminal portion 30. More specifically, the bonding layer 40 is interposed between the terminal layer 23 of the first terminal portion 20 and the terminal layer 33 of the second terminal portion 30, which face each other, and the terminal layer 23 of the first terminal portion 20 and the terminal layer 33 of the second terminal portion 30 are electrically connected via the bonding layer 40. The surfaces of the terminal layers 23 and 33 other than the bonding interface are covered by coating layers 24 and 34 even after bonding.

[0022] As shown in Figure 4, the bonding layer 40 includes a sandwich structure in which an intermediate layer 42 is interposed between a pair of IMC layers 41A and 41B.

[0023] Each IMC layer 41A, 41B is composed of an intermetallic compound (IMC), and in this embodiment, it is composed of an Ag-Sn intermetallic compound containing Sn from the terminal layers 23, 33 and Ag from the coating layers 24, 34. Such an intermetallic compound can be formed by the diffusion of elements constituting the terminal layers 23, 33 into the coating layers 24, 34, or by the diffusion of elements constituting the coating layers 24, 34 into the terminal layers 23, 33. The Ag in the coating layers 24, 34 readily forms an intermetallic compound with the Sn in the terminal layers 23, 33. For example, most of the constituent elements (Ag) of the coating layers 24, 34 before bonding (for example, 99 at% or more) become an intermetallic compound, constituting each IMC layer 41A, 41B. In this case, the Ag-Sn intermetallic compound in each IMC layer 41A, 41B (for example, Ag ３ Sn, Ag ０．９１８ Sn ０．０８２ The percentage of ) is approximately 75 at%, for example.

[0024] The intermediate layer 42 is made of a different material from the intermetallic compounds that make up the intermetallic compound layers 41A and 41B, and is made of a material mainly composed of Sn. More specifically, the intermediate layer 42 is made of a metal, alloy, or oxide mainly composed of Sn, and in this embodiment it is made of Sn. The intermediate layer 42 can be formed when the IMC layers 41A and 41B are formed. That is, the Sn that makes up each terminal layer 23 and 33 is diffused into the coating layers 24 and 34 to form the IMC layers 41A and 41B, but some of the diffused Sn aggregates between the IMC layers 41A and 41B to form the intermediate layer 42. Therefore, the proportion of Sn in the intermediate layer 42 is higher than the proportion of Sn in the IMC layers 41A and 41B. The proportion of Sn in the intermediate layer 42 may be the same as or lower than the proportion of Sn in the terminal layers 23 and 33.

[0025] The sandwich structure of the pair of IMC layers 41A, 41B and the intermediate layer 42 does not necessarily need to be formed continuously over the entire intermediate layer 42 as shown in FIG. 4, and may be formed partially on the intermediate layer 42. For example, as shown in FIG. 5, one IMC layer 41A (or IMC layer 41B) may be intermittent, and the sandwich structure may be formed intermittently. Further, as shown in FIG. 6, both IMC layers 41A and 41B may both be intermittent, and the sandwich structure may be formed intermittently. Furthermore, as shown in FIG. 7, both IMC layers 41A and 41B are partially connected, and the sandwich structure may be formed intermittently. In this case, the intermediate layer 42 also becomes intermittent.

[0026] The thickness of the bonding layer 40 can be 0.05 to 0.5 μm (for example, 0.15 μm). Thickness d of IMC layer 41A １ and thickness d of IMC layer 41B ２ can be 0.02 to 0.3 μm (for example, 0.05 μm). Thickness d of IMC layer 41A １ and thickness d of IMC layer 41B ２ total thickness (d １ +d ２ ) can be 1 μm or less (for example, 0.1 μm). Thickness d of IMC layer 41A １ and thickness d of IMC layer 41B ２ may be the same or different. The thickness of the intermediate layer 42 can be 0.02 to 0.3 μm (for example, 0.05 μm).

[0027] The bonded first terminal portion 20 and second terminal portion 30 are integrally covered with a coating resin 102. The coating resin 102 has insulating properties and is made of, for example, an epoxy resin or a polyurethane resin. The coating resin 102 may be an underfill material filled between the first base material 10A that is a wiring substrate and the second base material 10B that is an electronic component.

[0028] In the bond structure 100 described above, a bonding layer 40 is formed at the bonding interface between the terminal layer 23 of the first terminal portion 20 and the terminal layer 33 of the second terminal portion 30. The intermetallic compounds (Ag-Sn intermetallic compounds) that make up the IMC layers 41A and 41B contained in the bonding layer 40 are relatively brittle, which leads to a decrease in the strength of the bonding interface (i.e., bonding strength). One aspect of bonding strength is shear strength, and if the shear strength is low, separation or disconnection of the terminal layer 23 of the first terminal portion 20 and the terminal layer 33 of the second terminal portion 30 may occur. The bonding layer 40 includes an intermediate layer 42 between the IMC layers 41A and 41B, and the intermediate layer 42 is not made of the above-mentioned intermetallic compound, but is made of Sn. Therefore, due to the intermediate layer 42, the total thickness of the IMC layers 41A and 41B is thinner compared to when the bonding layer 40 is made up only of the IMC layers 41A and 41B. In other words, the thickness d of the IMC layer 41A １ and the thickness d of the IMC layer 41B ２ The sum of these is less than the thickness D of the bonding layer 40 (D > d １ +d ２ ). As the combined thickness of the fragile IMC layers 41A and 41B decreases, the bonding strength increases, and therefore, in the bonding structure 100, high bonding strength is achieved for the entire bonding layer 40.

[0029] The intermediate layer 42 does not necessarily have to be composed of pure Sn; even if it is composed of an alloy or oxide mainly composed of Sn, the IMC layers 41A and 41B can be thinned, and high bonding strength can be achieved for the entire bonding layer 40. When the intermediate layer 42 is composed of pure Sn, the intermediate layer 42 has high ductility and elasticity, so even higher bonding strength can be achieved for the entire bonding layer 40.

[0030] In the bonding layer 40, the thickness d of the IMC layer 41A １ and the thickness d of the IMC layer 41B ２ By keeping the total thickness to 1 μm or less, the proportion of Sn becomes relatively higher, reducing the concentration gradient that acts as a diffusion driving force, thereby suppressing Kirkendal voids, which can lead to a decrease in bonding strength.

[0031] Furthermore, the diffusion-suppressing layer 22 of the first terminal portion 20 is made of a material mainly composed of Ni or Pd (for example, Ni), which suppresses the diffusion of Sn in the terminal layer 23 and Cu in the base layer 21 to form intermetallic compounds, thereby suppressing the generation of Kirkendal voids associated with such diffusion. This further improves the bonding strength. Similarly, the UBM layer 32 of the second terminal portion 30 is also made of a material mainly composed of Ni or Pd (for example, Ni), which suppresses the diffusion of Sn in the terminal layer 33 and Cu in the electrode pad 31 to form intermetallic compounds, thereby suppressing the generation of Kirkendal voids associated with such diffusion. This further improves the bonding strength.

[0032] The inventors conducted the following experiment regarding the bonding strength between terminal layers. For the experiment, samples 1 to 3 were prepared in which the first terminal layer corresponding to the terminal layer 23 of the first terminal section 20 described above and the second terminal layer corresponding to the terminal layer 33 of the second terminal section 30 described above were bonded in different configurations, and the shear strength was measured for each of the samples 1 to 3.

[0033] Each of samples 1 to 3 used a 10 mm square first terminal layer (4 μm thick) held on a silicon substrate and a 2 mm square second terminal layer (4 μm thick) held on a Si substrate. Sample 1 is an example, in which both the first and second terminal layers are made of Sn, and a sandwich-structured bonding layer 40 shown in Figure 4 is provided at the bonding interface. Sample 2 is a comparative example, in which both the first and second terminal layers are made of Sn, and a bonding layer composed entirely of an Ag-Sn intermetallic compound is provided at the bonding interface. Sample 3 is a comparative example, in which both the first and second terminal layers are made of an Ag-Sn intermetallic compound, and the bonding layer at the bonding interface is also made of an Ag-Sn intermetallic compound.

[0034] To measure the shear strength, a Bond Tester (4000Plus) manufactured by Nordson Corporation was used, and the measurement was performed at a shear height of 145 μm at a speed of 150 μm / second. As a result, the shear strength of sample 1 was 166.42 g / mm². ２ The shear intensity of sample 2 was 103.72 g / mm². ２ The shear strength of sample 3 was 31.33 g / mm².２ and the shear strength of Sample 1 was significantly higher. From these results, it was confirmed that high shear strength can be obtained by providing the bonding layer including the above-described sandwich structure at the bonding interface.

[0035] As can be understood from the above description, the present specification discloses the following. [Supplementary Note 1] A bonding structure comprising: a pair of terminal layers containing Sn as a main component and bonded to face each other; and a bonding layer provided at a bonding interface between the pair of terminal layers, at least a part of the bonding layer including a sandwich structure in which an intermediate layer containing Sn as a main component and made of a material different from the intermetallic compound is interposed between a pair of intermetallic compound layers made of an Sn-containing intermetallic compound. [Supplementary Note 2] The bonding structure according to Supplementary Note 1, wherein the Sn-containing intermetallic compound is an Sn-Ag based intermetallic compound. [Supplementary Note 3] The bonding structure according to Supplementary Note 1 or 2, wherein the intermediate layer is made of pure Sn. [Supplementary Note 4] The bonding structure according to any one of Supplementary Notes 1 to 3, wherein the total thickness of the pair of intermetallic compound layers is 1 μm or less. [Supplementary Note 5] The bonding structure according to any one of Supplementary Notes 1 to 4, wherein the terminal layer is formed on an electrode, and the bonding structure further comprises an intermediate layer interposed between the electrode and the terminal layer and containing Ni or Pd as a main component.

[0036] 20…first terminal portion, 23…terminal layer, 24…covering layer, 30…second terminal portion, 33…terminal layer, 34…covering layer, 40…bonding layer, 100…bonding structure.

Claims

1. A bonding structure comprising a pair of terminal layers, each mainly composed of Sn and bonded to each other facing each other, and a bonding layer that at least partially includes a sandwich structure in which an intermediate layer, mainly composed of Sn and made of a material different from the intermetallic compound, is interposed between a pair of intermetallic compound layers, each composed of an intermetallic compound containing Sn, which is provided at the bonding interface between the pair of terminal layers.

2. The bonding structure according to claim 1, wherein the intermetallic compound containing Sn is a Sn-Ag intermetallic compound.

3. The bonding structure according to claim 1, wherein the intermediate layer is composed of pure Sn.

4. The bonding structure according to claim 1, wherein the total thickness of the pair of intermetallic compound layers is 1 μm or less.

5. The bonding structure according to any one of claims 1 to 4, wherein the terminal layer is formed on an electrode, and further comprises an intermediate layer interposed between the electrode and the terminal layer, the intermediate layer having Ni or Pd as its main component.

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