WO2010061259A1

WO2010061259A1 - Plating substrate having sn plating layer, and fabrication method therefor

Info

Publication number: WO2010061259A1
Application number: PCT/IB2009/007421
Authority: WO
Inventors: Takashi Nomura; Yasufumi Shibata
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2008-11-27
Filing date: 2009-11-12
Publication date: 2010-06-03
Also published as: JP2010126766A

Abstract

During the fabrication of a plating substrate 1 that has a base material 2 of which at least a surface is made of Cu or a Cu alloy, and a Pb-free Sn plating layer 3 on the surface of the base material 2, a heat treatment is performed at a temperature below the melting temperature of the plating layer 3 after the Pb-free Sn plating layer 3 is formed on the base material surface. The heat treatment is performed so as to produce a Sn-Cu compound layer between the formed plating layer 3 and the surface of the base material 2. In the thus-fabricated plating substrate 1, the growth of whiskers on the plating layer 3 is considerably restrained.

Description

PLATING SUBSTRATE HAVING Sn PLATING LAYER, AND FABRICATION

METHOD THEREFOR

BACKGROUND OF THE INVENTION 1. Field of the Invention

[0001] The invention relates to a plating substrate having a Pb-free Sn plating layer, and to a fabrication method for the plating substrate. 2. Description of the Related Art

[0002] In electronic component parts, such as semiconductor devices, and the like, a substrate of an external terminal is formed from copper (Cu), a Cu alloy, a 42-alloy (an alloy containing 42% by weight of nickel (Ni), and further containing at least iron), or the like. If the substrate is left as it is, the surface of the terminal oxidizes, giving rise to the possibility of conduction failure due to imperfect soldering or the like. Therefore, the terminal surface is usually subjected to plating or the like to form a protective film (plating layer) on the surface, in order to prevent the oxidation of the terminal surface.

[0003] In a related-art technology, when a material of a plating layer is a tin (Sn) alloy or the like, the material usually contains lead (Pb). However, in recent years, it has been demanded that the material of a plating layer be free from lead (Pb), from the viewpoint of reducing the environmental load. As for the material of the plating layer of the aforementioned terminal, a material that does not contain Pb has come to be used, for example, pure tin (Sn), or a Sn alloy such as Sn-Cu alloy, Sn-Bi alloy, Sn-Ag alloy, etc. However, if the terminal surface of an electronic component part is plated with a Pb-free material, needle-like whiskers of Sn crystal having a diameter of, for example, about 3 μm, are generated from the plating layer. [0004] In recent years, it has been demanded that electronic component parts, such as semiconductor devices in which an IC chip is mounted on a lead frame, be further reduced in size. As a result, the clearances between terminals have become as small as several hundred micrometers. Since whiskers as mentioned above sometimes grow to a length of several hundred micrometers, the thus-formed whiskers are likely to short-circuit terminals of an electronic component part if the terminal clearances are as small as several hundred micrometers as mentioned above. Therefore, a measure for restraining the generation of whiskers is demanded.

[0005] The generation and growth mechanism of needle-like whiskers has not been fully elucidated. However, from an understanding that a cause for the whisker generation is the internal stress accumulated in the plating layer, it has been proposed to remove the internal stress of the plating layer in order to restrain the generation of needle-like whiskers. Japanese Patent Application Publication No. 10-144839 (JP-A-10-144839) describes that a Sn alloy plating layer that does not contain Pb, after being plated, is heated at a temperature above its melting point so as to cause the reflowing and therefore release the internal stress, whereby the generation of whiskers can be restrained.

[0006] Besides, Japanese Patent Application Publication No. 11-343594 (JP-A-11-343594) describes a material for electric or electronic component parts, and its fabrication method in which a base of which at least a surface is made of Cu or a Cu alloy is electro-plated in a Sn plating bath or a Sn alloy plating bath so as to form a Cu-containing Sn layer as a surface layer, and subsequently a reflow process is performed to dissolve the whiskers present in the surface layer and form an intermediate layer that is made up of a Cu₃Sn (ε phase) layer and a Cu₆Sn₅ (η' phase) layer layered in that order between the surface layer and the substrate surface, so that the junction reliability in the solder junction can be improved.

[0007] Besides, it has also been proposed that the generation of whiskers can be restrained by controlling the crystal orientation plane of the plating layer and the orientation index thereof. Japanese Patent Application Publication No. 2006-249460 (JP-A-2006-249460) describes a technology of restraining the generation of whiskers by forming a Sn alloy phase in a crystal grain boundary of a Sn plating layer, and describes that the orientation indexes on the (220) plane and the (321) plane of the plating layer are increased in order to facilitate the formation of the Sn alloy phase.

[0008] None of the foregoing related-art technologies for restraining the generation of whiskers by relaxing the internal stress of the Pb-free plating layer cannot rightfully be said to have yielded sufficient results. Besides, if after a plating process is performed, the plating layer is caused to reflow as described in Japanese Patent Application Publication No. 10-144839 (JP-A-10-144839) or Japanese Patent Application Publication No. 11-343594 (JP-A-11-343594), there is possibility of deviation of the thickness of the plating, which needs to be bettered in view of means for preventing whisker generation. As for the technology of controlling the generation of whiskers by controlling the orientation index of a specific crystal orientation plane, it is difficult to find such a specific crystal orientation plane, and it is also not easy to control the orientation index of a crystal plane on the plating surface so that the specific crystal orientation plane is preferentially treated.

SUMMARY OF THE INVENTION

[0009] The invention provides a plating substrate having a Pb-free Sn plating layer which is capable of restraining the generation of whiskers by relatively easy means, and also provides a fabrication method for the substrate.

[0010] A first aspect of the invention relates to a fabrication method for a plating substrate having a base material whose surface is made of Cu or a Cu alloy. This fabrication method includes: forming a Sn plating layer that contains Sn and is free from Pb, on a surface of the base material; and heat-treating the base material and the plating layer at a temperature at which a Sn-Cu compound layer that contains a compound of Sn and Cu is produced between the plating layer formed and the surface of the base material, and which is lower than or equal to a temperature at which the plating layer melts. [0011] In the fabrication method in accordance with this aspect of the invention, the temperature at which the base material and the plating layer are heat-treated may be lower than the temperature at which the plating layer melts.

[0012] According to the foregoing construction, since the heat treatment is performed at a temperature below the melting point of the plating layer, the film thickness of the plating layer thus formed does not fluctuate, so that a good plating substrate will be obtained. Besides, since the Sn-Cu compound layer is formed between the plating layer and the base material surface, the generation of whiskers on the plating substrate is restrained. [0013] In the fabrication method in accordance with the foregoing aspect, heat-treating of the base material and the plating layer at the temperature may be performed within five hours after the Sn plating layer is formed, or may also be performed immediately after the Sn plating layer is formed.

[0014] In the fabrication method in accordance with this aspect, the base material and the plating layer may be heat-treated at the temperature until a thickness of the Sn-Cu compound layer produced becomes equal to or greater than 0.2 μm. According to the foregoing construction, the generation of whiskers on the plating substrate can be substantially completely restrained by forming the Sn-Cu compound layer whose thickness is greater than or equal to 0.2 μm. [0015] In the fabrication method according to the foregoing aspect, the Sn plating layer may consist of Sn, and the temperature at which the base material and the Sn plating layer are heat-treated may be lower than or equal to 230°C.

[0016] A second aspect of the invention relates to a plating substrate that is fabricated by using the fabrication method according to any one of claims 1 to 7, and that has: a base material of which at least a surface is made of Cu or a Cu alloy; a Pb-free Sn plating layer that is formed on the surface of the base material; and a Sn-Cu compound layer whose thickness is greater than or equal to 0.2 μm and which is provided between the surface of the base material and the Pb-free Sn plating layer, and over an entire area where the Sn plating layer is formed. [0017] According to this construction, since the Sn-Cu compound layer whose thickness is greater than or equal to 0.2 μm is formed over the entire area where the plating layer is formed, the generation of whiskers is greatly restrained.

[0018] In the plating substrate in accordance with this aspect, the Sn plating layer may have Sn crystal grains that are made of a Sn crystal, and a (110) plane of the Sn crystal grains may be present on a surface of the Sn plating layer, at a proportion of 10% or higher, and the Sn-Cu compound layer may have a thickness of 0.3 μm or greater.

[0019] According to the invention, when a plating substrate having a Pb-free Sn plating layer on a surface of the base material is fabricated, the generation of whiskers on the plating substrate can be certainly restrained. Besides, a good plating substrate whose plating layer has a film thickness that does not fluctuate can be obtained.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The foregoing and further objects, features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein:

FIG. 1 is a schematic diagram of a plated member according to an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

[0021] The present inventors, through many experiments on the generation of whiskers on a Pb-free Sn plating layer, have found that in the case where the base material is made of Cu or a Cu alloy, the generation of whiskers can be restrained, without causing deviation of the thickness of the plating layer, by subjecting the base material having a plating layer to a heat treatment at a temperature that is lower than the melting point of the plating layer. Although the reason of this phenomenon is not clear, it is considered that the heat treatment forms a Cu-Sn compound layer between the base material surface and the plating layer, and the thus-formed Cu-Sn compound layer restrains the generation of whiskers.

[0022] Hereinafter, an embodiment of the invention will be described. FIG. 1 shows an example of a plating substrate that is fabricated by using a method according to the embodiment.

[0023] In the example shown in FIG. 1, a plating substrate 1 includes a base material 2 made up of copper (Cu) or a Cu alloy, and a lead (Pb)-free tin (Sn) plating layer 3 containing Sn which is formed on the base material 2. Between the Sn plating layer 3 and the adjacent surface of the base material 2, a Sn-Cu compound layer 4 containing a compound of Sn and Cu which is formed by performing a heat treatment of the base material 2 and the Sn plating layer 3 at a temperature below the melting temperature of the Sn plating layer 3, for example, at a temperature of 230⁰C or lower. The Sn plating layer 2 is formed on the surface of the base material 2 by means such as electroplating or the like. The thickness of the Sn plating layer 2 is set appropriately according to the location of use of the plating substrate 1, or the purpose of use thereof, etc., but is usually about 5 to 10 μm. The plating solution is arbitrary, provided that the solution does not contain Pb. Known plating solutions can be appropriately used.

[0024] After the plating layer 3 is formed, the heating treatment at a temperature below the melting point of the plating layer 3 is performed from the plating layer 3 side, so as to form the foregoing Sn-Cu compound layer 4 between the surface of the base material 2 and the Sn plating layer 3. Since the thickness of the Sn-Cu compound layer 4 changes depending on the temperature and the time of the heat treatment, the temperature and the time of the heat treatment are set so that the Sn-Cu compound layer 4 having a desired thickness is formed by the heat treatment.

[0025] As shown in working examples later, the plating substrate 1 according to the invention more greatly restrains the growth of whiskers on the Sn plating layer 3 than a plating substrate that does not have the Sn-Cu compound layer 4.

[0026] As Example 1, many test pieces were prepared in which a Pb-free Sn plating layer 3 of 9 μm in thickness was formed on a surface of a base material 2, as shown in FIG. 1, by the electrolytic plating of the surface of the base material 2, which was a Cu alloy (concretely, C194), through the use of a Sn-1.5 Cu plating solution at a current density of 1.5 A/dm². On the plating layer surface of each of the test pieces, the occupancy proportion of the (110) plane of Sn crystal grains made of Sn crystal was 10% or greater.

[0027] Within five hours following the formation of the Sn plating layer, all the test pieces were subjected to a heat treatment at different heat treatment temperatures and for different treatment times (hours), and the thickness of the Sn-Cu compound layer 4 formed between the base material surface and the Sn plating layer of each test piece was measured. Results of the measurement are shown in Table 1. Incidentally, the heat treatment temperature was lower than the melting temperature of the Sn plating layer 3, for all the heat-treated test pieces. Besides, one test piece was not subjected to the heat treatment.

[0028] After the post-heat treatment test pieces and the test piece not subjected to the heat treatment were left at room temperature for 500 hours, the state of generation of whiskers on the Sn plating layer of each test piece was observed using a Scanning Electron Microscope (SEM). If whiskers were generated, a maximum value of the lengths of whiskers was measured. Results of the measurement are shown in Table 1. [Table 1]

[0029] As shown in Table 1, whiskers having great lengths grew in the test piece that were not heat-treated, and in the test pieces that were heat-treated but did not form the Sn-Cu compound layer 4. In the case where the heat treatment was performed so that a Sn-Cu compound layer was formed, whiskers grew if the thickness of the Sn-Cu compound layer was 0.2 μm or less, while the growth of whiskers was restrained at 15.0 μm if the compound layer was 0.2 μm thick. In the case where the thickness of the Sn-Cy compound layer was 0.3 μm or greater, no whisker grew.

[0030] . From these results, it is understood that the growth of whiskers can be restrained by performing the heat treatment at a temperature lower than the melting temperature of the plating layer, and that if the film thickness of the Sn-Cu compound layer produced between the plating layer and the base material is 0.2 μm or greater, the growth of whiskers can be restrained to such a degree that the whiskers do not pose a problem in practical use, and that the generation of whiskers can be completely prevented by growing the Sn-Cu compound layer to a film thickness of 0.3 μm or greater,.

[0031] In Example 2, many test pieces substantially the same as those in Example 1 were made. However, Example 2 is different from Example 1 in that the electric current density during the plating was 0.5 A/dm², and the occupancy proportion of the (110) plane of Sn crystal grains on the plating layer surface in each test piece was less than 10%.

[0032] Within five hours following the formation of the Sn plating layer, each test piece was subjected to the heat treatment at different heat treatment temperatures for different treatment times (hours), and the thickness of the Sn-Cu compound layer 4 formed between the base material surface and the Sn plating layer of each test piece was measured. Results of the measurement are shown in Table 2. Incidentally, the heat treatment temperature was lower than the melting temperature of the Sn plating layer 3, for all the heat-treated test pieces. Besides, one test piece was not subjected to the heat treatment. [0033] After the post-heat treatment test pieces and the test piece not subjected to the heat treatment were left at room temperature for 500 hours, the state of generation of whiskers on the Sn plating layer of each test piece was observed using a Scanning Electron Microscope (SEM). If whiskers were generated, a maximum value of the lengths of whiskers was measured. Results of the measurement are also shown in Table 2.

[Table 2]

[0034] As shown in Table 2, whiskers having great lengths grew in the test piece that was not heat-treated, and in the test pieces that were heat-treated but did not form the Sn-Cu compound layer 4. However, since the occupancy proportion of the (110) plane where whiskers are likely to grow was less than 10%, the whisker length was short, comparison with Example 1. In the case where the Sn-Cu compound layer was formed by performing the heat treatment, the growth of whiskers was restrained at a length of 27.5 μm if the layer thickness was 0.1 μm or less. Furthermore, since the occupancy proportion of the (110) plane where whiskers are likely to grow was less than 10%, no whisker grew if the thickness of the Sn-Cu compound layer was greater than 0.2 μm. [0035] From these results, it is understood that in the case where the occupancy proportion of the (110) plane where whiskers are likely to grow is less than 10%, the growth of whiskers can be restrained to such a degree that the whiskers do not pose a problem in practical use if the film thickness of the Sn-Cu compound layer produced between the plating layer and the base material surface is 0.1 μm or greater, and that the generation of whiskers can be completely prevented by growing the Sn-Cu compound layer to a film thickness of 0.2 μm or greater.

[0036] Incidentally, the present inventors performed the X-ray diffraction on many Pb-free plating layer surfaces by an EBSP method, and made an analysis about the crystal orientation planes where whiskers were generated. As a result, the inventors have found that the probability of whiskers growing from a crystal orientation plane of high atom density of crystal grains of a plating material is higher than the probability of whiskers growing from a crystal orientation plane of low atom density. As for the Sn crystal grains, the (100) plane is a plane having relatively high atom density, and therefore, it is considered that the probability of whiskers growing from the (100) plane of crystal grains is the highest, in the case where the plating material is a Sn-based plating material. Besides, due to the existence of the (100) plane of crystal grains in the plating surface, it is possible to estimate the existence of the (110) plane, which has a high probability of whiskers growing on the plating surface. [0037] As shown in the foregoing examples, even in the case where the proportion of the (110) plane that has high probability of whisker growth to the total Sn plating layer surface is 10% or greater, the generation of whiskers can be substantially completely restrained by growing the Sn-Cu compound layer to a thickness of 0.3 μm or greater. Besides, it was also recognized that the generation of whiskers can be restrained if the thickness of the Sn-Cu compound layer is 0.2 μm or greater in the case where the occupancy proportion of the (110) plane to the plating layer formation plane is 10% or less.

[0038] Incidentally, in the embodiment, the tin (Sn) that forms the Sn plating layer 3 may be pure tin (Sn), or may also be a Sn alloy that does not contain lead (Pb), such Sn-Cu alloy, Sn-Bi alloy, or Sn-Ag alloy. Besides, in the embodiment, the base material 2 may be a base material that is made entirely of copper (Cu) or a Cu alloy, or may also be a base material in which a core material is an Fe-based material such as carbon steel, Fe-Ni-based alloy, Fe-Ni-Co-based alloy, stainless steel, etc., and the surface of the core material is coated with Cu or a Cu alloy. [0039] While some embodiments of the invention have been illustrated above, it is to be understood that the invention is not limited to details of the illustrated embodiments, but may be embodied with various changes, modifications or improvements, which may occur to those skilled in the art, without departing from the scope of the invention.

Claims

1. A fabrication method for a plating substrate having a base material whose surface is made of Cu or a Cu alloy, characterized by comprising: forming a Sn plating layer that contains Sn and is free from Pb, on a surface of the base material; and heat-treating the base material and the plating layer at a temperature at which a Sn-Cu compound layer that contains a compound of Sn and Cu is produced between the plating layer formed and the surface of the base material, and which is lower than or equal to a temperature at which the plating layer melts.

2. The fabrication method according to claim 1, wherein the temperature at which the base material and the plating layer are heat-treated is lower than the temperature at which the plating layer melts.

3. The fabrication method according to claim 1 or 2, wherein the base material and the plating layer is heat-treated at the temperature until a thickness of the Sn-Cu compound layer produced becomes equal to or greater than 0.2 μm.

4. The fabrication method according to any one of claims 1 to 3, wherein heat-treating of the base material and the plating layer at the temperature is performed within five hours after the Sn plating layer is formed.

5. The fabrication method according to any one of claims 1 to 4, wherein heat-treating of the base material and the plating layer at the temperature is started immediately after the Sn plating layer is formed.

6. The fabrication method according to any one of claims 1 to 5, wherein the Sn plating layer consists of Sn.

7. The fabrication method according to claim 6, wherein the temperature at which the base material and the Sn plating layer are heat-treated is lower than or equal to 230°C.

8. A plating substrate characterized by being fabricated by using the fabrication method according to any one of claims 1 to 7, and comprising: a base material whose surface is made of Cu or a Cu alloy; a Sn plating layer that is formed on the surface of the base material, and that contains Sn, and that is free from Pb; and a Sn-Cu compound layer whose thickness is greater than or equal to 0.2 μm and which is provided between the surface of the base material and the Sn plating layer, and over an entire area where the Sn plating layer is formed.

9. The plating substrate according to claim 8, wherein: the Sn plating layer has Sn crystal grains that are made of a Sn crystal; a (110) plane of the Sn crystal grains is present on a surface of the Sn plating layer, at a proportion of 10% or higher; and the Sn-Cu compound layer has a thickness of 0.3 μm or greater.