Bump junction structure

Abstract

In this bump junction structure, surfaces of bump layers of a first bump structure and a second bump structure are covered with cover layers, whereby whiskers are less likely to be generated from the surfaces of the respective bump layers. That is, by covering, with the cover layers, at least portions of the surfaces of the bump layers, it is possible to inhibit generation of whiskers from the surfaces of the bump layers in regions covered with the cover layers.

Description

Bump bonding structure

[0001] The present disclosure relates to a bump bonding structure.

[0002] Conventionally, tin-lead solder has been used for mounting components such as electronic components. However, lead was designated as a hazardous substance in the RoHS Directive enacted in 2006, and accordingly, the transition from tin-lead solder to lead-free solder has been strongly required. Since lead-free solder has a higher proportion of Sn, bonding at a higher temperature than conventional solders is required, and many effects of the change from conventional solders to lead-free solder have been confirmed, such as the generation of whiskers from the surface. Regarding the generation of whiskers, which is one of the confirmed effects, in recent years, component mounting with finer wiring has narrower inter-terminal distances, so electrical short circuits caused by whiskers connecting terminals are conceivable.

[0003] Japanese Patent Application Laid-Open No. 2003-286531

[0004] Through intensive research, the inventors have newly found a technique capable of suppressing whiskers in component mounting.

[0005] One aspect of the present disclosure aims to provide a bump bonding structure in which the generation of whiskers is suppressed.

[0006] A bump bonding structure according to one aspect of the present disclosure includes a pair of bump layers containing Sn as a main component and bonded to face each other, and a coating layer covering at least a part of each surface of the bump layers.

[0007] In the above bump bonding structure, when the pair of bump layers are bonded to each other, the coating layer covering at least a part of the surface of each bump layer makes it difficult for whiskers to generate from the surface.

[0008] According to various aspects of the present disclosure, a bump bonding structure in which the generation of whiskers is suppressed is provided.

[0009] This is a schematic cross-sectional view showing a bump joint structure according to one embodiment. This is a schematic cross-sectional view showing the configuration of each bump structure shown in Figure 1 before joining. This is a flowchart showing the procedure for manufacturing the bump structure shown in Figure 2. This is a diagram showing the steps of the manufacturing method for the bump structure shown in Figure 2. This is an enlarged cross-sectional view of a main part showing a coating layer of a different form than that shown in Figure 2.

[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 bump joint structure 100 according to one embodiment will be described with reference to Figure 1.

[0012] In the bump joint structure 100, a first bump structure 1A provided on a first substrate 10A and a second bump structure 1B provided on a second substrate 10B are joined together and electrically connected to each other. The first substrate 10A may be, for example, an electronic component or a mounting substrate, and similarly, the second substrate 10B may also be an electronic component or a mounting substrate. In this embodiment, the first bump structure 1A and the second bump structure 1B have the same structure. The structure of the first bump structure 1A and the second bump structure 1B may be different.

[0013] The structures of both the first bump structure 1A and the second bump structure 1B, as shown in Figure 2, include a bump layer 20 formed on an electrode pad 12 provided on the flat substrate surface 10a of the first substrate 10A or the second substrate 10B. Both substrates 10A and 10B are insulating and may be, for example, a silicon substrate or an organic substrate.

[0014] The electrode pad 12 is in the form of a film and has a thickness of, for example, 0.1 to 30 μm (10 μm as an example). The electrode pad 12 can be made of a metal film or an alloy film, for example, Cu, Cu alloy, Al, Al alloy, Au, or Au alloy. The electrode pad 12 according to this embodiment is made of Cu. The electrode pad 12 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 12 may be an elliptical or polygonal shape with a width of 2 μm or more when viewed in plan.

[0015] On the surface 10a of the substrate 10, the electrode pad 12 is surrounded by a frame 14. The frame 14 is insulating and can be made of a resist material containing, for example, an insulating resin. The frame 14 has an opening 14a through which the surface of the electrode pad 12 is exposed. In this embodiment, the opening 14a of the frame 14 is a perfect circle with a diameter of about 1 μm or more. Since the diameter of the opening 14a of the frame 14 is smaller than the diameter of the electrode pad 12, only a part of the surface of the electrode pad 12 is exposed through the opening 14a, not the entire surface. The frame 14 may cover the entire outer edge of the electrode pad 12 or it may cover it partially. The opening 14a of the frame 14 may be an elliptical or polygonal shape with a width of about 1 μm or more in a plan view. If the opening 14a 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 14a.

[0016] The bump layer 20 integrally covers the electrode pad 12 and the frame 14. The bump layer 20 is substantially dome-shaped, with its upper surface curved in a convex manner. In this embodiment, the bump layer 20 is a perfect circle with a diameter of 15 μm or less (8 μm as an example) in plan view. The bump layer 20 may be elliptical or polygonal in plan view. If the bump layer 20 is polygonal in plan view, the average value of the distances between substantially opposite sides in the polygon can be considered as the diameter of the bump layer 20. Since the diameter of the bump layer 20 is larger than the diameter of the opening 14a of the frame 14, the bump layer 20 covers the upper surface 14b in the peripheral region of the opening 14a. The bump layer 20 may cover the entire peripheral region of the opening 14a or it may cover only a portion of it. By designing the bump layer 20 to have a diameter of 15 μm or less, the bump structure 1A and 1B as a whole can be miniaturized, thereby enabling a narrower pitch and higher density arrangement by bringing adjacent bump structures 1A and 1B closer together on the substrates 10A and 10B.

[0017] The bump layer 20 is made of a material mainly composed of Sn. The bump layer 20 according to this embodiment is substantially made of pure Sn. The bump layer 20 made of pure Sn can be dissolved by reflow soldering to remove whiskers that form on the surface of the bump layer 20. The bump layer 20 has a thickness of, for example, 2 μm or more (8 μm as an example). By making the bump layer 20 bulky with a thickness of 2 μm or more, sufficient clearance can be ensured during bonding.

[0018] The bump layer 20 is covered by a coating layer 24. Specifically, the coating layer 24 covers the surface of the bump layer 20 (i.e., the convex curved surface). The coating layer 24 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 24 is made of Ag. The coating layer 24 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 24 may be made of a single layer, or it may be made of multiple layers with different materials and compositions.

[0019] In this embodiment, since the bump layer 20 is composed of pure Sn, whiskers are prone to forming on its surface. When forming the bump layer 20, the whiskers that form on the surface of the bump layer 20 can be temporarily removed by dissolving the coating by reflow. However, whiskers may still form even after reflow, depending on conditions related to the internal stress of the bump layer 20, conditions related to the external pressure applied to the bump layer 20 from the outside, and conditions related to temperature. The coating layer 24 has at least the function of preventing whiskers. The coating layer 24 can also function as an antioxidant film that suppresses oxidation of the surface of the bump layer 20.

[0020] In this embodiment, an underbump layer 30 (hereinafter also referred to as the UBM layer) is interposed between the electrode pad 12 and the bump layer 20. The UBM layer 30 covers the entire area of ​​the electrode pad 12 exposed from the opening 14a within the opening 14a of the frame 14. The UBM layer 30 in this embodiment directly covers the surface of the electrode pad 12. The UBM layer 30 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 30 can be 3 μm or less. The UBM layer 30 is composed of a material mainly composed of Pd or Ni. The UBM layer 30 in this embodiment is composed of pure Pd. The UBM layer 30 may contain trace amounts of P or B.

[0021] In this embodiment, an IMC layer 22, composed of an intermetallic compound (IMC), is interposed between the bump layer 20 and the UBM layer 30. Specifically, the IMC layer 22 is composed of an intermetallic compound containing the metals that constitute the bump layer 20 and the metals that constitute the UBM layer 30. For example, if the bump layer 20 is composed of pure Sn and the UBM layer 30 is composed of Pd, the IMC layer 22 is composed of a Sn-Pd intermetallic compound. Also, if the bump layer 20 is composed of pure Sn and the UBM layer 30 is composed of Ni, the IMC layer 22 is composed of a Sn-Pd intermetallic compound. The IMC layer 22 is in the form of a film and has a thickness of, for example, 0.5 to 3 μm (1.5 μm as an example).

[0022] Next, the manufacturing methods for the bump structures 1A and 1B described above will be explained with reference to Figures 3 and 4.

[0023] When manufacturing each bump structure 1A and 1B, first, each base material 10A and 10B is prepared (step S1 in Figure 3). In this step, the electrode pads 12 and frame 14 may be placed on the base material surface 10a of each base material 10A and 10B, or the base materials 10A and 10B may be brought in with the electrode pads 12 and frame 14 already placed on the base material surface 10a. In this state, as shown in Figure 4(a), a portion of the surface of the electrode pad 12 is exposed through the opening 14a of the frame 14.

[0024] Each substrate 10A and 10B can be subjected to known pretreatment as needed before forming the UBM layer 30. Examples of pretreatment include surface modification by excimer irradiation, degreasing, and acid cleaning.

[0025] Next, a UBM layer 30 is formed on the surface of the electrode pad 12 in the area exposed from the opening 14a of the frame 14 (step S2 in Figure 3). In this step, the UBM layer 30 is formed by reducing electroless plating. For this electroless plating, a plating solution containing a palladium compound, a complexing agent, and a reducing agent is used. From the viewpoint of improving the workability of electroless plating (bath stability, deposition rate), a plating solution containing hypophosphorous acid as a reducing agent can be used. As a result, as shown in Figure 4(b), the surface of the electrode pad 12 is directly covered by the UBM layer 30 within the opening 14a.

[0026] Furthermore, a bump layer 20 is formed on the electrode pad 12 via the UBM layer 30 (step S3 in Figure 3). In this step, the bump layer 20 is formed by reductive electroless plating. For this electroless plating, a plating solution containing a tin compound, a complexing agent, and a reducing agent is used, and buffering agents, antioxidants, surfactants, brighteners, etc., can be included as needed. After electroless plating, a reflow treatment is performed. The reflow treatment is, for example, a high-temperature treatment in a nitrogen atmosphere, in which the bump layer 20 is melted and then rapidly cooled and solidified to obtain a dome-shaped bump layer 20. There are no particular restrictions on the reflow conditions, but for example, the atmosphere is an oxygen concentration of 1000 ppm or less, the temperature is 200 to 300°C, and the holding time is 5 to 120 seconds. At this time, as shown in Figure 4(c), the opening 14a is completely filled by the UBM layer 30 and the upper surface 14b in the peripheral region of the opening 14a is covered.

[0027] Subsequently, a coating layer 24 is formed on the bump layer 20 (step S4 in Figure 3), completing the bump structures 1A and 1B shown in Figure 2. The coating layer 24 is formed by reducing electroless plating. For this electroless plating, a plating solution containing a silver compound, a complexing agent, and a reducing agent is used. After electroless plating, a reflow process is performed. There are no particular restrictions on the reflow conditions, but for example, the atmosphere is an oxygen concentration of 1000 ppm or less, the temperature is 200 to 300°C, and the holding time is 5 to 120 seconds. During the reflow process, the Ag in the coating layer 24 can diffuse into the bump layer 20. In this case, the Ag in the coating layer 24 and the Sn in the bump layer 20 are alloyed.

[0028] The first bump structure 1A and the second bump structure 1B 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 bump structure 1A and the second bump structure 1B against each other at a temperature below the melting point in a vacuum or an inert gas atmosphere. At this time, each bump layer 20 of the first bump structure 1A and the second bump structure 1B does not substantially melt, and their shape is substantially maintained before and after bonding. During solid-phase diffusion bonding, the coating layer 24 covering the surface of each bump layer 20 of the first bump structure 1A and the second bump structure 1B is fed out from the bonding interface 5, and at the bonding interface 5, the bump layer 20 of the first bump structure 1A and the bump layer 20 of the second bump structure 1B are in direct contact. Therefore, in the bump bonding structure 100, the bump layers 20 facing each other are joined at the bonding interface 5. The surface of the bump layer 20 other than the bonding interface 5 is covered by the coating layer 24 even after the first bump structure 1A and the second bump structure 1B are bonded together.

[0029] The joined first bump structure 1A and second bump structure 1B are integrally covered by a coating resin 102. The coating resin 102 has insulating properties and is composed of, for example, an epoxy resin or a polyurethane resin. If one of the base materials 10A and 10B is an electronic component and the other is a mounting substrate, the coating resin 102 may be an underfill material filled between the electronic component and the mounting substrate.

[0030] In the bump joint structure 100 described above, the coating layer 24 covering the surface of each bump layer 20 of the first bump structure 1A and the second bump structure 1B makes it difficult for whiskers to be generated from the surface of each bump layer 20. That is, by the coating layer 24 covering at least a portion of the surface of the bump layer 20, the generation of whiskers from the surface of the bump layer in the region covered by the coating layer 24 can be suppressed.

[0031] The coating layer 24 is composed of a material mainly composed of Ag, Au, or Pd, which effectively suppresses whiskers in the bump layer 20 composed of pure Sn. The coating layer 24 may be composed of a single layer, multiple layers, or a mixture of single-layer and multi-layer sections. As shown in Figure 5, the coating layer 24 can be a two-layer structure, for example, a first layer 25 and a second layer 26. The second layer 26 is interposed between the first layer 25 and the bump layer 20, and the proportion of the main component metal differs between the first layer 25 and the bump layer 20. Specifically, if the first layer 25 is composed of pure Ag, the second layer 26 is composed of an intermetallic compound of Ag from the first layer 25 and Sn from the bump layer 20, with a lower proportion of Ag compared to the first layer 25. Furthermore, if the first layer 25 is composed of a Sn-Ag alloy with a relatively high Ag content (Ag-rich), it may be composed of a Sn-Ag alloy with a relatively low Ag content (Sn-rich).

[0032] Furthermore, in the bump joint structure 100 described above, the UBM layer 30 is interposed between the electrode pad 12 and the bump layer 20 in each bump structure 1A and 1B. Sn, the main component of the bump layer 20, tends to diffuse into the electrode pad 12 under high temperatures such as those in the reflow process described above, but the UBM layer 30 functions as a diffusion prevention layer, suppressing such Sn diffusion. As a result, erosion of the electrode pad 12 due to Sn diffusion is less likely to occur, and bump structures 1A and 1B with high dimensional accuracy can be realized.

[0033] When the main component metal of the UBM layer 30 is Pd, the UBM layer 30 is not limited to being composed of pure Pd; any material containing 90 wt% or more of Pd as the main component can function as a diffusion barrier layer. In this case, the UBM layer 30 may contain trace amounts of P as additives (sub-components). If the UBM layer 30 has a thickness of 0.1 μm or more, it can further suppress the diffusion of Sn, and if it has a thickness of 1.0 μm or more, it can almost completely suppress the diffusion of Sn. Since increasing the thickness of the UBM layer 30 leads to increased costs, the thickness of the UBM layer 30 can be kept to 3 μm or less.

[0034] The bump layer 20 and UBM layer 30 according to this embodiment are formed by electroless plating. In the fine bump structures 1A and 1B described above, it is difficult to form the bump layer 20 and UBM layer 30 by electroplating. Electroplating requires a seed layer, but it is difficult to pattern the seed layer with high positional accuracy in the narrow area on the surface of the electrode pad 12 surrounded by the frame 14. Even if it were possible to provide it, the resist used for patterning may become a residue, leading to a decrease in yield and an increase in cost. In this embodiment, the bump layer 20 and UBM layer 30 are formed by electroless plating, eliminating the need for a seed layer. This allows for the formation of the bump layer 20 and UBM layer 30 in the fine bump structures 1A and 1B described above with high yield and low cost. When the bump layer 20 is formed by electroless plating, compared to when it is formed by electroplating, components in the plating solution are less likely to be mixed into the bump layer 20 as impurities, and high crystallinity can be achieved.

[0035] As can be understood from the above description, this specification discloses the following: [Appendix 1] A bump joint structure comprising a pair of bump layers, which are mainly composed of Sn and are joined facing each other, and a coating layer that covers at least a portion of the surface of each of the bump layers. [Appendix 2] The bump joint structure according to Appendix 1, wherein the coating layer contains Ag, Au, or Pd as the main component metal. [Appendix 3] The bump joint structure according to Appendix 2, wherein the coating layer is composed of a plurality of layers with different proportions of the main component metal. [Appendix 4] The bump joint structure according to any one of Appendixes 1 to 3, further comprising a resin coating that integrally covers the pair of bump layers and the coating layer.

[0036] 1A, 1B... Bump structure, 5... Bonding interface, 10A, 10B... Substrate, 12... Electrode pad, 14... Frame, 20... Bump layer, 24... Coating layer, 100... Bump bonding structure, 102... Coating resin, S1 to S4... Process.

Claims

1. A bump joint structure comprising a pair of bump layers, each mainly composed of Sn and joined to each other facing each other, and a coating layer covering at least a portion of the surface of each of the bump layers.

2. The bump joint structure according to claim 1, wherein the coating layer contains Ag, Au, or Pd as the main component metal.

3. The bump joint structure according to claim 2, wherein the coating layer is composed of multiple layers with different proportions of main component metal.

4. The bump joint structure according to claim 1, further comprising a resin coating that integrally covers the pair of bump layers and the covering layer.

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