Metal treatment – Stock
Reexamination Certificate
2000-06-28
2001-10-02
Ip, Sikyin (Department: 1742)
Metal treatment
Stock
C420S560000, C420S561000, C228S056300
Reexamination Certificate
active
06296722
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the composition of a novel lead-free solder alloy.
2. Description of the Related Art
In the solder alloy, lead has been conventionally an important metal for diluting tin to improve flow factor and wettability. Obviating the use of lead, a toxic, heavy metal, is preferred in consideration of working environments in which soldered products are used, and the earth friendly to which solder is released. Avoiding the use of lead in solder alloy is thus noticeable practice.
When a lead-free solder alloy is formed, the alloy is required to have wettability to metals to be soldered. Tin having such wettability is an indispensable metal as a base material. In the formation of a lead-free solder alloy, it is important to fully exploit the property of tin and to determine the content of an additive metal for the purpose of imparting, to the lead-free solder alloy, strength and flexibility as good as those of the conventional tin-lead eutectic alloy.
SUMMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a lead-free solder alloy having tin as a base material with other additive materials that are easily gettable as good as the conventional tin-lead eutectic alloy, and offers a stable and liable solder joint.
To achieve the object of the present invention, the solder alloy is preferably formed of three metals of 0.1-2 weight percent (hereinafter wt %) Cu, 0.002-1 wt % Ni and the remaining wt % Sn. Of these elements, tin has a melting point of about 232C, and is an indispensable metal to impart wettability of the alloy against the metals to be soldered. A tin-based alloy, without lead of a large specific gravity, is light in its molten state, and cannot offer enough flowability to be appropriate for a nozzle-type soldering operation. The crystalline structure of such solder alloy is too soft and not mechanically strong enough. By additive of copper the alloy reinforces strongly. The addition of approximately 0.7% copper added to tin forms an eutectic alloy having a melting point of approximately 227° C., which is lower than that of tin alone by approximately 5° C. The addition of copper restrains copper leaching in which copper, a typical base material of lead wire, leaches out of the surface of the lead wire in the course of soldering operations. At a soldering temperature of 260° C., for example, the copper leaching rate of the copper-added alloy is half as high as the copper leaching rate in the tin-lead eutectic solder. Restraining the copper leaching reduces a copper density difference present in a soldering area, thereby slowing the growth of a brittle compound layer.
The addition of copper is effective to prevent a rapid change in composition in the alloy itself when using a long period on a dipping method.
The optimum amount of additive copper is within a range of 0.3-0.7 wt %, and if more copper is added, the melting temperature of the solder alloy rises. The higher the melting point, the higher the soldering temperature needs to be. A high soldering temperature is not preferable to thermally weak electronic components. Typical soldering temperature upper limit is considered to be 300° C. or so. With the liquidus temperature of 300° C., the amount of additive copper is about 2 wt %. The preferable value and limits are set as the above.
In the present invention, not only a small amount of copper is added to tin as a base material, but also 0.002-1 wt % nickel is added. Nickel controls intermetallic compounds such as Cu
6
Sn
5
and Cu
3
Sn, which are developed as a result of reaction of tin and copper, and dissolves the developed compounds. As such intermetallic compounds have a high temperature melting point, they hinder flowability of milting solder and make solder function declined. Therefore, if these intermetallic compounds remain on patterns at a soldering operation, these become to be so-called bridge that shorts conductors. Namely, needle-like projections remains when leaving from melting solder. To avoid such problems, nickel is added. Although nickel itself produces intermetallic compound with tin, copper and nickel are always solid soluble at any ratio. Therefore, nickel cooperates with the development of Sn—Cu intermetallic compounds. Since the addition of copper to tin helps the alloy to improve its property as a solder compound in the present invention, a large amount of Sn—Cu intermetallic compounds is not preferable. For this reason, nickel, in an all-ratio solid soluble relationship with copper, is thus employed to control the reaction of copper with tin.
The liquidus temperature rises if nickel is added because a melting point of nickel is high. In consideration of the typical permissible upper temperature limit, the amount of additive nickel is limited to 1 wt %. It was learned for an inventor that the amount of additive nickel as low as or greater than 0.002 wt % held a good flowability and solderablity showed a sufficient strength of a soldered joint. According to the present invention, a lower limit of the amount of additive nickel is thus 0.002 wt %.
In the above process, nickel is added to the Sn—Cu alloy. Alternatively, Cu may be added to an Sn—Ni alloy. When nickel alone is slowly added to tin, according to the raising up of a melting point, the flow factor drops in its molten state by reason of producing intermetallic compounds. By adding copper, the alloy has a smooth property with an improved flow factor but some degree of viscosity. In either process, the interaction of copper and nickel helps create a preferable state in the alloy. The same solder alloy is therefore created not only by adding Ni to the Sn—Cu base alloy but also by adding Cu to the Sn—Ni base alloy.
Referring to
FIG. 1
a range of 0.002-1 wt % nickel and a range of 0.1-2 wt % copper result in a good solder joint. When the base alloy is Sn—Cu, the content of copper represented by the X axis is limited to a constant value within a range of 0.1-2 wt %. If the content of nickel is varied within a range of 0.002-1 wt % with the copper content limited to within a range of 0.1-2 wt %, a good solder alloy is obtained. When the base alloy is Sn—Ni, the content of nickel represented by the Y axis is limited to a constant value within a range of 0.002-1 wt %. If the content of copper is varied within a range of 0.002-1 wt %, a good solder alloy is obtained. These ranges remain unchanged even if an unavoidable impurity, which obstructs the function of nickel, is mixed in the alloy. Gallium has a melting point of 30° C. and its radius is slightly smaller than that of coppers. Therefore, when it joints, the speed of spread of wettability become faster and the strength of joint improves. By adding gallium, the amount of oxidizable dregs in melting solder alloy reduces. However, if gallium is added much, solidus temperature plummets, which affects reliability and rises the cost problem. In consideration of cost and efficiency, the upper limit of additive amount of gallium in the present invention is set.
REFERENCES:
patent: 5837191 (1998-11-01), Gickler
patent: 5863493 (1999-01-01), Achari et al.
patent: 19816671 (1998-10-01), None
Ip Sikyin
Nihon Superior Sha Co., Ltd.
Thompson Hine LLP
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