JP2011134925A - Electrode structure - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a full bonding strength using lead-free solder, without depending on an underlying Ni layer. <P>SOLUTION: In an electrode structure 10, an Ni-plating layer 12 is formed on an underlying electrode base 11, and a Co thin film layer 13 is formed on the Ni-plating layer 12. A solder layer 20 is formed not on the Ni-plating layer 12 but on the Co thin film layer 13. A sample to be bonded 30 is bonded to the electrode structure 10 by means of the solder layer 20. The Co thin-film layer 13 has a thickness of approximately 10 to 800 nm, and the Co thin-film layer 13 of this thickness can be formed by a vapor deposition method or a sputtering method. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an electrode structure joined using lead-free solder.

  Joining wires or the like to electrodes using solder has been conventionally performed, but conventionally used solder (PbSn: alloy of lead (Pb) and tin (Sn)) In recent years, the inclusion of harmful lead components has become a problem. For this reason, lead-free solder containing no lead component is used, but it is difficult to obtain performance equivalent to that of lead-containing solder in terms of bonding strength and the like.

  As lead-free solder materials, for example, SnAgCu-based, SnZnBi-based, SnCu-based, etc., which have tin (Sn) as a main component are known. When such a lead-free solder is used, an intermetallic compound is formed between the electrode material and the lead-free solder, thereby joining them. A metal that can be soldered can be used for the outermost surface of the electrode, and nickel (Ni) plating is often applied to the outermost surface. In this case, an intermetallic compound of Sn and Ni is formed.

  However, although bonding is performed by forming this intermetallic compound, as described in Patent Document 1, when this intermetallic compound is formed thick, its bonding strength (or durability) is It is known to be lower. In a lead-free solder having a high joining temperature, an intermetallic compound is likely to grow, and this point is more remarkable than a conventional lead-containing solder. In order to prevent the intermetallic compound from becoming thick, Patent Document 1 describes that when phosphorus (P) is added to the Ni plating layer, phosphorus (P) is also added to the solder. Yes. In forming the Ni plating layer used here, for example, hypophosphorous acid is added to the plating solution, and P is also added to the Ni plating layer. In the technique described in Patent Document 1, when the influence of P on the growth of intermetallic compounds was examined, it was shown that the growth of intermetallic compounds was suppressed by adding P to the solder itself. Therefore, by using P-added lead-free solder, it is possible to obtain a joint with high joint strength and high durability.

International Publication No. W02006 / 131979

  However, the technique described in Patent Document 1 utilizes the fact that P is added to the Ni plating layer that is the outermost surface to be soldered. For this reason, the effect is not acquired when Ni plating layer to which P is not added is used. Alternatively, in this technique, good bonding is obtained by optimizing the combination of the Ni plating layer material which is the outermost surface and the lead-free solder material.

  On the other hand, if we can devise only the outermost surface to be soldered and obtain a good joint using lead-free solder on it, the degree of freedom in the manufacturing process will increase, and this structure will be used. Can be manufactured at low cost. In the technique described in Patent Document 1, since both the outermost surface to be soldered and the solder are adjusted, it is difficult to manufacture a device using this at low cost.

  That is, it is difficult to obtain a sufficient bonding strength using lead-free solder regardless of the underlying Ni layer.

  The present invention has been made in view of such problems, and an object thereof is to provide an invention that solves the above problems.

In order to solve the above problems, the present invention has the following configurations.
The electrode structure of the present invention is an electrode structure that is joined by solder containing tin (Sn) as a main component, the electrode base, a nickel (Ni) layer formed on the electrode base, and the Ni layer And a cobalt (Co) layer having a thickness of 10 nm to 800 nm, which is formed on and is in direct contact with the solder.
In the electrode structure of the present invention, the cobalt layer is formed by any one of vapor deposition, sputtering, and plating.
In the electrode structure of the present invention, the electrode base is made of copper (Cu) or a copper alloy.
In the electrode structure of the present invention, the solder is an alloy of tin (Sn), silver (Ag), and copper (Cu).

  Since the present invention is configured as described above, lead-free solder can be used, and sufficient bonding strength can be obtained regardless of the underlying Ni layer.

It is sectional drawing of the electrode structure which concerns on embodiment of this invention. It is a photograph of the torn surface at the time of using Co thin film layer (Example). It is a photograph of the fracture surface when not using Co thin film layer (comparative example).

  Hereinafter, an electrode structure according to an embodiment of the present invention will be described. This electrode structure is designed on the assumption that it is joined by lead-free solder mainly composed of tin (Sn). This cross-sectional structure is shown in FIG. In this electrode structure 10, a Ni plating layer 12 is formed on the electrode base 11, and a Co thin film layer 13 is formed on the Ni plating layer 12. The solder layer 20 is not formed on the Ni plating layer 12 but on the Co thin film layer 13. The sample 30 to be joined is joined to the electrode structure 10 by the solder layer 20.

  Here, as the electrode base 11, copper (Cu) or a copper alloy which is a general electrode material and has high electric conductivity is used. The Ni plating layer 12 is used as a layer to be soldered similarly to the case of using lead-containing solder. Therefore, the electrode substrate 11 does not need to be made of a material that can be soldered, and may be made of a material that can form the Ni plating layer 12 on the surface.

  The Ni plating layer 12 is formed on the electrode substrate 11 (copper) with a thickness of about 5 μm, for example, by electroless plating. Here, as a reducing agent in a plating solution in electroless plating, in addition to hypophosphorous acid to which phosphorus (P) is added as described in Patent Document 1, boron (B Dimethylamine borane to which) is added, hydrazine from which almost pure Ni is formed, and the like can also be used. Further, electrolytic plating may be used.

  The Co thin film layer 13 has a thickness of about 10 nm to 800 nm, and is formed on the Ni plating layer 12 to be soldered. The Co thin film layer 13 having this thickness can be formed by vapor deposition or sputtering. It can also be obtained by an extremely short time plating process (flash plating).

  On the electrode structure 10, a solder layer 20 containing lead-free solder as a main component is bonded, so that the sample 30 to be bonded is bonded to the electrode structure 10. The sample 30 to be joined is sufficient if it is made of a material that can be joined by the solder layer 20, and may have the same structure as the electrode structure 10. As the lead-free solder, for example, SnAgCu, SnZnBi, SnCu, or the like can be used. All of these have tin (Sn) as a main component, and their melting point (joining temperature) is higher than that of conventional lead-containing solder (PbSn alloy).

  When the solder layer 20 is formed on the electrode structure 10, bonding is performed by forming an intermetallic compound of Sn, which is the main component in the solder layer 20, and Ni, which is the main component of the Ni plating layer 12. Made. However, since the growth of the intermetallic compound is suppressed by the presence of the Co thin film layer 13 and the film thickness can be kept thin, the bonding strength and reliability of this bonding are improved. That is, the Co thin film layer 13 functions as a barrier layer.

  Below, the result of investigating the joining when Co is formed on the Ni plating layer 12 and the solder layer 20 (lead-free solder) is fused thereon will be described.

  Here, Cu was used for the electrode substrate 11, and a Ni plating layer 12 having a thickness of 5 μm was formed on the surface thereof by electroless plating. A structure in which cobalt (Co), indium (In), germanium (Ge), and copper (Cu) thin films are formed on this structure is composed of lead-free solder SnAg3Cu0.5 (Sn as a main component, Ag and Cu added amount) Are 3 mass% and 0.5 mass%, respectively. Each thin film layer had a thickness of 500 nm and was formed by vapor deposition. Table 1 shows the joining situation immediately after joining. From this result, it is only Co that can be used as a thin film layer between the solder layer 20 and the Ni plating layer 12, and the bonding itself is impossible except for Co. In addition, there was no thin film layer, and it was also possible to join the solder layer 20 directly to the Ni plating layer 12.

  The fact that the bonding was possible means that an intermetallic compound of Sn as the main component of the solder layer 20 and Ni as the main component of the Ni plating layer 12 was formed. Therefore, the intermetallic compound formed was investigated for Co that could be bonded when used as a material for the thin film layer.

  First, the cross section of the bonding interface was filled with resin, polished with abrasive paper and alumina powder having a particle size of 0.3 μm, and then observed with a laser microscope. The result is shown in FIG. From this result, it can be confirmed that an intermetallic compound is formed, but the film thickness is not uniform. Since Co is extremely thin, the thin film layer cannot be confirmed by this cross-sectional photograph.

  When the thin film layer is Co and when there is no thin film layer and the solder layer 20 is directly joined to the Ni plating layer 12, the results of measuring the thickness of the intermetallic compound and the average calculated at the 8 positions calculated from the cross-sectional photograph are shown. It is shown in 2. Here, when there is no thin film layer, the result immediately after joining and after performing a high temperature holding test at 175 ° C. for 24 h is shown.

  From this result, it can be confirmed that the thickness of the intermetallic compound is 0.68 times in the case of using Co compared with the case of not using the thin film layer, and becomes thinner. This indicates that Co functions as a barrier layer that suppresses diffusion of Ni. Moreover, when there was no thin film layer, it has also confirmed that an intermetallic compound became thick after a high temperature holding test (24h). Therefore, when the Co thin film layer 13 in which the intermetallic compound is thin is used, the bonding strength between the Ni plating layer 12 and the solder layer 20 is increased.

  In order to investigate this point, comparative observations were made of the fracture surfaces when the Co thin film layer 13 was used (Example) and when it was not used (Comparative Example). Here, the Ni plating layer 12, the Co thin film layer 13, and the solder layer 20 are the same as those in Table 1. Here, when the Co thin film layer 13 is used, it is a fracture surface generated when the stress amplitude σa = 15 MPa and the number of repetitions N = 1348518 for the same sample as in Table 1, and the Co thin film layer When 13 is not used, the fracture surface is generated when σa = 12.5 MPa and N = 1484487. FIG. 3A is a photograph of a fractured surface when the Co thin film layer 13 is used, and FIG. 3B is a photograph of a fractured surface when this is not used. When the Co thin film layer 13 was used, the fracture surface was a non-smooth surface formed over two layers (the solder layer 20 and the Ni plating layer 12). On the other hand, when the Co thin film layer 13 was not used, the fracture surface was mottled, and the mottled portion was almost equal to the interface between the solder layer 20 and the Ni plating layer 12. This result shows that by using the Co thin film layer 13, the bonding state between the solder layer 20 and the Ni plating layer 12 is good, and a strong bonding strength can be obtained.

  The thickness of the Co thin film layer 13 is preferably 10 nm or more in order to exhibit sufficient barrier properties. In addition, although it is preferable that the intermetallic compound is thin, bonding is not performed unless the intermetallic compound of Ni and Sn is formed. The thickness is preferably 800 nm or less.

  For example, unlike the technique described in Patent Document 1, when the Co thin film layer 13 is used, Ni diffusion is suppressed by the Co thin film layer 13. Can be made thinner. Therefore, instead of the Ni plating layer 12, a Ni layer formed by a method other than plating can be used. Also, when using plating, electroless plating to which phosphorus (P) is added can be used, and electroless plating and electrolytic plating to which other components are added can also be used. That is, if the same Co thin film layer is formed on the structure in which the Ni layer is formed on the electrode base, the same effect can be obtained.

  The Co thin film layer 13 may be formed by any method as long as it can form the above-described thickness. However, it is necessary that the Ni plating layer 12 and the electrode substrate 11 are not adversely affected. For this reason, vapor deposition, sputtering, flash plating, and the like that can be formed at a low temperature are particularly preferable.

  In the above example, Cu is used as the electrode substrate 11. However, it is obvious that the material is the same as long as the Ni plating layer 12 can be formed thereon.

DESCRIPTION OF SYMBOLS 10 Electrode structure 11 Electrode base 12 Ni plating layer 13 Co thin film layer 20 Solder layer 30 To-be-joined sample

Claims (4)

It is an electrode structure to be joined by solder mainly composed of tin (Sn),
An electrode substrate;
A nickel (Ni) layer formed on the electrode substrate;
A cobalt (Co) layer having a thickness of 10 nm to 800 nm formed on the Ni layer and in direct contact with the solder;
An electrode structure comprising:   The electrode structure according to claim 1, wherein the cobalt layer is formed by any one of vapor deposition, sputtering, and plating.   The electrode structure according to claim 1, wherein the electrode base is made of copper (Cu) or a copper alloy.   The electrode structure according to any one of claims 1 to 3, wherein the solder is an alloy of tin (Sn), silver (Ag), and copper (Cu).