Patent Application
20070295528
The present invention will be explained in more detail in conjunction with appended drawings, wherein:
Preferred embodiments according to the present invention will be explained in detail hereinafter by referring to the appended drawings.
Sn is used as a base metal of a Sn-plating which is usually used as a plating material of a wiring material. Sn has a two crystal structure types: βSn having a body-centered tetragonal crystal structure (white tin, density of 7.3 g/cm3); and αSn having a diamond type crystal structure (gray tin, density of 5.75 g/cm3). Since an allotropic transformation point where βSn transforms into αSn (hereinafter, referred as “β to α transformation”) is around 13° C. (or less), βSn when manufactured transforms into αSn when used at a temperature not more than the allotropic transformation point. Further, there are two types of Sn oxides each having an oxidation number of 2 and an oxidation number of 4, namely SnO (tin (II) oxide, density of 6.45 g/cm3) which is a black tetragonal crystal, and SnO2 (tin (IV) oxide, density of 6.95 g/cm3) which is a colorless tetragonal crystal.
The whisker is a needle like crystal of Sn as described above. Inventors of the present invention zealously studied this problem. As a result of the studies, as for the generation of the whisker at a surface of the Sn-plating film, it is founded one of the causes of the whisker is a volume expansion in accordance with the β to α transformation or the oxidation of Sn. In particular, the β to α transformation easily occurs, so that 27% of volume expansion is caused at a region of the Sn-plating film to which an external force is applied. Under a high temperature and high humidity condition or the like, Sn is oxidized to form an oxide, so that 28% of volume expansion is caused when the tin oxide is SnO and 33% of volume expansion is caused when the time oxide is SnO2. In accordance with the volume expansion, Sn atoms having nowhere to go are grown to be columnar outside the Sn-plating, thereby forming a whisker. Accordingly, the Inventors found that the generation of the whisker can be suppressed by retarding the β to α transformation or the oxidation of Sn.
As an element for retarding the β to α transformation (transformation retardant element), Pb, Sb, Bi, Cd, In, Ag, Au, and Ni are known, as described in for example, W. Lee Williams, “GRAY TIN FORMATION IN SOLDERED JOINTS STORED AT LOW TEMPERATURE”, SYMPOSIUM ON SOLDER, Alfred Bornemann, “TIN DISEASE IN SOLDER TYPE ALLOYS”, SYMPOSIUM ON SOLDER (1956), and C. E. Hormer and H. C. Watkins, “Transformation of Tin at Low Temperatures”, THE METAL INDUSTRY, 1942, vol. 60, pp. 364-366 and the like. It is assumed that each of these elements except Ni has an effect of suppressing the β to α transformation which involves the volume expansion, since each of these elements has an atomic radius greater than that of Sn. Other than these elements, Ti, Zr, and Hf are elements each having an atomic radius greater than that of Sn. In the present invention, it is premised that the wiring material should be Pb-free, Sb, Bi, Cd, In, Ag, Au, Ni, Ti, Zr, and Hf are used as the transformation retardant element.
As for an element suppressing oxidation (oxidation control element), it is possible to use Ge, P, K, Zn, Cr, Mn, Na, V, Si, Ti, Al, Li, Mg, Ca, and Zr each having an oxidative tendency greater than that of Sn as read from Elingham diagram. It is assumed that each of these elements has an effect of suppressing the oxidation of Sn which involves the volume expansion, since each of these elements has an oxidative tendency greater than that of Sn.
Next, a Pb-free Sn-based material and a wiring conductor in a first preferred embodiment will be explained.
A Pb-free Sn-based material in the first preferred embodiment according to the invention comprises a base metal composed of Sn-based material doped with a transformation retardant element (first additive component element) for retarding a transformation of a crystal structure, and an oxidation control element (second additive component element) for suppressing an oxidation. The transformation retardant element and the oxidation control element are different from each other.
A wiring conductor in the first preferred embodiment is a metal conductor consisted of the Pb-free Sn-based material, or the metal conductor covered with the Pb-free Sn-based material at its surface. The wiring conductor here is a metal conductor such as wiring material, cable conductor, printed circuit board and the like.
In more concrete, as shown in
As for the wiring conductor, a wiring member comprising a core composed of a Cu-based material, and a coating layer composed of a Sn-based material part and provided around the core, a wiring member totally composed of the Sn-based material part (solder material or brazing-filler material), or the like may be proposed. As for the wiring member, for example, various wiring members for electronic devices such as a flexible flat cable (FFC), a flexible printed circuit (FPC), a multi frame joiner (MFJ) that is a printed circuit board in which an insulator is applied on a metal, a printed circuit board, a power supply board (PSB) that is a member in which a wiring is installed on an insulator, a small diameter coaxial cable, antenna cable, or the like may be proposed.
A base metal of the Sn-based material part may be any one of a pure Sn and a Sn alloy. Further, a doping ratio of each of the transformation retardant element and the oxidation control element doped to the Sn-based material part base metal is from 0.001 to 10 wt %, and preferably around 0.1 wt % (or from 0.01 to 1.0 wt %). When the doping ratio of the transformation retardant element or the oxidation control element in the Sn-based material part is less than 0.001 wt %, the effect of retarding the β to α a transformation or the effect of suppressing the oxidation cannot be sufficiently realized. On the contrary, when the doping ratio of the transformation retardant element or the oxidation control element in the Sn-base material part base metal is greater than 10 wt %, there will be defects such as generation of cracks, deterioration of solderability, or the like.
For suppressing the generation of the whisker under conditions of a normal room temperature leaving test (3000 hr), a thermal shock test (3000 cycles), and a humidity resistance leaving test (3000 hr), it is requested that the doping amount of the oxidation control element doped to the Sn-based material part base metal is not less than 0.01 wt %, and particularly the doping amount of the transformation retardant element is greater than that of the oxidation control element. In more concrete, the doping amount of the transformation retardant element is preferably not less than 0.1 wt %, and more preferably not less than 1.0 wt %.
The reason of determining the above ranges may be explained as follows. The oxidation control element can exert the oxidation control effect with a very small amount, since it is sufficient to dope the amount necessary for modifying only a surface of the Sn-plating. On the other hand, the transformation retardant element can exert the effect of retarding the β to α transformation when a doping amount of the transformation retardant element is considerable, since the Sn-plating should be totally doped with the transformation retardant element.
It is preferable that the transformation retardant element and the oxidation control element doped to the Sn-based material part base metal are selected, with considering the work environment and security in manufacturing. As for the transformation retardant element, Sb, Bi, Ag, Au, Ni, Ti, Zr and Hf are more preferable. As for the oxidation control element, Ge, Zn, P, K, Mn, V, Si, Al, Mg, and Ca are more preferable.
As the Sn-based material part base metal, a Pb-free solder alloy base metal may be used. A Pb-free solder alloy (solder material or brazing-filler material) can be obtained, by doping the aforementioned transformation retardant element with a doping ratio of not more than 10 wt % and the oxidation control element with a doping ratio of not more than 10 wt % to the Pb-free solder alloy base metal.
As for the Pb-free solder alloy base metal, for example, Sn-0.1 to 5 wt % Ag-0.1 to 5 wt % Cu alloy (namely, a Sn—Ag—Cu solder alloy comprising Ag of 0.1 to 5 wt % and Cu of 0.1 to 5 wt %) may be used, however, the present invention is not limited thereto. Any existing Pb-free solder alloy is applicable.
Here, In may be doped to the Sn-based material part base metal as the transformation retardant element, so that the β to α transformation can be delayed as well as a melting point of the wiring conductor can be lowered. According to this structure, it is possible to improve a metal flow property and a joint property of the wiring conductor when the wiring conductor is joined to the solder material or the brazing-filler material.
Further, Cu with a doping ratio of e.g. 0.1 to 5.0 wt % may be doped to the Sn-based material part base metal as a dopant in addition to the transformation retardant element and the oxidation control element. According to this structure, it is possible to suppress a solder leach (dissolution of metallization) of the wiring conductor when the wiring conductor is joined to the solder material by solder joint.
Next, a function of the wiring conductor in the first preferred embodiment according to the invention will be explained below.
In a case where the wiring conductor in the first preferred embodiment is a wiring member to be used as a conductor of the FFC, a wiring member comprising a core composed of Cu-based conductor, and a Sn-plating film provided around a periphery of the core, in which the Sn-plating film comprises a Sn-plating base metal doped with a transformation retardant element with a doping ratio of 0.001 to 10 wt % and an oxidation control element with a doping ratio of 0.001 to 10 wt % may be used as the wiring conductor. The wiring conductor according to this structure satisfies the request of realizing the Pb-free Sn plating film, and has a whisker resistance property similar to that of a wiring conductor comprising Sn—Pb alloy (solder) plating film that has an actual performance of the whisker resistance property.
In more concrete, as shown in
Further, even if the wiring conductor comprising the aforementioned Sn-plating film is used in cold climates (at a temperature lower than the allotropic transformation point) or at a high temperature (for example, at 85° C. and 85% RH, which is often used in the high temperature test), the β to α transformation and the oxidation which involve a volume variation can be suppressed. Accordingly, the generation of the whisker can be suppressed in the terminal connecting part, and a generation and a residue of a strain energy within the wiring member (wiring conductor) can be suppressed, so that a flex resistance of the terminal connecting part can be kept good.
Next, the Pb-free solder alloy in the first preferred embodiment is a solder material (or a brazing-filler material) for electrically connecting metal conductors, which comprises a solder material base metal doped with a transformation retardant element with a doping ratio of 0.001 to 10 wt % and an oxidation control element with a doping ratio of 0.001 to 10 wt %. In a terminal connecting part in which the metal conductors are electrically connected to each other by using the aforementioned solder material (brazing-filler material), a joint part has a whisker resistance property similar to a joint part comprising Sn—Pb alloy (solder) plating film that has an actual performance of the whisker resistance property. Accordingly, even if the wiring conductor comprising the aforementioned Sn-plating film is used in cold climates (at a temperature lower than the allotropic transformation point) or at a high temperature, the generation of the whisker can be suppressed at the joint part, and it is possible to avoid defects such as the short circuit between adjacent conductors, thereby improving a connecting reliability of the joint part.
Next, a wiring conductor in a second preferred embodiment will be explained.
A wiring conductor 10 in the second preferred embodiment comprises a metal conductor 1, and a Pb-free Sn coating layer 2′ provided at an entire surface of the metal conductor 1. The Pb-free Sn coating layer 2′ is formed by providing a Pb-free Sn-based plating film 2a at an entire surface (or at least at a part of the surface) of the metal conductor 1, and a transformation retardant element layer (transformation retardant plating film) 3 as well as an oxidation control element layer (oxidation control plating film) 4 on the Pb-free Sn-plating film 2a as shown in
The Pb-free Sn coating layer 2′ is a layer mainly composed of the transformation retardant element, the oxidation control element, and a Sn-alloy. The Pb-free Sn coating layer 2′ may be totally composed of an alloy. Further, the Pb-free Sn coating layer 2′ may partially comprise a residue of at least one of the transformation retardant element layer 3, the oxidation control element layer 4, and the Sn-plating film 2a.
A weight ratio of the transformation retardant element layer 3 to that of the Sn-plating film 2a is from 0.001 to 10 wt %, preferably around 0.1 wt % (or from 0.01 to 1.0 wt %). Similarly, a weight ratio of the oxidation control element layer 4 to that of the Sn-plating film 2a is from 0.001 to 10 wt %, preferably around 0.1 wt % (or from 0.01 to 1.0 wt %).
For suppressing the generation of the whisker under conditions of a normal room temperature leaving test (3000 hr), a thermal shock test (3000 cycles), and a humidity resistance leaving test (3000 hr), it is requested that the doping amount of the oxidation control element is not less than 0.01 wt %, and particularly the doping amount of the transformation retardant element is greater than that of the oxidation control element. In more concrete, the doping amount of the transformation retardant element is preferably not less than 0.1 wt %, and more preferably not less than 1.0 wt %.
In the second preferred embodiment, the transformation retardant element layer 3 and the oxidation control element layer 4 are provided on the Sn-plating film 2a. As shown in
As shown in
Next, a method for fabricating a wiring conductor in the second preferred embodiment will be explained.
As shown in
Then, as shown in
As shown in
As shown in
After appropriately conducting a rolling process, an area reduction process or the like on the metal conductor 1 provided with the Sn-plating film 2a, the transformation retardant plating film 3, and the oxidation control plating film 4, a reflow process (annealing by energization) is conducted thereon. By conducting the reflow process, Sn in the Sn-plating film 2a, the transformation retardant elements in the transformation retardant plating film 3, and the oxidation control elements in the oxidation control plating film 4 are diffused.
As a result, as shown in
Annealing temperature and annealing time of the reflow process are such determined that the temperature and time are enough to diffuse Sn in the Sn-plating film 2, the transformation retardant elements in the transformation retardant plating film 3, and the oxidation control elements in the oxidation control plating film 4. Since the annealing temperature and time are varied in accordance with the transformation retardant element and the oxidation control element to be used, the annealing temperature and time are appropriately adjusted in accordance with the oxidation control element to be used.
The present invention is not limited to the preferred embodiments as described above, and other variations can be expected.
Next, the present invention will be explained in conjunction with following Examples however the present invention is not limited thereto.
Samples of wiring member were prepared by conducting a fusion welding of a pure Sn doped with following elements. In the sample, a pure Sn is doped with:
(a) 0.01 wt % of a transformation retardant element (any one of Sb, Bi, In, Ag, Au, Ni, Ti, Zr, and Hf) and 0.01 wt % of an oxidation control element (any one of Ge, P, K, Zn, Mn, V, Si, Mg, Al, and Ca), respectively;
(b) 0.01 wt % of a transformation retardant element (Bi), 0.01 wt % of another transformation retardant element (Ni), and 0.01 wt % of an oxidation control element (any one of P and Zn), respectively;
(c) 1 wt % of a transformation retardant element (any one of Sb, Bi, In, Ag, and Au) and 0.01 wt % of an oxidation control element (any one of P, K, Zn, Mn, and V), respectively;
(d) 0.1 wt % of a transformation retardant element (any one of Ni, Ti, Zr, and Hf) and 0.01 wt % of an oxidation control element (any one of Si, P, Zn, Ge, Mg, Al, and Ca), respectively;
(e) 1.0 wt % of a transformation retardant element (Bi), 0.1 wt % of another transformation retardant element (Ni), and 0.01 wt % of an oxidation control element (any one of P and Zn), respectively;
(f) 0.01 wt % of only a transformation retardant element;
(g) 0.01 wt % of only an oxidation control element; and
(h) no dopant.
Samples of wiring member were prepared by conducting a fusion welding of a Sn-3 wt % Ag-0.5 wt % Cu alloy which is a Pb-free solder material doped with following elements. In the sample, the Sn-3 wt % Ag-0.5 wt % Cu alloy is doped with:
(i) 0.01 wt % of a transformation retardant element (any one of Sb, Bi, In, Ag, Au, Ni, Ti, Zr, and Hf) and 0.01 wt % of an oxidation control element (any one of Ge, P, K, Zn, Mn, V, Si, Mg, Al, and Ca), respectively;
(j) 0.01 wt % of a transformation retardant element (Bi), 0.01 wt % of another transformation retardant element (Ni), and 0.01 wt % of an oxidation control element (any one of P and Zn), respectively;
(k) 1 wt % of a transformation retardant element (any one of Sb, Bi, In, Ag, and Au) and 0.01 wt % of an oxidation control element (any one of P, K, Zn, Mn, and V), respectively;
(l) 0.1 wt % of a transformation retardant element (any one of Ni, Ti, Zr, and Hf) and 0.01 wt % of an oxidation control element (any one of Si, P, Zn, Ge, Mg, Al, and Ca), respectively;
(m) 1.0 wt % of a transformation retardant element (Bi), 0.1 wt % of another transformation retardant element (Ni), and 0.01 wt % of an oxidation control element (any one of P and Zn), respectively;
(n) 0.01 wt % of only a transformation retardant element;
(o) 0.01 wt % of only an oxidation control element; and
(p) no dopant.
In a state where each of the wiring members is fitted into and contacted with a connector, a normal room temperature leaving test (25° C.×1000 hr), a thermal shock test (−55° C. to 125° C.×1000 cycles), and a humidity resistance leaving test (55° C., 95% RH×1000 hr) were carried out.
In addition, for the Examples 17 to 32 and Examples 49 to 64 to which the transformation retardant element of not less than 0.01 wt % is doped, a normal room temperature leaving test (25° C.×3000 hr), a thermal shock test (−55° C. to 125° C.×3000 cycles), and a humidity resistance leaving test (55° C., 95% RH×3000 hr) were carried out.
Thereafter, each of the wiring members was detached from the connector, and a status of generation of whisker at a plating film surface in a connector fitting part (connecting part) was observed by means of electron microscope.
TABLE 1 and TABLE 2 show an evaluation result of whisker resistance property of the wiring members after respective tests. In TABLE 1 and TABLE 2, ⋆ indicates “no whisker” (normal room temperature leaving test: 3000 hr, thermal shock test: 3000 cycles, humidity resistance leaving test: 3000 hr), ⊚ indicates “no whisker” (normal room temperature leaving test: 1000 hr, thermal shock test: 1000 cycles, humidity resistance leaving test: 1000 hr), ◯ indicates that a length of the whisker is less than 50 μm (normal room temperature leaving test: 1000 hr, thermal shock test: 1000 cycles, humidity resistance leaving test: 1000 hr), and X indicates a length of the whisker is not less than 50 μm (normal room temperature leaving test: 1000 hr, thermal shock test: 1000 cycles, humidity resistance leaving test: 1000 hr).
As shown in TABLE 1 and TABLE 2, in the Conventional arts 1 and 2 using the wiring member comprising a pure Sn doped with no dopant and the wiring member comprising the Sn-3 wt % Ag-0.5 wt % Cu alloy doped with no dopant, respectively, the maximum length of whisker is not less than 50 μm. The whisker suppressing effect cannot be obtained at all.
On the other hand, in the Comparative Examples 1 to 36 using the wiring members doped with any one of the transformation retardant element and the oxidation control element, the maximum length of whisker is less than 50 μm, namely the length of the whisker in the respective wiring members is shortened compared with the Conventional arts 1 and 2. The whisker suppressing effect can be obtained in the all of the Comparative Examples 1 to 36.
In comparison, in the Examples 1 to 64 using the wiring members doped with both of the transformation retardant element and the oxidation control element, no whisker was generated after the respective tests for evaluating the whisker resistance property. Compared with the Comparative Examples 1 to 36, a higher whisker suppressing effect can be obtained in the Examples 1 to 64.
Particularly in the Examples 17 to 32 and the Examples 49 to 64 using the wiring member doped with 0.1 wt % or more of the transformation retardant element, no whisker was generated although the respective testing times and testing cycles tripled (normal room temperature leaving test: 3000 hr, thermal shock test: 3000 cycles, humidity resistance leaving test: 3000 hr). Therefore, it is confirmed that the whisker suppressing effect is significantly high.
Although the invention has been described with respect to the specific embodiments for complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art which fairly fall within the basic teaching herein set forth.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-175279 | Jun 2006 | JP | national |
| 2007-045927 | Feb 2007 | JP | national |
