US20070178007A1

US20070178007A1 - Lead-free solder, solder joint product and electronic component

Info

Publication number: US20070178007A1
Application number: US11/616,507
Authority: US
Inventors: Long Thantrong; Yuuji Hisazato
Original assignee: Toshiba Corp
Current assignee: Toshiba Corp
Priority date: 2005-12-27
Filing date: 2006-12-27
Publication date: 2007-08-02
Also published as: DE102006061636A1; JP2007196289A; KR20070069069A; KR100829465B1; CN101209516A; CN101209516B

Abstract

This invention provides lead-free solders that have excellent oxidation resistance and can be plastically worked easily and well. The lead-free solders and molded products of the lead-free solders can provide solder joint products, particularly electronic components, which are highly reliable, for example, in mechanical strength and joint strength. The lead-free solders, solder joint product, and electronic component are as follows. A lead-free solder comprising a tin (Sn)-base alloy having a tantalum (Ta) content of not less than 0.005% by weight and not more than 2.0% by weight. A lead-free solder comprising a tin (Sn)-base alloy comprising not less than 0.005% by weight and not more than 2.0% by weight of tantalum (Ta) and not less than 0.1% by weight and not more than 10.0% by weight of zinc (Zn) with the balance consisting of tin (Sn) and unavoidable impurities. A lead-free solder comprising a tin (Sn)-base alloy comprising not less than 0.005% by weight and not more than 2.0% by weight of tantalum (Ta) and not less than 0.1% by weight and not more than 60.0% by weight of bismuth (Bi) with the balance consisting of tin (Sn) and unavoidable impurities. A lead-free solder comprising a tin (Sn)-base alloy comprising not less than 0.005% by weight and not more than 2.0% by weight of tantalum (Ta) and not less than 0.1% by weight and not more than 10.0% by weight of indium (In) with the balance consisting of tin (Sn) and unavoidable impurities. A lead-free solder comprising a tin (Sn)-base alloy comprising not less than 0.005% by weight and not more than 2.0% by weight of tantalum (Ta) and not less than 0.01% by weight and not more than 7.5% by weight of copper (Cu) with the balance consisting of tin (Sn) and unavoidable impurities. A lead-free solder comprising a tin (Sn)-base alloy comprising not less than 0.005% by weight and not more than 2.0% by weight of tantalum (Ta) and not less than 0.01% by weight and not more than 5.0% by weight of silver (Ag) with the balance consisting of tin (Sn) and unavoidable impurities. A lead-free solder comprising a tin (Sn)-base alloy comprising not less than 0.005% by weight and not more than 2.0% by weight of tantalum (Ta), not less than 0.01% by weight and not more than 5.0% by weight of silver (Ag), and not less than 0.01% by weight and not more than 7.5% by weight of copper (Cu) with the balance consisting of tin (Sn) and unavoidable impurities. A solder joint product produced by jointing with the above lead-free solder. An electronic component produced by jointing with the above lead-free solder.

Description

BACKGROUND OF THE INVENTION

1. Field of Invention
The present invention relates to a lead-free solder, which has excellent oxidation resistance, mechanical properties and wetting properties and can easily be plastically worked, a solder joint product produced by jointing with the lead-free solder, and an electronic component.
2. Background Art
In recent years, there is a growing concern about environmental problems from the viewpoint of global environmental protection. Under such a circumstance, an increase in discarded amount of industrial wastes is becoming a serious problem. Solder is used, for example, in substrates in electric power control computers, home electric appliances, and personal computers included in industrial wastes. Harmful heavy metals such as lead sometimes flow out from this solder. For example, when lead flows out, the lead acts on acid rain and the like to produce a lead-containing aqueous solution, and the aqueous solution often penetrates groundwater.
In Japan, Home Appliance Recycling Law was established in 1998, and the recovery of the spent appliances was required for home electric appliances in 2001. In Europe, the use of lead has been prohibited as a specific substance since 2004 by Directive of the European Parliament and of the Council on Waste Electrical and Electronic Equipment. In this way, legal regulation regarding the use of lead has been tightened, and the development of lead-free solder has been urgently needed (see, for example, non-patent document 1).
Solder plays an important role in mechanically and electrically connecting a plurality of element components used under severe environments involving heat cycles, mechanical impact, mechanical vibration and the like. Also in lead-free solder, a lead-free solder, which has a melting point similar to that of conventional tin (Sn)-lead (Pb) solder, has excellent mechanical properties and wetting properties and has excellent plastic workability good enough to be molded into a ribbon or filament form, has been demanded.
In conventional lead-free solder, however, it is common practice to adopt Sn (tin)-Zn (zinc) or Sn (tin)-Bi (bismuth) solder in order to provide a melting point similar to that of Sn (tin)-37 wt % pb (lead) solder (melting point 183° C.), or to add a large amount of In (indium) or Bi (bismuth), for example, to Sn (tin)-Cu (copper), Sn (tin)-Ag (silver), or Sn (tin)-Cu (copper)-Ag (silver) solder in order to lower the melting point. For example, Sn (tin)-9.0 wt % Zn (zinc) (melting point 199° C.), Sn (tin)-58.0 wt % Bi (bismuth) (melting point 138° C.), Sn (tin)-0.5 wt % Cu (copper)-4.0 wt % Ag (silver)-8 wt % In (indium) (melting point 208° C.) are mentioned as such solders. Since, however, these solders contain a large amount of elements causative of embrittlement, for example, mechanical properties and plastic workability are deteriorated, making it difficult to ensure satisfactory solder bonding strength and reliability. Further, these lead-free solder is likely to cause brittle failure or the like upon plastic working, and, thus, it is very difficult for these lead-free solders to successfully undergo extrusion, rolling, wire drawing and the like. Accordingly, molded products in a ribbon or filament form could not have been substantially produced. For the above reason, applications of lead-free solder have been remarkably limited. [Non-patent document 1] Proposal for a Directive of the European Parliament and of the Council on Waste Electrical and Electronic Equipment, Commission of the European Communities, Brussels, 13 Jun. 2000

SUMMARY OF THE INVENTION

As described above, since conventional lead-free solders contain a large amount of elements causative of embrittlement, for example, mechanical properties and plastic workability are deteriorated, making it difficult to ensure satisfactory solder bonding strength and reliability. Further, these lead-free solder cause brittle failure or the like upon plastic working, and, thus, it is very difficult for these lead-free solders to successfully undergo extrusion, rolling, wire drawing and the like. Accordingly, molded products of solder in a ribbon or filament form could not have been substantially produced. For the above reason, applications of lead-free solder have been remarkably limited.
The present invention has been made with a view to solving the above problems of the prior art, and an object of the present invention is to provide a lead-free solder, which has excellent oxidation resistance, mechanical properties, and plastic workability, can ensure satisfactory solder bonding strength and reliability, and can be molded into a ribbon or a filament, and to provide a highly reliable solder joint product produced by jointing with the lead-free solder, and an electronic component.
The above object can be attained by a lead-free solder comprising a tin (Sn)-base alloy having a tantalum (Ta) content of not less than 0.005% by weight and not more than 2.0% by weight.
According to the present invention, there is also provided a lead-free solder comprising a tin (Sn)-base alloy comprising not less than 0.005% by weight and not more than 2.0% by weight of tantalum (Ta) and not less than 0.1% by weight and not more than 10.0% by weight of zinc (Zn) with the balance consisting of tin (Sn) and unavoidable impurities.
According to the present invention, there is also provided a lead-free solder comprising a tin (Sn)-base alloy comprising not less than 0.005% by weight and not more than 2.0% by weight of tantalum (Ta) and not less than 0.1% by weight and not more than 60.0% by weight of bismuth (Bi) with the balance consisting of tin (Sn) and unavoidable impurities.
According to the present invention, there is also provided a lead-free solder comprising a tin (Sn)-base alloy comprising not less than 0.005% by weight and not more than 2.0% by weight of tantalum (Ta) and not less than 0.1% by weight and not more than 10.0% by weight of indium (In) with the balance consisting of tin (Sn) and unavoidable impurities.
According to the present invention, there is also provided a lead-free solder comprising a tin (Sn)-base alloy comprising not less than 0.005% by weight and not more than 2.0% by weight of tantalum (Ta) and not less than 0.01% by weight and not more than 7.5% by weight of copper (Cu) with the balance consisting of tin (Sn) and unavoidable impurities.
According to the present invention, there is also provided a lead-free solder comprising a tin (Sn)-base alloy comprising not less than 0.005% by weight and not more than 2.0% by weight of tantalum (Ta) and not less than 0.01% by weight and not more than 5.0% by weight of silver (Ag) with the balance consisting of tin (Sn) and unavoidable impurities.
According to the present invention, there is also provided a lead-free solder comprising a tin (Sn)-base alloy comprising not less than 0.005% by weight and not more than 2.0% by weight of tantalum (Ta), not less than 0.01% by weight and not more than 5.0% by weight of silver (Ag), and not less than 0.01% by weight and not more than 7.5% by weight of copper (Cu) with the balance consisting of tin (Sn) and unavoidable impurities.
In a preferred embodiment of the lead-free solder according to the present invention, the tin-base alloy further comprises at least one additive element (Y) selected from the group consisting of indium and bismuth.
In a further preferred embodiment of the lead-free solder according to the present invention, in said tin-base alloy, the content of the additive element (Y) selected from the group consisting of indium and bismuth is not more than 10% by weight for indium and not more than 60% by weight for bismuth.
In a preferred embodiment of the lead-free solder according to the present invention, the tin-base alloy further comprises at least one additive element (X) selected from the group consisting of cobalt (Co), titanium (Ti), nickel (Ni), palladium (Pd), antimony (Sb), and germanium (Ge).
In a further preferred embodiment of the lead-free solder according to the present invention, in said tin-base alloy, the content of the additive element (X) selected from the group consisting of cobalt, titanium, nickel, lead, antimony, and germanium is such that, for each of the additive elements (X), the content is not more than 0.5% by weight and, when a plurality of the additive elements (X) are contained, the total content of the plurality of the additive elements (X) is not more than 1.0% by weight.
The lead-free solder according to the present invention may be in a cream, ribbon, filament or rod form.
According to the present invention, there is also provided a solder joint product produced by jointing with any one of the above lead-free solders.
According to the present invention, there is provided an electronic component produced by jointing with any one of the above lead-free solders.
The lead-free solder according to the present invention has excellent oxidation resistance and can realize plastic working such as extrusion, rolling, and wire drawing very easily and well. Further, the lead-free solder according to the present invention, when used as solder, is excellent in various properties required of solder, for example, mechanical properties and wetting properties. Thus, the present invention can provide a lead-free solder, which has excellent oxidation resistance, mechanical properties, and plastic workability, can ensure satisfactory solder bonding strength and reliability, and can be plastically worked into a ribbon or a filament.
Since the lead-free solder according to the present invention has improved wetting properties which could have been realized without a substantial melting point rise, a significant improvement in solder bonding strength and reliability can be realized while effectively preventing a deterioration in an object to be joined by heat. Accordingly, the present invention can provide solder joint products having excellent bonding strength and reliability, preferably electronic components with various electronic elements, for example, LED light emitting elements, SEDs (surface-conduction electron-emitter displays), or mounted substrates joined thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph showing the state of a thin film of a melt of an Sn-3Ag-0.5Cu-4In-0.05Ta solder according to the present invention formed on a substrate;
FIG. 2 is a photograph showing the state of a thin film of an Sn-3Ag-0.5Cu solder formed on a substrate;
FIG. 3 is a photograph showing the state of a thin film of an Sn-3Ag-0.5Cu-4In solder formed on a substrate;
FIG. 4 is a diagram showing the shape of a droplet of an Sn-3Ag-0.5Cu-4In-0.05Ta solder according to the present invention formed in the evaluation of the surface tension of the Sn-3Ag-0.5Cu-4In-0.05Ta solder by a dropping method;
FIG. 5 is a diagram showing the shape of a droplet of an Sn-3Ag-0.5Cu solder formed in the evaluation of the surface tension of the Sn-3Ag-0.5Cu solder by a dropping method;
FIG. 6 is a diagram showing the shape of a droplet of an Sn-3Ag-0.5Cu-4In solder formed in the evaluation of the surface tension of the Sn-3Ag-0.5Cu-4In solder by a dropping method; and
FIG. 7 is a drawing briefly illustrating “thermal cycle test” carried out in Examples 1 and 2.

DETAILED DESCRIPTION OF THE INVENTION

<Lead-Free Solder>
The lead-free solder according to the present invention comprises an Sn (tin)-base alloy containing not less than 0.005% by weight and not more than 2.0% by weight of Ta (tantalum). The term “lead-free solder” as used herein generally refers to an alloy having a Pb (lead) content of not more than 1000 ppm.
Lead-free solders containing a predetermined amount of tantalum according to the present invention can be classified into (i) Sn—Ta-base alloys, (ii) Sn—Zn—Ta-base alloys, (iii) Sn—Bi—Ta-base alloys, (iv) Sn—Cu—Ta-base alloys, (v) Sn—Ag—Ta-base alloys, and (vi) Sn—Cu—Ag—Ta-base alloys.
In the present invention, even when a lead-free solder belongs to any one of the alloys (i) to (vi), tantalum should be contained as an indispensable component in an amount of not less than 0.005% by weight and not more than 2.0% by weight.
Tantalum is a component that is important mainly for improving the wetting properties of the solder and the mechanical properties of the solder joint part. The tantalum as the additive element reduces the surface tension of a melt of tin or tin alloy to improve the capability of the solder to wet the member to be soldered. Further, the tantalum causes nucleation of an Sn—Ta intermetallic compound in a solidification process of tin or tin alloy and thus contributes to refinement of the crystal structure. When the tantalum content is less than 0.005% by weight, the effect of improving the wetting properties derived from surface tension reduction cannot be attained although satisfactory effect of refining the solidification structure can be attained. On the other hand, when the tantalum content is more than 2.0% by weight, in some cases, a coarse Sn—Ta intermetallic compound is formed under some cooling conditions, resulting in lowered solder strength although excellent wetting properties can be realized. The content of tantalum in the lead-free solder according to the present invention is preferably in the range of 0.05 to 1.0% by weight, particularly preferably in the range of 0.1 to 0.50% by weight, because the best balance between the wetting properties and the mechanical properties can be achieved and the best soldering properties can be realized.
The Sn—Ta-base lead-free solder (i) according to the present invention comprises tin, the predetermined amount of tantalum, optional various additive elements (which will be described in detail) and unavoidable impurities. Since the Sn—Ta-base lead-free solder according to the present invention contains a small amount of tantalum, this lead-free solder has excellent wetting properties and mechanical properties simultaneously, without varying the melting point 232° C. of pure tin, in this melting point region.
The eutectic composition of Sn-9 wt % Zn has the lowest melting point (melting point 198° C.) and is commonly used as a low-melting point lead-free solder. This solder, however, has a problem that, due to the high Zn content, a coarse eutectic structure is likely to be formed and, consequently, satisfactory solder strength and reliability cannot be provided. The Sn—Zn—Ta-base lead-free solder (ii) according to the present invention is most effective particularly in solving the above problem. Specifically, tantalum contained in the solder reduces the surface tension, and significantly refines the Sn—Zn eutectic structure to realize a refined and uniform Sn—Zn eutectic structure. By virtue of this, mechanical properties are improved. In particular, brittleness is significantly improved, and the occurrence of cracks upon solidification can be suppressed. This mechanism is also very effective in an Sn—Zn hypereutectic composition having a zinc content of more than 9% by weight and a hypoeutectic composition having a zinc content of less than 9% by weight, and the composition is not particularly limited.
The Sn—Bi—Ta-base lead-free solder (iii) according to the present invention is similar to the Sn—Zn—Ta-base lead-free solder (ii) according to the present invention, and an Sn-57 wt % Bi eutectic composition (melting point 139° C.) is commonly used as a low-melting point lead-free solder. Although a part of bismuth is dissolved in tin to prepare a solid solution, a major part of the bismuth forms, as a simple substance, a eutectic structure which is coarsened under some cooling conditions. The presence of this coarse eutectic structure is a main cause of brittle fracture in the solder part. When the solder strength and the reliability should be ensured, the bismuth content should be not more than 5% by weight. The bismuth content is particularly preferably in the range of 0.1 to 1.0% by weight.
The Sn—Cu—Ta-base lead-free solder (iv),
Sn—Ag—Ta-base lead-free solder (v), and Sn—Cu—Ag—Ta-base lead-free solder (vi) according to the present invention also have tantalum incorporation effect common to the Sn—Zn—Ta-base lead-free solder and Sn—Bi—Ta-base lead-free solder according to the present invention. In general, a composition close to Sn-0.5 to 0.75 wt % Cu eutectic composition and a composition close to Sn-0.5 to 0.75 wt % Cu-3.0 to 3.5 wt % Ag ternary eutectic composition are used. In order to raise the melting point of solder (liquidus line temperature), a hypereutectic composition having a copper content of more than 7.5% by weight, which is much higher than the copper content of Sn-0.7 wt % Cu eutectic composition, or a hypoeutectic composition having a copper content of less than 0.5% by weight, which is much lower than the copper content of Sn-0.7 wt % Cu eutectic composition is also used. In all the compositions of the Sn—Cu-base lead-free solder, the tantalum incorporation effect can be attained. This is true of Sn—Ag-base lead-free solders and Sn—Cu—Ag-base lead-free solders.
In the lead-free solders (i) to (vi) according to the present invention, as described above, various additive elements may if necessary be used. At least one additive element (X) selected from the group consisting of cobalt (Co), titanium (Ti), nickel (Ni), lead (Pd), antimony (Sb) and germanium (Ge) may be exemplified as a specific preferred example of the additive element. The use of the additive element (X) can reduce the surface tension of the melt lead-free solder and can improve wettability. For example, Japanese Patent Application No. 65858/2004 describes that the surface tension is reduced by adding cobalt (Co), nickel (Ni), and palladium (Pb).
For all of cobalt, titanium, nickel, lead, antimony and germanium as the additive element in the lead-free solder according to the present invention, the addition amount is preferably not less than 0.005% by weight and not more than 0.5% by weight. When the addition amount is less than 0.005% by weight, satisfactory surface tension reduction cannot be ensured. On the other hand, when the addition amount exceeds 0.5% by weight, a coarse intermetallic compound is likely to be formed under some cooling conditions and, thus, the mechanical properties are sometimes deteriorated. When a plurality of additive elements (X) are contained in the lead-free solder, the total content of the additive elements (X) is preferably not more than 1.0% by weight, particularly preferably not more than 0.7% by weight.
When the lead-free solder according to the present invention is the Sn—Cu—Ta-base lead-free solder (iv), Sn—Ag—Ta-base lead-free solder (v), or Sn—Cu—Ag—Ta-base lead-free solder (vi), in addition to the above additive elements (X), other additive elements may be optionally used. At least one additive element (Y) selected from the group consisting of indium (In) and bismuth (Bi) may be exemplified as a specific preferred example of the other additive element. When the additive element (Y) is used in the lead-free solder according to the present invention, the additive element (X) may be present or absent. Further, in the lead-free solder according to the present invention, additional additive elements other than the additive element (X) and the additive element (Y) may be present so far as the effect of the present invention is attained.
Indium (In) is mainly a component useful for lowering the melting point of the lead-free solder and is properly specified by taking the balance between the allowable temperature range of an electronic component to be applied and the material cost into consideration. In general, the addition amount of indium is preferably not more than 10.0% by weight, particularly preferably not more than 50% by weight, from the viewpoint of oxidation resistance.
Bismuth (Bi) is mainly a component useful for lowering the melting point of the lead-free solder, and the composition of bismuth is not particularly limited. As described above, however, when the solder strength and the reliability should be ensured, the bismuth content should be not more than 5% by weight, particularly preferably in the range of 0.1% by weight to 1.0% by weight. When bismuth together with indium is contained, in particular, the bismuth content is preferably in the range of 0.1% by weight to 1.0% by weight.
The lead-free solders (i) to (vi) according to the present invention and the lead-free solders optionally containing the additive element (X) and the additive element (Y) according to the present invention can undergo plastic working, for example, extrusion, rolling, and wire drawing very easily and well. Further, the lead-free solders (i) to (vii) according to the present invention and the lead-free solders optionally containing the additive element (X) and the additive element (Y) according to the present invention, when used as solder, are particularly excellent in various properties required of solder, for example, bonding strength, mechanical properties, and wetting properties.
Accordingly, the present invention can provide solder joint products, preferably electronic components with various electronic elements, for example, LED light emitting elements, SEDs (surface-conduction electron-emitter displays), or mounted substrates joined thereto, possessing excellent bonding strength and reliability.
The lead-free solder according to the present invention can be produced by any desired method without particular limitation. For example, the lead-free solder according to the present invention can be produced by melt kneading indispensable components, i.e., tin (Sn) and tantalum (Ta), and zinc (Zn), bismuth (Bi), copper (Cu), or silver (Ag), optional components, i.e., an additive element (X) or an additive element (Y) and, if necessary, other components so as to provide a lead-free solder having a contemplated composition, at a temperature at or above the melting point of each of the components, and then cooling the mixture. The lead-free solder according to the present invention is particularly preferably produced by previously alloying tin in a required amount or a smaller amount than required for constituting the contemplated lead-free solder, with one or at least two of the above indispensable components and/or optional components, and then alloying the alloyed product (prealloyed product) with the remaining amount of the required amount of tin and the remaining amount of the indispensable components and/or optional components so as to provide the contemplated lead-free solder. Thus, a solder comprising tin and the above metal components, which have been dispersed homogeneously and intimately, can easily be produced. In order to prevent a deterioration in properties by oxidation of the lead-free solder, preferably, the lead-free solder and its constituent components are handled in an inert atmosphere, for example, in a nitrogen, argon, or helium gas atmosphere.
The lead-free solder according to the present invention is a solid or paste form at room temperature depending, for example, upon the composition of the lead-free solder, specific production conditions, and other conditions.
The lead-free solder according to the present invention thus produced can undergo plastic working, for example, extrusion, rolling, and wire drawing very easily and well. Further, the lead-free solder according to the present invention, when used as solder, is excellent in various properties required of solder, for example, bonding strength, mechanical properties, and wetting properties.
The wetting properties of the lead-free solder according to the present invention is such that the percentage wet spreading (%) specified in JIS Z 3198-3 is 75 to 80% for (i) Sn—Ta-base lead-free solder, 60 to 70% for (ii) Sn—Zn—Ta-base lead-free solder, 80 to 90% for (iii) Sn—Bi—Ta-base lead-free solder, 70 to 85% for (iv) Sn—Cu—Ta-base lead-free solder, 75 to 85% for (v) Sn—Ta—Ag-base lead-free solder, and 75 to 85% for (vi) Sn—Cu—Ag—Ta-base lead-free solder. For each of these lead-free solders, a 15% or more improvement in wetting properties (percentage spreading) can be realized over the same solders as described above except that tantalum is not contained.
The wetting properties of the lead-free solder can also be evaluated by placing a solder layer having a predetermined thickness on a substrate, heating the assembly in an air atmosphere to a temperature at or above the melting point of the solder and visually observing the state of a solder film formed from the melt solder.
FIG. 1 is a diagram showing the state of a ribbon solder (2) provided by providing a ribbon solder of Sn-3Ag-0.5Cu-4In-0.05Ta having a size of 25 mm in length×25 mm in width×150 μm in thickness as the solder according to the present invention, placing the ribbon solder on an oxygen-free copper plate (1) coated with flux, heat treating the assembly at a temperature of 250° C. to melt the solder and then solidifying the solder. FIG. 2 is a diagram showing the same ribbon solder as in FIG. 1 except that Sn-3Ag-0.5Cu was used instead of the above solder according to the present invention. FIG. 3 is a diagram showing the same ribbon solder as in FIG. 1 except that Sn-3Ag-0.5Cu-4In was used instead of the above solder according to the present invention.
As is apparent from these drawings, the lead-free solder according to the present invention has good wetting properties and can form a solder film having substantially even thickness on a substrate. On the other hand, for other solders, since the wetting properties are unsatisfactory, a part of the solder is repelled in a molten state by the substrate. As a result, an even solder film cannot be formed, and a solder non-adhered part 3 or an excessively protuberant part 4 of the solder film occurs. Further, the contour of the solder film is unclear. In the solder having poor wettability, satisfactory solder bonding strength cannot be realized, and, further, there is a high possibility that the solder leaks out into an unintended region. Accordingly, such solder is not suitable particularly in the field of applications where highly accurate solder joint quality is required, for example, as solder for joining or wiring of highly precise electronic elements.
Further, the wetting properties of the solder can also be evaluated by measuring the surface tension of the solder in a molten state by a dropping method. The dropping method is a method for measuring the surface tension using such a property that, when a liquid is dropped through a circular tube opening, a solder droplet is dropped as a result that the weight of the solder droplet overcomes the surface tension of the droplet.
FIG. 4 shows the shape of a solder droplet just before dropping in the case where a melt of Sn-3Ag-0.5Cu-4In-0.05Ta solder according to the present invention gradually flows out through a circular tube opening having a diameter of 0.7 mm. FIG. 5 shows the shape of the same solder droplet as in FIG. 4 except that Sn-3Ag-0.5Cu is used instead of the solder in FIG. 4. FIG. 6 shows the shape of the same solder droplet as in FIG. 4 except that Sn-3Ag-0.5Cu-4In is used instead of the solder in FIG. 4. In each drawing, the surface tension γ of the molten solder was calculated from the relationship between the maximum diameter de of the solder droplet in a horizontal direction just before dropping of the solder droplet and the diameter ds of the solder droplet in a horizontal direction at a position of distance de from the front end of the solder droplet
γ=g·ρ·(de)·² /H
wherein γ represents surface tension; g represents gravitational constant; de represents maximum diameter; and H represents correction factor (H=ds/de).
As derived from the above equation, the larger the ds value, the lower the surface tension. The lower the surface tension of the solder, the better the wettability.
The surface tension of Sn-3Ag-0.5Cu-4In-0.1Ta solder according to the present invention shown in FIG. 4 is 0.40 to 0.42 N/m,
the surface tension of conventional Sn-3Ag-0.5Cu solder shown in FIG. 5 is 0.38 to 0.40 N/m, and
the surface tension of conventional Sn-3Ag-0.5Cu-4In solder shown in FIG. 6 is 0.43 to 0.46 N/m.
From the above results, it is apparent that the incorporation of 4In in conventional Sn-3Ag-0.5Cu solder raises the surface tension, and the incorporation of 0.1Ta in the solder reduces the surface tension to realize good wetting properties.
The lead-free solder according to the present invention has excellent oxidation resistance, and, thus, a deterioration in various properties based on oxidation has been highly suppressed and, further, is much less likely to produce the so-called “dross” composed mainly of oxide of the solder. For example, in the lead-free solder according to the present invention, the amount of dross produced is reduced to about one-fifth of that in the case of tantalum-free lead-free solder.
<Process for Producing Lead-Free Solder Molded Product>
The production process of a lead-free solder molded product according to the present invention is characterized by comprising an ingot casting step of dissolving and casting the lead-free solder according to the present invention to form an ingot and a plastic working step of plastically working this ingot to prepare a molded product.
In general, the conventional lead-free solder cannot be plastically worked without difficulties, and, even when plastic working can be successful carried out, it is difficult to realize mechanical properties, wetting properties and other properties which are satisfactory as a solder. On the other hand, the lead-free solder according to the present invention can be plastically worked very easily and well and, even after the plastic working, has good mechanical properties, wetting properties, solder bonding strength and other properties which are preferred as solder.
According to the production process of a lead-free solder molded product according to the present invention, a solder molded product having desired form and size, which has properties preferred as a solder, can easily be produced. In the present invention, a lead-free solder molded product in a form which could not have hitherto been realized by using a conventional general lead-free solder, for example, a ribbon, filament or rod form, can be produced.
Specific examples of preferred ribbon-shaped lead-free solder molded products according to the present invention include those having a thickness of 50 to 500 μm, particularly preferably 100 to 150 μm. Specific examples of preferred filament-shaped lead-free solder molded products according to the present invention include those having a filament diameter of 0.1 mm to 1 mm, particularly preferably 0.2 mm to 0.5 mm. The shape and dimension of the rod-shape lead-free solder molded product according to the present invention is not particularly limited. In order to minimize the segregation of the chemical constituents in the cast rod-shaped lead-free solder molded product, however, the cooling rate is preferably not less than 1° C./sec. Further, examples of rod-shaped lead-free solder molded products include rod-shaped lead-free solder molded products having a homogeneous structure produced by extruding a cast ingot, subjecting the extrudate to plastic working such as rolling, or rod-shaped lead-free solder molded products produced by directly rolling molten lead-free solder.
The lead-free solder molded product according to the present invention is particularly suitable as solder molded products for solder joining of, for example, electronic components with various electronic elements, for example, LED light emitting elements, and SEDs (surface-conduction electron-emitter displays) and mounted substrates.
The lead-free solder and lead-free solder molded product according to the present invention have substantially the same melting point as the conventional lead-containing tin solder and have excellent solder joint strength and reliability which are equal to or superior to those of the conventional lead-free solder and led-free solder molded product.

EXAMPLES

Example 1

A lead-free solder composed of tin-9.0 wt % zinc-0.1 wt % tantalum was molten, and the melt was cast into a billet having a diameter of 100 mm and a length of 300 mm. Next, the billet was extruded into a rod material having a thickness of 10 mm and a width of 70 mm. The rod material was then rolled to prepare a ribbon having a thickness of 100 μm and a width of 70 mm.
Next, as shown in FIG. 7, flux was coated onto a surface of a copper plate 100 having a thickness of 3 mm, a width of 50 mm, and a length of 60 mm as shown in FIG. 7, and a ribbon solder 102 having a thickness of 100 μm, a width of 40 mm and a length of 50 mm was then placed on the coated copper plate 100. Next, a copper metallized SiN substrate 101 having a thickness of 0.5 mm, a width of 30 mm, and a length of 40 mm was placed on the upper part of the ribbon solder 102. The assembly was heated in a nitrogen gas atmosphere at a temperature of 230° C. for 45 sec for reflowing. The joint product thus obtained was subjected to a thermal cycle test under conditions of −25° C. to 125° C. After the 2000-cycle thermal cycle test, the joint product was inspected by an ultrasonic flaw inspection test. As a result, neither crack nor separation was observed.

Example 2

A lead-free solder composed of tin-0.5 wt % copper-2.5 wt % silver-4.0 wt % In-0.1 wt % tantalum-0.1 wt % cobalt was molten, and the melt was cast into a billet having a diameter of 100 mm and a length of 300 mm. Next, the billet was extruded into a rod material having a thickness of 10 mm and a width of 70 mm. The rod material was then rolled to prepare a ribbon having a thickness of 100 μm and a width of 70 mm.
Next, as shown in FIG. 7, flux was coated onto a surface of a copper plate 100 having a thickness of 3 mm, a width of 50 mm, and a length of 60 mm, and a ribbon solder 102 having a thickness of 100 μm, a width of 40 mm and a length of 50 mm was then placed on the coated copper plate 100. Next, a copper metallized SiN substrate 102 having a thickness of 0.5 mm, a width of 30 mm, and a length of 40 mm was placed on the upper part of the ribbon solder 101. The assembly was heated in a nitrogen gas atmosphere at a temperature of 250° C. for 45 sec for reflowing. The joint product thus obtained was subjected to a thermal cycle test under conditions of −25° C. to 125° C. After the 4000-cycle thermal cycle test, the joint product was inspected by an ultrasonic flaw inspection test. As a result, neither crack nor separation was observed.

Comparative Example 1

A lead-free solder composed of tin-9.0 wt % zinc was molten, and the melt was cast into a billet having a diameter of 100 mm and a length of 300 mm. The billet was then extruded into a section having a thickness of 10 mm and a width of 70 mm. As a result, it was found that a number of cracks occurred in the section in its edge part in a direction perpendicular to the extrusion direction. The extruded material was rolled to prepare a ribbon solder having a thickness of 100 μm, a width of 40 mm and a length of 50 mm which was then cut and used as a sample.
Next, in the same manner as in Example 1, flux was coated onto a surface of a copper plate 100 having a thickness of 3 mm, a width of 50 mm, and a length of 60 mm, and a ribbon solder 102 having a thickness of 100 μm, a width of 40 mm and a length of 50 mm was then placed on the coated copper plate 100. Next, a copper metallized SiN substrate 102 having a thickness of 0.5 mm, a width of 30 mm, and a length of 40 mm was placed on the upper part of the ribbon solder 101. The assembly was heated in a nitrogen gas atmosphere at a temperature of 230° C. for 45 sec for reflowing. The joint product thus obtained was subjected to a thermal cycle test under conditions of −25° C. to 125° C. After the 750-cycle thermal cycle test, the joint product was inspected by an ultrasonic flaw inspection test. As a result, a number of cracks were observed at four corners of the joint product.

Claims

1. A lead-free solder comprising a tin (Sn)-base alloy having a tantalum (Ta) content of not less than 0.005% by weight and not more than 2.0% by weight.

2. A lead-free solder comprising a tin (Sn)-base alloy comprising not less than 0.005% by weight and not more than 2.0% by weight of tantalum (Ta) and not less than 0.1% by weight and not more than 10.0% by weight of zinc (Zn) with the balance consisting of tin (Sn) and unavoidable impurities.

3. A lead-free solder comprising a tin (Sn)-base alloy comprising not less than 0.005% by weight and not more than 2.0% by weight of tantalum (Ta) and not less than 0.1% by weight and not more than 60.0% by weight of bismuth (Bi) with the balance consisting of tin (Sn) and unavoidable impurities.

4. A lead-free solder comprising a tin (Sn)-base alloy comprising not less than 0.005% by weight and not more than 2.0% by weight of tantalum (Ta) and not less than 0.1% by weight and not more than 10.0% by weight of indium (In) with the balance consisting of tin (Sn) and unavoidable impurities.

5. A lead-free solder comprising a tin (Sn)-base alloy comprising not less than 0.005% by weight and not more than 2.0% by weight of tantalum (Ta) and not less than 0.01% by weight and not more than 7.5% by weight of copper (Cu) with the balance consisting of tin (Sn) and unavoidable impurities.

6. A lead-free solder comprising a tin (Sn)-base alloy comprising not less than 0.005% by weight and not more than 2.0% by weight of tantalum (Ta) and not less than 0.01% by weight and not more than 5.0% by weight of silver (Ag) with the balance consisting of tin (Sn) and unavoidable impurities.

7. A lead-free solder comprising a tin (Sn)-base alloy comprising not less than 0.005% by weight and not more than 2.0% by weight of tantalum (Ta), not less than 0.01% by weight and not more than 5.0% by weight of silver (Ag), and not less than 0.01% by weight and not more than 7.5% by weight of copper (Cu) with the balance consisting of tin (Sn) and unavoidable impurities.

8. The lead-free solder according to claim 1, wherein said tin-base alloy further comprises at least one additive element (Y) selected from the group consisting of indium and bismuth.

9. The lead-free solder according to claim 8, wherein, in said tin-base alloy, the content of the additive element (Y) selected from the group consisting of indium and bismuth is not more than 10% by weight for indium and not more than 60% by weight for bismuth.

10. The lead-free solder according to claim 1, wherein said tin-base alloy further comprises at least one additive element (X) selected from the group consisting of cobalt (Co), titanium (Ti), nickel (Ni), palladium (Pd), antimony (Sb), and germanium (Ge).

11. The lead-free solder according to claim 10, wherein, in said tin-base alloy, the content of the additive element (X) selected from the group consisting of cobalt, titanium, nickel, palladium, antimony, and germanium is such that, for each of the additive elements (X), the content is not more than 0.5% by weight and, when a plurality of the additive elements (X) are contained, the total content of the plurality of the additive elements (X) is not more than 1.0% by weight.

12. The lead-free solder according to claim 1, which is in a cream, ribbon, filament or rod form.

13. A solder joint product produced by jointing with a lead-free solder according to claim 1.

14. An electronic component produced by jointing with a lead-free solder according to claim 1.