1. Field of the Invention
The present invention generally relates to electronic devices, and more particularly, to an electronic device including a part processed by a solder-paste and solder-packaging.
2. Description of the Related Art
Soldering is utilized for packaging parts on a substrate with respect to various electronic devices. Particularly, a surface mount technology has been utilized in recent electronic devices in order to realize high-density implementation of parts on a wiring substrate. A solder paste is used for the surface mount of parts.
The solder paste is a material typically obtained by mixing solder alloy particles with a flux vehicle containing rosin, organic solvents, thixotropic agents, and activators. The solder paste is usually screen-printed on the wiring substrate when it is used. While the mixture ratio of ingredients of the solder paste varies depending on the type of the solder alloy composition, a typical ratio between the solder alloy particles and the flux vehicle is roughly nine to one in weight.
The solder alloy particles are manufactured by a gas atomize method, a disk atomize method or the like for pulverizing melting solder into particles. For instance, the solder alloy particles having a grain diameter of 20-40 xcexcm are utilized for packaging of quad flat package (QFP) having a pitch of 0.5 mm.
Conventionally, an alloy in a Sn (tin)xe2x80x94Pb (lead) system was generally utilized as the solder alloy. However, because the Pb has a toxicity, a Pb-free solder alloy such as an alloy in a Snxe2x80x94Agxe2x80x94Cu system or an alloy in a Snxe2x80x94Zn (zinc)xe2x80x94Bi (bismuth) system has been used recently. An alloy having the composition of Sn-3.0 Ag-0.5 Cu, namely an alloy including Ag of 3 mass % of and Cu of 0.5 mass %, is used as the alloy in the Snxe2x80x94Agxe2x80x94Cu system. An alloy having the composition of Sn-8 Zn-3 Bi, namely an alloy including Zn of 8 mass % and Bi of 3 mass %, is utilized as the alloy in the Snxe2x80x94Znxe2x80x94Bi system.
However, the above-mentioned alloy in the Snxe2x80x94Agxe2x80x94Cu system has a melting point of 218xc2x0 C. while the melting point of a conventional solder alloy in the Snxe2x80x94Pb system has a melting point of 183xc2x0 C. That is, the melting point of the alloy in the Snxe2x80x94Agxe2x80x94Cu system is 35xc2x0 C. higher than the melting point of the conventional solder alloy in the Snxe2x80x94Pb system. Hence, there is a risk of damaging the parts during reflow soldering process with Snxe2x80x94Agxe2x80x94Cu solder. Therefore, it is required for designing a printed wiring board that parts having low heat resistance be arranged close to parts having a large heat capacity. Furthermore, if the solder alloy in the Snxe2x80x94Agxe2x80x94Cu system is used, it is necessary to heighten the soldering temperature, as the melting point of the solder is higher than that of conventional solder. Therefore, it may be necessary to renew a furnace for uniform heating if the solder alloy in the Snxe2x80x94Agxe2x80x94Cu system is to be used.
On the other hand, the above-mentioned solder alloy in the Snxe2x80x94Znxe2x80x94Bi system has a melting point of 193xc2x0 C. This melting point is only 10xc2x0 C. higher than a melting point of a conventional solder in the Snxe2x80x94Pb system, namely 183xc2x0 C. Therefore, there is no disadvantage regarding the heat resistance of the parts. Hence, it may be possible to keep utilizing the conventional equipment for soldering when the solder alloy in the Snxe2x80x94Znxe2x80x94Bi system is used.
FIG. 1 is a view illustrating a conventional soldering process in which the solder alloy in a Snxe2x80x94Znxe2x80x94Bi system is used.
Referring to FIG. 1-(A), a copper (Cu) wiring pattern 11 is formed on a printed wiring board forming an electronic device. A solder paste 13 is screen-printed on the Cu wiring pattern 11. The solder paste 13 includes solder alloy particles 12 in the Snxe2x80x94Znxe2x80x94Bi system. A lead terminal 14 of the QFP is mounted on the solder paste 13. The solder alloy particles 12 are heated at a temperature of approximately 210xc2x0 C. that is higher than the solder melting point of 193xc2x0 C., thereby the solder alloy particles 12 are melted and the solder alloy layer 12A is formed as shown in FIG. 1-(B).
When the Cu wiring pattern 11 of the electronic device is soldered by such the solder alloy in the Snxe2x80x94Znxe2x80x94Bi system, a Znxe2x80x94Cu compound layer 11A is apt to be formed on an interface between the solder alloy layer 12A and the Cu wiring pattern 11. This is because there is a high affinity between Zn and Cu.
The Znxe2x80x94Cu compound layer 11A grows as time passes, as shown in FIG. 1-(C), because of the solid-phase-diffusing of Zn in the solder alloy layer 12A. During actual device operation, the electronic device generally has a temperature of approximately 80xc2x0 C. Therefore, a heat stress accompanying such a temperature increase promotes the solid-phase-diffusing of Zn in the solder alloy layer 12A.
When the Znxe2x80x94Cu compound layer 11A is grown thick on the interface between the Cu wiring pattern 11 and the solder alloy layer 12, bonding strength of the Znxe2x80x94Cu compound layer 11A and the solder alloy layer 12 is reduced. Hence, breaking between the Znxe2x80x94Cu compound layer 11A and the solder alloy layer 12, is apt to occur as shown in FIG. 2, and thereby the bonded lead terminal is separated from the substrate.
Accordingly, it is a general object of the present invention is to provide a novel and useful solder paste and electric device in which one or more of the problems described above are eliminated.
Another and more specific object of the present invention is to provide a solder paste which can provide enduring stability for a solder part for which a Pb free solder in a Snxe2x80x94Zn system is used, and an electronic device having such a solder part.
The above objects of the present invention are achieved by a solder paste, including a flux, a solder alloy particle scattered or mixed in the flux and including Sn and Zn as composition elements, and a metal particle scattered or mixed in the flux and including an element in the IB group in the periodic table as a composition element. The above objects of the present invention are also achieved by an electronic device, including a terminal of which at least a surface is made of Cu, and a solder layer covering the terminal and including a solder alloy including Sn and Zn as composition elements, and a particle having a diameter of 5 xcexcm or more and including an element in the IB group as a composition element.
According to the above invention, the alloy particles are scattered or mixed in the flux of the solder paste, with the particles including an element in the IB group as a composition element. Accordingly, after soldering is completed, it is possible to fix Zn on the particles including the element in the IB group. Hence, as a result of this, it is possible to decrease a reduction of the strength of the soldering part. It may be possible to select Cu, Au, or Ag as the element in the IB group.
Other objects, features, and advantages of the present invention will be more apparent from the following detailed description when read in conjunction with the accompanying drawings.