JP2003211283A - Lead-free solder material - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an Sn-Ag binary lead-free solder alloy material which is made finer in the solidified structure not depending upon a cooling rate during packaging and is reduced in the consumption of Ag by subjecting a rough and large primary crystal β-Sn phase to grain refining. <P>SOLUTION: Al is added to the Sn-Ag binary alloy to induce crystal formation in a manner of multiple acts by solidification accompanied by heteronucleation, by which the grain refining of the material structure is performed. As the more specific composition, 0.1 to 5 wt.% Al is incorporated into the alloy consisting of 1 to 5 wt.% Ag and the balance Sn. Further, 40 to 60 wt.% Bi or 40 to 50 wt.% In is incorporated in addition to the above composition into the alloy. The above composition is otherwise formed of 0.1 to 3 wt.% Cu and further, 0.5 to 5 wt.% one or 2 kinds or more of the elements selected from In, Bi, Ni, Au, Sb, Zn, Mg, La, and Ce are incorporated into the alloy. In the structure of Figure (a), the primary crystal β-Sn phase is made finer in the grains and the gaps thereof are packed with fine eutectic structures but in a comparison example not containing Al of (b) and (c), the rough and large primary crystal β-Sn phase occupies much thereof. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a so-called solder material, which is a soft brazing material used for circuit connection of electric / electronic parts and joining of metal members in the electronics industry, and more particularly to a so-called lead-free solder material from which lead is eliminated.

[0002]

2. Description of the Related Art A solder material has been widely used as a means for electrical connection and mechanical fixing, such as mounting and connecting electronic parts in the electronics industry. -Pb-based alloys contain lead, which poses a toxicity problem not only in the working environment but also in the disposal and recovery of used products.
In recent years, in order to reduce or eliminate the load on the environment, development of so-called lead-free solder material containing no lead as an alternative material for these materials has been under way. These solder materials are required not to cause heat damage to the electronic components, resist materials, etc. targeted for the electronic mounting, or to prevent distortion of a substrate containing resins, etc., and therefore Sn-Pb. Similar to solders, it must have a low melting point and the strength and reliability of the connecting / fixing parts. Particularly, in recent electronic packaging, these requirements have become more severe with the progress of miniaturization of parts and the like and high density surface mounting. Therefore, in the new lead-free solder material, Sn is the main component with the aim of lowering the melting point and refining the crystal structure that meet these conditions,
The composition of the eutectic alloy system containing Ag and other constituent elements is selected. As these solder materials, Sn-Ag
It has been reported in Japan, the United States and Europe that the eutectic and further Cu, Bi and In are added individually or in combination to lower the melting point and improve the mechanical properties. (See JP-A-200-94181 and JP-A-7-51883.)

However, when these Sn-based solder alloys are generally applied to soldering in mounting electronic parts,
Despite the eutectic composition, the actually formed metallographic structure is unlikely to be a fine eutectic structure and is usually coarse primary β-
There is a problem that the Sn phase develops to form a two-phase mixed structure of the Sn phase and the eutectic phase, resulting in deterioration of mechanical properties such as material strength. In addition, under such circumstances, Sn-3.
Even in 0Ag-0.5Cu alloy and the like, it is unavoidable that the content of expensive Ag is large and the cost cost becomes significantly high. Therefore, it is desired to reduce the content of Ag. If the initial eutectic composition is deviated, the intended fine eutectic structure cannot be obtained, and desired mechanical properties cannot be obtained.

Under these circumstances, when applying these solder alloy materials to the mounting of electronic parts, both the flow method and the reflow method are heated including the mounted electronic parts and substrate, and then cooled or released. Although the melting and solidification of the solder material progresses during the cooling process, this cooling rate varies significantly depending on the mounting components and process, and therefore the eutectic structure as intended cannot be obtained at any cooling rate under the mounting conditions. The quality of the obtained metal structure is not stable because it does not always have a uniform structure.

[0005]

In the Sn-based lead-free solder material, the fineness of the material structure and the improvement of the mechanical properties due to it, in particular, the formation of the fine material structure which is not affected by the cooling rate under the mounting conditions and the like. The mechanical properties are improved, and the mechanical properties are maintained / improved in a wide range of component compositions including a hypoeutectic composition with a reduced Ag content.

[0006]

The present invention is a lead-free solder material in which a Sn—Ag binary alloy is added with Al and the material structure is refined by solidification accompanied by heterogeneous nucleation. 1 wt% to 5 wt% Ag, also C
u: 0.1 to 3 wt%, with the balance being Sn, Al: 0.1 to
This is a lead-free solder alloy material containing 5 wt%. In addition to the above composition, Bi: 40 to 60 wt% or In: 40 to 50 wt% is added, or in the above composition, Cu: 0.1 to 3 wt%, and I:
n, Bi, Ni, Au, Sb, Zn, Mg, La, Ce
0.1 to 5 wt% of any one or more of the elements selected from
% This is a lead-free solder material that contains all the elements.

The alloy composition of the present invention has a coarse primary crystal β-Sn.
Although the phase is in the range where it crystallizes, a small amount of Al is added, so that the cooling rate is not affected and the refinement of the metal structure can be achieved at the cooling rate under the solder mounting condition. Regarding the mechanism, the inventors of the present invention did not refine the material structure under the mounting conditions even though these Sn-based solder alloys had a eutectic structure, and formed a coarse primary crystal β-Sn phase. The cause of this is examined, and unlike the solidification process under equilibrium conditions, when given a certain cooling rate, the eutectic composition shifts to the high Ag side and the actual solidification structure does not become the eutectic composition. I focused on. Its structure is primary β-S
The eutectic of the n-phase, β-Sn phase and Ag ₃ Sn intermetallic compound crystallizes out, and the β-Sn phase that crystallizes in the primary crystal is softer than the eutectic phase and has a large grain size, so the material strength is high. Was coming of decline. Therefore, if the β-Sn phase that crystallizes in the primary crystal is refined and a uniform structure is obtained, the material strength is also improved, but in the conventional Sn-Ag based alloy, the composition region of the eutectic is controlled to form the entire structure. Since it is difficult to form a eutectic phase, the present inventors have conceived that heterogeneous nuclei are formed in the molten metal and crystals are crystallized simultaneously in the solidification process.

Namely, a part the addition of Al in air to Sn-Ag alloy melt is changed to Al ₂ O _3, Al ₂
Since O ₃ is generally known to function as a strong heterogeneous nucleus, it was considered whether it functions as a heterogeneous nucleus in a Sn-based alloy. As a basis for this, when looking at the degree of mismatch between the Sn and Al ₂ O ₃ crystal planes, the mismatch is large, and
It is expected that Al ₂ O ₃ will function as a strong foreign nucleus for n. In fact, when Al is added to the Sn-Ag alloy and solidified from the molten metal, the cooling curve obtained for the Sn-Ag binary alloy shows a latent heat phenomenon in the cooling curve because homogeneous nucleation occurs with large supercooling. However, in the sample in which Al was added to the Sn—Ag alloy, supercooling was suppressed and the crystallization transformation point of the primary crystal did not appear. From this, it was found that Al functions as a heterogeneous nucleus and nucleation is performed simultaneously.

The Sn-Ag alloy is characterized by a large amount of supercooling due to the absence of heterogeneous nuclei during solidification as described above. Therefore, in the actual soldering operation, the cooling rate at which the solder solidifies is Under each condition, the composition shown in the equilibrium diagram shifts to the high alloy side, and even if it is a eutectic composition, it shows a hypoeutectic structure, and the primary grain β-Sn phase with a large grain size and fine grains It has a mixed structure with various eutectic crystals. In the present invention, the Sn-Ag binary alloy is A
The addition of 1 part of Al combines with oxygen during melting to form Al ₂ O ₃ , and this Al ₂ O ₃ functions as a heterogeneous nucleus to simultaneously crystallize crystals during solidification. Therefore, the structure is made finer. Therefore, the β-Sn phase crystallized as a primary crystal is finely divided over a wide composition range from hypereutectic to hypoeutectic of these Sn-Ag binary alloys. And mechanical properties such as material strength can be improved. This means that at the same time, in these Sn-based eutectic alloys, the refinement of the material structure can be achieved without depending on the cooling rate, and the alloy composition has a wide composition range from Ag hypereutectic composition to hypoeutectic composition. It means that the crystal structure can be finely controlled. Therefore, in the present invention, the material structure can be refined under a wide cooling rate condition under the mounting condition, and the Ag content can be reduced without lowering the mechanical characteristics.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Aspects of the present invention will be specifically described below. In the present invention, as a basic composition of Sn, Ag
Content from 1.0wt% to 5wt%, Al content 0.1wt%
% To 5 wt%, which is achieved. Here, in claim 2, (1) the basis for setting the Ag content from 0.1 wt% to 5 wt%,
The minimum content of 0.1 wt% is the minimum condition for maintaining the strength of the solder material, and the maximum content of 5 wt% is eutectic from the hypoeutectic composition when the cooling rate is accelerated during mounting. It is the boundary composition within the range of transformation into composition. (2) Cu content The minimum content of 0.1 wt% is the minimum condition for maintaining Cu addition ability, that is, improvement of wettability and material strength, and the maximum content of 3 wt% maintains material strength, Note that this is a limit region in which a rapid increase in melting temperature can be suppressed. (3) Regarding the Al content, 0.1 wt% of the minimum content
Is the minimum condition that Al changes into Al ₂ O ₃ in the atmosphere and functions as a heterogeneous nucleus, and the maximum content of 5 wt% is excessive Al ₂ O ₃ generated as a heterogeneous nucleus, which impairs the wettability of the material. There is no marginal area. Further, the reason for limiting the contents of Bi and In in claim 3 is as follows: (4) Regarding the minimum Bi content of 40 wt%, the primary crystal is crystallized from 43 wt% of the eutectic to lower the material strength. Therefore, it is a practically determined limit concentration, and 60 wt% is a limit point for suppressing a sharp rise in melting point. (5) Regarding the In content The minimum content of 40 wt% is the minimum condition for changing the phase structure, and 50 wt% is the minimum limit composition for suppressing the rise of the melting point. (6) Other alloying elements Furthermore, the additive elements of claim 4 have the role of lowering the melting point and improving the wettability. In, Bi,
Sb and Zn are elements that lower the melting point, and if the concentration is 0.1 wt% or less, the effect cannot be exhibited, and if the concentration is 5 wt% or more, a remarkable effect cannot be expected with a concentration lower than that. In addition, if added in an amount of 5 wt% or more, the wettability is poor. Ni, Cu, and Au are mixed in the molten metal during mounting because they are used as electronic component substrate materials during mounting. The effect of these elements is to improve wettability. Therefore, the minimum content of 0.1 wt% is the minimum condition under which the effect can be exhibited, and 5 wt% is the limit concentration at which the effect continues. On the other hand, addition of 5 wt% or more adversely affects the wettability and lowers the material strength. Mg, La, and Ce are added as elements that strengthen the grain boundary gap portion of the material structure and enhance the material strength. Therefore, the minimum concentration of 0.1 wt% is the minimum content at which the effect can be exhibited, and 5 wt% is the limit concentration at which the effect can be exhibited. On the contrary, with the addition of more than this, the intermetallic compound becomes coarser. This leads to a decrease in material strength.

[0011]

[Examples] Specific data will be described below. (A) Sn-2.0wt so that the total sample weight is 200g
% Ag hypoeutectic alloy was prepared and held at 573 K for 1.8 ks, then 0.9 ks was added while stirring while adding and stirring Al so that the content of Al in the molten metal was 0.1 wt%, and the cooling rate was from 10 K / s to 130 K / s. s was given to solidify. As a comparison material, (b) Sn-2.0 wt% so that the total amount is 200 g.
Ag and (c) Sn-3.5 wt% Ag eutectic alloys were prepared and solidified under the same conditions as above.

FIGS. 1 (a), (b) and (c) are micrographs of Example (a) and Comparative Examples (b) and (c) solidified at the cooling rate of 10 K / s. The composition of each contained element is shown in mass% for convenience of notation in the figure, but in reality
It is no different from wt%. It is understood that in the Sn-2.0 mass% Ag hypoeutectic alloy of the present invention prepared by adding 0.1 mass% of Al of (a), the primary crystal β-Sn phase is uniformly finely formed. . On the other hand, in the comparative examples of (b) and (c), Sn-2.0 mass% Ag and Sn-
The structure of the 3.5 mass% Ag eutectic alloy is formed by coarsening of the primary crystal β-Sn phase, during which the eutectic structure is finely crystallized. As can be seen from the scales of 10 μm and 50 μm inserted in the figure, the grain size of the primary β-Sn phase in these structures is 10 compared with the primary β-Sn phase of the present invention.
It is more than doubled and noticeable differences are seen. Thus, the eutectic structure of the β-Sn phase and the Ag ₃ Sn phase was densely formed in the gap between the primary crystal β-Sn phases. From this result, the primary crystal β-
The Sn-phase and eutectic phase do not have a structure morphology having a certain area, but the grain size of the primary crystal β-Sn phase is refined, so that the eutectic phase forms a structure in which the grain boundary distribution is reinforced. It became clear. This has great significance for improving the material strength.

FIG. 2 shows the relationship between the grain size of the primary β-Sn phase and the cooling rate in the alloys of the present invention and comparative examples. Comparative example
(b) Sn-2.0 mass% Ag hypoeutectic alloy, and (c) Sn-3.
With 5 mass% Ag eutectic alloy, the cooling rate was 10 K / s in all cases.
There is a tendency for the grain size to become finer from 40 μm to about 20 μm as it increases from 10 K / s to 130 K / s. In the alloy of the present invention, the cooling rate is 6 to 7 μm even in the range of 10 K / s to 130 K / s. It can be seen that the particle size remains extremely fine and the cooling rate is almost independent. In this figure, the cooling rate (2k /
s to 10 K / s), the grain size of the alloy of the present invention was 1/7 of that of the comparative alloy, and it was remarkably refined.

FIG. 3 shows the results of measuring the concentration distribution in the structure of the alloy of the present invention in which 0.1 wt% of Al was added to the hypoeutectic alloy of Sn-2.0 wt% Ag. The main point of this figure is to grasp the grain size of the primary crystal β-Sn phase and the grain size and phase structure of the intermetallic compound existing in the intergranular gap by measuring the concentration distribution. Looking at the concentration composition of the Sn phase, the place where the concentration sharply decreases corresponds to the grain boundary gap portion, and the high concentration portion is the primary β-Sn phase. From the Ag and Al concentration distributions, the Ag and Al concentrations are increased where the Sn concentration changes. Further, the changes in the concentrations of Ag and Al show the same transition. Therefore, the phase existing in the intergranular space is the primary crystal β-S.
It is understood that it is an intermetallic compound different from the n phase. Because Ag and Al are not dissolved in the primary crystal β-Sn phase, the above fact can be understood from this fact as well. From the above, when Al is added to the Sn-Ag alloy, Al has no solubility in Sn and forms an intermetallic compound in the intergranular intergranular part by combining with Ag, or by contacting with air in the atmosphere to form Al ₂ It can be inferred that it changes to O ₃ .

The alloy of the present invention has a structure in which Sn-based solder alloy has a structure in which heterogeneous nuclei are formed in a molten metal by Al ₂ O ₃ formed by addition of Al, and crystals are simultaneously crystallized during solidification process. Is to be miniaturized,
The effect can be exhibited in a wide range of Ag content with respect to Sn, from a hypoeutectic composition to a hypereutectic composition. In addition, as for other components, Cu, Bi, In, Ni, Au, etc., which have been conventionally added to the Sn-based solder alloy in order to lower the melting point, improve the wettability as a solder material, and maintain the material strength. It is possible to suppress the coarsening of the primary crystal β-Sn phase and achieve the refinement of the structure by the same mechanism for solder alloys containing elements such as Al, Sb, Zn, Mg, La, and Ce. Within the range of the above component, in the solder alloy containing one or more of these elements, it is possible to exert the action and effect of the present invention together with the characteristic improvement due to these contained components.

[0016]

As described above, the lead-free solder alloy material of the present invention improves the mechanical properties of the conventional Sn-Ag binary lead-free solder material and is affected by changes in the solder cooling rate. Since the material structure can be made finer and the mechanical properties can be improved without increasing the density,
The present invention contributes to electronic solder mounting, in which the miniaturization of components is remarkable, and also contributes to the development of these industries by promoting the spread of the components by reducing the cost because the expensive Ag content can be reduced.

[Brief description of drawings]

FIG. 1 is an enlarged view of microstructures of a solder alloy of the present invention (a) and comparative alloys (b) and (c) based on micrographs.

FIG. 2 is a graph showing the relationship between the cooling rate and the crystal grain size of the solder alloy of the present invention and the alloy of Comparative Example.

FIG. 3 is a concentration distribution diagram of each contained element in the solder alloy of the present invention.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Taiki Kobayashi             Hokkaido, Sapporo, Kita-ku, Kita 13-jo Nishi 8-chome, Hokkaido             University Graduate School of Engineering F-term (reference) 5E319 BB08 GG11

Claims (4)

[Claims] 1. A lead-free solder material in which a material structure is refined by solidification with heterogeneous nucleation in an Sn-Ag binary alloy. 2. The above Ag: 1 wt% to 5 wt%, also C
u: 0.1 to 3 wt%, Al: 0.1 to the alloy consisting of the balance Sn
The lead-free solder alloy according to claim 1, wherein the lead-free solder alloy contains 5 to 5 wt%. 3. In addition to the above composition, Bi: 40-60 wt%
Alternatively, the lead-free solder material according to claim 1 or 2, wherein In: 40 to 50 wt% is contained. 4. In the above composition, Cu: 0.1-5 wt%
And In, Bi, Ni, Au, Sb, Zn, M
3. The lead-free solder material according to claim 1, which contains 0.1 to 5 wt% of one or more elements selected from g, La, and Ce.