LEAD-FREE SOLDER ALLOY COMPOSITION AND METHOD FOR PREPARING LEAD-FREE SOLDER ALLOY

Abstract

This invention relates to a lead-free solder alloy composition and a method of preparing a lead-free solder alloy, wherein the lead-free solder alloy composition includes a ceramic powder added to a lead-free solder of Sn-(0.1 to 2) wt % Cu, Sn-(0.5 to 5) wt % Ag, or Sn-(0.1 to 2) wt % Cu-(0.5 to 5) wt % Ag. According to this invention, a novel lead-free solder alloy, which functions as a replacement for a conventional lead-free solder, is provided, thus exhibiting superior spreadability, wettability, and mechanical properties than a conventional lead-free solder.

Claims

1. A lead-free solder alloy composition, wherein a ceramic powder is added to a lead-free solder of Sn-(0.1 to 2) wt % Cu, Sn-(0.5 to 5) wt % Ag, or Sn-(0.1 to 2) wt % Cu-(0.5 to 5) wt % Ag.

2. The lead-free solder alloy composition of claim 1, wherein an additive added to the ceramic powder includes at least one selected from among La.sub.2O.sub.3, SiC, Cu-coated CNT (Cu-CNT), and ZrO.sub.2.

3. The lead-free solder alloy composition of claim 2, wherein contents of the ceramic powder are 0.01 wt % to 1.0 wt % of La.sub.2O.sub.3, 0.01 wt % to 1.0 wt % of SiC, and 0.005 wt % to 1.0 wt % of Cu-CNT.

4. The lead-free solder alloy composition of claim 1, wherein the ceramic powder has a size of 10 μm or less.

5. A method of preparing a lead-free solder alloy, comprising: a step of mixing at least one solder powder selected from among Sn—Cu, Sn—Ag, and Sn—Cu—Ag base; a step of melting the mixed solder powder; and a step of adding an additive to the melted solder powder.

6. The method of claim 5, wherein in the step of adding the additive, the additive is at least one ceramic powder selected from among La.sub.2O.sub.3, SiC, Cu-coated CNT (Cu-CNT) and ZrO.sub.2.

7. The method of claim 6, wherein in the step of adding the additive, the contents of ceramic powder are 0.01 wt % to 1.0 wt % of La.sub.2O.sub.3, 0.01 wt % to 1.0 wt % of SiC, and 0.005 wt % to 1.0 wt % of Cu-CNT.

Description

DESCRIPTION OF DRAWINGS

[0027] FIG. 1 is a mimetic diagram schematically showing the principle of inhibiting an amount of oxide generated in a lead-free solder composition according to Prior Art 2;

[0028] FIG. 2 shows images illustrating the average thickness of the intermetallic compound of solders containing nanopowder and a solder containing no nanopowder when a lead-free solder alloy composition according to the present invention is used;

[0029] FIG. 3 shows images illustrating the average grain size of the solders containing nanopowder and the solder containing no nanopowder when the lead-free solder alloy composition according to the present invention is used;

[0030] FIG. 4 is a graph comparing the spreadability of Sn—Cu, Sn—Ag and Sn—Cu—Ag lead-free solders and Sn—Cu, Sn—Ag and Sn—Cu—Ag lead-free solders containing additives, when the lead-free solder alloy composition according to the present invention is used;

[0031] FIG. 5 is a graph showing the results of the measured wettability of the Sn—Cu—Ag lead-free solder and the Sn—Cu—Ag lead-free solder containing an additive, when the lead-free solder alloy composition according to the present invention is used;

[0032] FIG. 6 is a graph showing the results of the measured hardness of the Sn—Cu—Ag lead-free solder and the Sn—Cu—Ag lead-free solder containing an additive, when the lead-free solder alloy composition according to the present invention is used;

[0033] FIG. 7 is a block diagram showing the process of preparing a lead-free solder alloy according to the present invention; and

[0034] FIG. 8 is a block diagram showing the process of preparing a lead-free solder paste using the lead-free solder alloy composition according to the present invention.

BEST MODE

[0035] The above-identified objective is achieved by the present invention, through a lead-free solder alloy composition wherein a ceramic powder is added to a lead-free solder of Sn-(0.1 to 2) wt % Cu, Sn-(0.5 to 5) wt % Ag, or Sn-(0.1 to 2) wt % Cu-(0.5 to 5) wt % Ag.

[0036] Also, in the present invention, the additive added to the ceramic powder may include at least one selected from among La.sub.2O.sub.3, SiC, Cu-coated CNT (Cu-CNT) and ZrO.sub.2.

[0037] Also, the contents of the ceramic powder of the present invention are 0.01 wt % to 1.0 wt % of La.sub.2O.sub.3, 0.01 wt % to 1.0 wt % of SiC, and 0.005 wt % to 1.0 wt % of Cu-CNT.

[0038] Also, in the present invention, the ceramic powder has a size of 10 μm or less.

[0039] Also, the present invention addresses a method of preparing a lead-free solder alloy, comprising: a step of mixing at least one solder powder selected from among Sn—Cu, Sn—Ag and Sn—Cu—Ag base, a step of melting the mixed solder powder, and a step of adding an additive to the melted solder powder.

[0040] Also, in the step of adding the additive, the additive may be at least one ceramic powder selected from among La.sub.2O.sub.3, SiC, Cu coated CNT (Cu-CNT), and ZrO.sub.2.

[0041] Also, in the step of adding the additive, the contents of ceramic powder are 0.01 wt % to 1.0 wt % of La.sub.203, 0.01 wt % to 1.0 wt % of SiC, and 0.005 wt % to 1.0 wt % of Cu-CNT.

Mode for Invention

[0042] The terminologies or words used in the description and the claims of the present invention should be construed based on the meanings and concepts consistent with the technical idea of the invention based on the principle that the inventors can appropriately define the terms in order to describe the invention in the best way.

[0043] In the overall specification, when any part “includes” any element, this means that another element is not excluded, but may be further included unless otherwise specifically mentioned.

[0044] Hereinafter, a detailed description will be given of a lead-free solder alloy composition and a method of preparing a lead-free solder alloy according to embodiments of the present invention, with reference to the accompanying drawings.

[0045] FIG. 2 shows images illustrating the average thickness of the intermetallic compound of solders containing nanopowder and a solder containing no nanopowder in a lead-free solder alloy composition according to the present invention, FIG. 3 shows images illustrating the average grain size of the solders containing nanopowder and the solder containing no nanopowder in the lead-free solder alloy composition according to the present invention, FIG. 4 is a graph showing the results of a comparison of the spreadability of Sn—Cu, Sn—Ag and Sn—Cu—Ag lead-free solders and Sn—Cu, Sn—Ag and Sn—Cu—Ag lead-free solders containing additives, in the lead-free solder alloy composition according to the present invention, FIG. 5 is a graph showing the results of measurement of wettability of the Sn—Cu—Ag lead-free solder and the Sn—Cu—Ag lead-free solders containing additives, in the lead-free solder alloy composition according to the present invention, and FIG. 6 is a graph showing the results of measurement of hardness of the Sn—Cu—Ag lead-free solder and the Sn—Cu—Ag lead-free solders containing additives, in the lead-free solder alloy composition according to the present invention.

[0046] According to the above drawings, the lead-free solder alloy composition of the present invention comprises adding a ceramic powder to a lead-free solder of Sn-(0.1 to 2) wt % Cu, Sn-(0.5 to 5) wt % Ag, or Sn-(0.1 to 2) wt % Cu-(0.5 to 5) wt % Ag.

[0047] Here, the additive added to the ceramic powder may include at least one selected from among La.sub.2O.sub.3, SiC, and Cu coated CNT (Cu-CNT), and the amount of La.sub.2O.sub.3 is 0.01 wt % to 1.0 wt %, the amount of SiC is 0.01 wt % to 1.0 wt %, and the amount of Cu-CNT is 0.005 wt % to 1.0 wt %. The ceramic powder preferably has a size of 10 m or less.

[0048] That is, the developed solder alloy is improved in wettability and spreadability, and is composed of Sn—Cu and an additive, Sn—Ag and an additive, or Sn—Ag—Cu and an additive. The selected additives are a nanometer-sized ceramic powder such as La.sub.2O.sub.3, SiC, and Cu-CNT (CNT: Carbon Nanotube). A solder joint including a solder alloy of Sn—Ag—Cu, Sn—Ag, or Sn—Cu base which is a typical lead-free solder, a solder ball or a solder paste, and the process produces a solder joint including a reinforced intermetallic compound such as Ag.sub.3Sn, Cu.sub.6Sn.sub.5 or the like.

[0049] Here, in the lead-free solder alloy composition, the Sn—Cu solder alloy is configured such that the amount of Cu is 0.1 wt % to 2 wt % (preferably 0.7 wt %). The Sn—Ag alloy includes Ag in an amount of 0.5 wt % to 5 wt % (preferably 3.5 wt %). Also, the Sn—Ag—Cu alloy includes Ag in an amount of 0.5 wt % to 5 wt % (preferably 3 wt %) and Cu in an amount of 0.1 wt % to 2 wt % (preferably 0.5 wt %). Additives added to the solder are La.sub.2O.sub.3, SiC, and Cu-coated CNT (Carbon Nanotube). The composition of each additive is 0.01 wt % to 1.0 wt % (preferably 0.05 wt %) of La.sub.203, 0.01 wt % to 1.0 wt % (preferably 0.05 wt %) of SiC, and 0.005 wt % to 1.0 wt % (preferably 0.01 wt %) of Cu-CNT.

[0050] The lead-free solder alloy is composed of about 96.5 wt % of Sn, 3.0 wt % of Ag, 0.5 wt % of Cu, and a nanometer-sized powder reinforcing agent as an additive. The reinforcing agent includes 0.01 wt % to 1.0 wt % (preferably 0.05 wt %) of La.sub.203, 0.01 wt % to 1.0 wt % (preferably 0.05 wt %) of SiC, and 0.005 wt % to 1.0 wt % (preferably 0.01 wt %) of Cu-CNT. Compared to the Sn-3.0Ag-0.5Cu (SAC305) solder alloy containing no additive, it is preferred that the zero cross time is short in order to improve wettability. However, the zero cross time of the Sn-3.0% Ag-0.5% Cu alloy was 1.08 sec., about 73.6% (0.285 sec) when La.sub.2O.sub.3 was added to the Sn-3.0% Ag-0.5% Cu alloy, 76.8% (0.25 sec) upon the addition of SiC, and 27.7% (0.78 sec) upon the addition of Cu-CNT.

[0051] Further, the lead-free solder alloy is composed of about 96.5 wt % of Sn, 3.0 wt % of Ag, 0.5 wt % of Cu, and a nanopowder reinforcing agent as an additive, such as La.sub.2O.sub.3, SiC, and Cu-CNT. A solder using only Sn-3.0 wt % Ag-0.5 wt % Cu was observed to have an average grain size of about 30 μm. When the solder containing the additive is compared with the Sn-3.0 wt % Ag-0.5 wt % Cu solder, the average grain size was decreased by 33.5% to 20 μm when La.sub.2O.sub.3 was added, decreased by 40% to 17.69 μm when SiC was added, and decreased by 28.64% to 21.07 μm when Cu-CNT was added. That is, the grain size of the solder becomes micronized according to the addition of the additive. Typically, when the grain size of a metal is micronized, yield strength and tensile strength are increased according to the following Hall-Petch Equation.

[00001]${\sigma }_y={\sigma }_0+\frac{K}{\sqrt{d}}$

[0052] σ.sub.y: yield strength, d: average grain diameter, σ.sub.0 and K: constant

[0053] When the solder is manufactured using the composition of the present invention, it is known that the solder alloy is configured to include an intermetallic compound (IMC) having a fine thickness therein. The average thickness of the intermetallic compound is about 2.7 μm in the case of the Sn-3.0 wt % Ag-0.5 wt % Cu (SAC3055) solder alloy containing no additive. When the solder containing the additive is compared with the Sn-3.0 wt % Ag-0.5 wt % Cu solder, it is seen that the average thickness of the intermetallic compound decreased by 18% to 2.2 μm when La.sub.2O.sub.3 is added, decreased by 37% to 1.84 μm, when SiC is added, and decreased by 7.8% to 2.71 μm when Cu-CNT is added.

[0054] The Sn-3.0Ag-0.5Cu solder alloy containing the ceramic nanopowder exhibits considerably high spreadability compared to that of the Sn-3.0Ag-0.5Cu solder. The spreadability thus increased is highly favorable in solder bonding of electronic circuits and electric systems. The solder having superior spreadability may be easily spread on a sensitive electronic part or circuit board upon soldering, whereby a solder joint may be efficiently formed, thus decreasing the incidence of defects and increasing strength in the solder joints.

[0055] When the solder, to which the nano-sized La.sub.2O.sub.3, SiC, and Cu-CNT are added, is melted and then solidified in the soldering process, La.sub.2O.sub.3, SiC, and Cu-CNT, having a melting point much higher than that of Sn(231), are present in the form of a fine nano-sized solid, and function as a solid nucleation site (seed) upon solidification of the powder added thereto. Thereby, the added nanopowder provides many nucleation sites on which solid crystals are produced, and thus the grain size is micronized compared to the Sn-3.0Ag-0.5Cu solder containing no additive. In addition, the nano-sized La.sub.2O.sub.3, SiC, and Cu-CNT hinder the formation of the intermetallic compound (IMC), such as Ag.sub.3Sn, Cu.sub.6Sn.sub.5 or the like, in the solder, whereby the intermetallic compound is micronized, thus exhibiting higher strength and superior properties of the solder.

[0056] In the present invention, the composition of the reinforcing agent is diverse, including 0.01 wt % to 1.0 wt % La.sub.2O.sub.3 (preferably 0.05 wt %), 0.01 wt % to 1.0 wt % of SiC (preferably 0.05 wt %), and 0.005 wt % to 1.0 wt % of Cu-CNT (preferably 0.01 wt %). If the amounts of La.sub.2O.sub.3 and SiC are less than 0.01 wt %, soldering properties are not improved. On the other hand, if the amounts thereof exceed 1.0 wt %, soldering properties may deteriorate and dewetting, which is a wetting defect, may occur.

[0057] Likewise, when Cu-CNT is added in an amount of 0.005 wt % or less, wettability is not changed, and if the amount of Cu-CNT exceeds 1.0 wt %, dewetting may occur.

[0058] The composition and conditions of the solder of the present invention are given in Table 1 below.

TABLE-US-00001 TABLE 1 Component Amount Sn—Cu 0.1~1.0 wt % Cu, remainder of Sn Sn—Ag 0.5~4.0 wt % Ag, remainder of Sn Sn—Ag—Cu 0.5~4.0 wt % Ag, 0.1~1.0 wt % Cu, remainder of Sn La.sub.2O.sub.3 0.01~1.0 wt % SiC 0.01~1.0 wt % Cu-CNT 0.005~1.0 wt %  ZrO.sub.2 0.01~1.0 wt % La.sub.2O.sub.3, SiC 0.01~0.5 wt % La.sub.2O.sub.3, 0.01~0.5 wt % SiC SiC, Cu-CNT 0.01~0.5 wt % SiC, 0.005~0.5 wt % Cu-CNT La.sub.2O.sub.3, Cu-CNT 0.01~0.5 wt % La.sub.2O.sub.3, 0.005~0.5 wt % Cu-CNT La.sub.2O.sub.3, SiC, Cu-CNT 0.01~0.5 wt % La.sub.2O.sub.3, 0.01~0.5 wt % SiC, 0.005~0.5 wt % Cu-CNT La.sub.2O.sub.3, ZrO.sub.2 0.001~1.0 wt % La.sub.2O.sub.3, 0.01~1.0 wt % ZrO.sub.2 SiC, ZrO.sub.2 0.01~1.0 wt % SiC, 0.01~1.0 wt % ZrO.sub.2 Cu-CNT, ZrO.sub.2 0.005~1.0 wt % Cu-CNT, 0.01~1.0 wt % ZrO.sub.2 La.sub.2O.sub.3, SiC, Cu-CNT, 0.001~ 0.1 wt % La.sub.2O.sub.3, and ZrO.sub.2 0.01~1.0 wt % SiC, 0.005~1.0 wt % Cu-CNT, and 0.01~1.0 wt % ZrO.sub.2 Sn Remainder SAC305 Particle size 20 to 38 μm La.sub.2O.sub.3 Particle size 30 nm SiC Particle size 70 nm Cu-CNT diameter 20 nm ZiO.sub.2 10 nm

[0059] As is apparent from Table 1, two kinds of ceramic nanopowder are mixed and added as follows.

[0060] (1) When La.sub.2O.sub.3 and SiC are mixed and added, the amount of La.sub.2O.sub.3 is 0.01 wt % to 0.5 wt % (preferably 0.02 wt %), and the amount of SiC is 0.01 wt % to 0.5 wt % (preferably 0.03 wt %).

[0061] (2) When SiC and Cu-CNT are mixed and added, the amount of SiC is 0.01 wt % to 0.5 wt % (preferably 0.04 wt %), and the amount of Cu-CNT is 0.005 wt % to 0.5 wt % (preferably 0.01 wt % Cu-CNT).

[0062] (3) When La.sub.2O.sub.3 and Cu-CNT are mixed and added, the amount of La.sub.2O.sub.3 is 0.01 wt % to 0.5 wt % (preferably 0.04 wt % La.sub.2O.sub.3), and the amount of Cu-CNT is 0.005 wt % to 0.5 wt % (preferably 0.01 wt %).

[0063] (4) When La.sub.2O.sub.3 and ZrO.sub.2 are mixed and added, the amount of La.sub.2O.sub.3 is 0.01 wt % to 0.1 wt % (preferably 0.005 wt %), and the amount of ZrO.sub.2 is 0.01 wt % to 1.0 wt % (preferably 0.5 wt %).

[0064] (5) When SiC and ZrO.sub.2 are mixed and added, the amount of SiC is 0.01 wt % to 1.0 wt % (preferably 0.05 wt %), and the amount of ZrO.sub.2 is 0.01 wt % to 1.0 wt % (preferably 0.5 wt % ZrO.sub.2).

[0065] (6) When Cu-CNT and ZrO.sub.2 are mixed and added, the amount of Cu-CNT is 0.005 wt % to 1.0 wt % (preferably 0.01 wt %), and the amount of ZrO.sub.2 is 0.01 wt % to 1.0 wt % (preferably 0.5 wt % ZrO.sub.2).

[0066] Next, the amount of three kinds of ceramic nanopowder mixed and added are as follows.

[0067] When La.sub.2O.sub.3, SiC and Cu-CNT are mixed and added, the amount of La.sub.2O.sub.3 is 0.01 wt % to 0.5 wt % (preferably 0.02 wt %), the amount of SiC is 0.01 wt % to 0.5 wt % (preferably 0.02 wt %), and the amount of Cu-CNT is 0.005 wt % to 0.5 wt % (preferably 0.01 wt %).

[0068] Also, the amount of four kinds of ceramic nanopowder mixed and added are as follows.

[0069] When four additives La.sub.2O.sub.3, SiC, Cu-CNT and ZrO.sub.2 are mixed, the amount of La.sub.2O.sub.3 is 0.001 wt % to 0.1 wt % (preferably 0.005 wt %), the amount of SiC is 0.01 wt % to 1.0 wt % (preferably 0.05 wt %), the amount of Cu-CNT is 0.005 wt % to 1.0 wt % (preferably 0.01 wt %), and the amount of ZrO.sub.2 is 0.01 wt % to 1.0 wt % (preferably 0.4 wt % ZrO.sub.2).

[0070] As the main component of the present invention, there are Sn-containing lead-free solders Sn—Cu, Sn—Ag, and Sn—Ag—Cu alloys, Sn—Cu and an additive, Sn—Ag and an additive, and Sn—Ag—Cu and an additive. The addition of an additive increases wettability and spreadability of the solder.

[0071] In the lead-free solder alloy composition of the present invention, the Sn—Cu alloy includes 0.1 wt % to 2 wt % of Cu (preferably 0.7 wt %), the Sn—Ag alloy includes 0.5 wt % to 5 wt % of Ag (preferably 3.5 wt %), and the Sn—Ag—Cu alloy includes basically 0.5 wt % to 5 wt % of Ag (preferably 3 wt %) and 0.1 wt % to 2 wt % of Cu (preferably 0.5 wt %).

[0072] This solder alloy includes, as the additive, ceramic nanoparticles or carbon nanotubes. The selected additive may include lanthanum oxide (La.sub.2O.sub.3), silicon carbide (SiC), and copper-coated carbon nanotube (Cu-CNT).

[0073] For a solder paste resulting from the alloy of the present invention, a powder alloy having a solder powder size of 20 μm to 38 μm (classified as Type 4 in the solder paste standard) is used. The nanoparticle additive includes La.sub.2O.sub.3, SiC, and Cu-CNT, each having an average diameter of about 30 nm, 70 nm, and 20 nm. The additive improves the properties of the microstructure of the solder and micronizes the size of intermetallic compounds (IMC) such as Ag.sub.3Sn, Cu.sub.6Sn.sub.5, etc. in the solder, thereby increasing the mechanical strength and wettability of the solder.

[0074] In order to improve wettability necessary for optimal soldering of the solder, the composition of the solder containing the additive has to be an optimal composition.

[0075] As an example, the optimal compositions for each solder are as follows.

[0076] The solder material is composed of a basic material and an additive, and each added material must be included at a predetermined ratio. 0.01 wt % to 1.0 wt % (preferably 0.05 wt %) of La.sub.2O.sub.3 is included, 0.01 wt % to 1.0 wt % (preferably 0.05 wt %) of SiC is included, and 0.005 wt % to 1.0 wt % (preferably 0.01 wt %) of Cu-CNT is included.

[0077] If the ratio of La.sub.2O.sub.3 or SiC is less than 0.01 wt %, the IMC of the solder material is thicker than the IMC of the Sn-3.0 wt % Ag-0.5 wt % Cu alloy solder. On the other hand, if La.sub.2O.sub.3 or SiC exceeds 1.0 wt %, the solder may become brittle, thus generating cracks in the faying surface and deteriorating the soldering properties. Likewise, if the amount of Cu-CNT is less than 0.005 wt %, there are no changes in wettability, and if the amount of Cu-CNT exceeds 1.0 wt %, liquidness fluidity may decrease and soldering properties may be deteriorated.

[0078] Preparation and testing of properties of lead-free solder alloy of the present invention—in order to evaluate the microstructure and the IMC distribution, a scanning electron microscope (SEM) was used. Also to evaluate soldering properties, spreadability, wettability, and micro hardness testing were performed.

[0079] 1. To manufacture solder alloys of the present invention, SnAgCu, SnAg, SnCu and additives were melted at 500° C. for 30 min.

[0080] 2. Observation of microstructure: Grain size, IMC size

[0081] 3. Measurement of hardness

[0082] 4. Wetting balance test: Zero cross time, at 250

[0083] 5. Spreading test: JIS-Z-3197

[0084] The preparation conditions for melting are shown in Table 2.

TABLE-US-00002 TABLE 2 Melting temperature of mixed solder powder 500° C. Duration at 500° C., heating rate 30 min, 10° C./min

[0085] <Test>

[0086] *Spreading Test

[0087] Spreading test was performed according to JIS-Z-3197 Standard. First, a copper piece having a size of 30 mm×30 mm×0.3 mm was polished and then washed with alcohol. After drying, it was heated at 150° C. for 1 hr to form a uniform oxide film.

[0088] 0.3 g of a solder powder was mixed with 0.03 g of a flux, and placed at the center of the copper piece. The piece was placed in a solder bath heated to 300° C. and melted. After a while, the solder powder, positioned at the center of the copper piece, began to melt. When the copper piece was maintained for 30 seconds in the solder bath melted at 300° C. so that the solder powder completely melts and spread, the copper piece was taken out of the solder bath and cooled at room temperature. The spreading test was conducted using the solder spread on the cooled copper plate.

[0089] *Wetting Balance Test

[0090] To measure the wettability of the alloy of the present invention, a wetting balance tester (RESCA SAT 5000) was used, and for a test sample a 99.99% oxygen-free copper plate having a size of 10 mm×1 mm×30 mm was used. The copper test sample was mechanically polished to remove a surface oxide layer and external impurities using a silicon carbide sandpaper (particle size #1200 & #2400), and then ultrasonically cleaned. Before the wetting balance test, the copper sample was slightly coated with a BGA-type flux (SENJU, Sparkle Flux WF-6063M5) and further activated for 30 sec in a solder melt. The copper sample was immersed in a melted solder (the solder of the present invention) at 250 for 5 sec at a rate of 2.5 mm/s to a depth of 2 mm. In the wetting balance test, the average zero cross time of each sample was measured.

[0091] <Description of Developed Technique>

[0092] La.sub.2O.sub.3 having an average particle size of 30 nm, SiC having an average particle size of 70 nm, and Cu-CNT having an average particle size of 20 nm were added to Sn-0.7 wt % Cu, Sn-3.5 wt % Ag and Sn-3.0 wt % Ag-0.5 wt % Cu solders. The resulting mixtures were placed in an alumina crucible and melted at 500° C. for 1 hr in a furnace heated at a heating rate of 10/min. The solders were solidified, sampled, polished, and etched, and then the microstructures thereof were observed.

[0093] -Measurement Results

[0094] Microstructure: when the microstructure of the Sn-3.0 wt % Ag-0.5 wt % Cu solder containing nanoparticles was compared with that of the Sn-3.0 wt % Ag-0.5 wt % Cu solder containing no nanoparticles, there was a clear difference in the grain size, precipitates, and matrix dispersion. The Ag.sub.3Sn intermetallic compound of the La.sub.2O.sub.3-added Sn-3.0 wt % Ag-0.5 wt % Cu became finer and dispersed well. On the other hand, the Ag.sub.3Sn intermetallic compound of the Sn-3.0 wt % Ag-0.5 wt % Cu solder containing no nanoparticles became coarse.

[0095] The micronization of the microstructure of commercially available Sn-3.0 wt % Ag-0.5 wt % Cu solder show improved physical properties thereof by the addition of ceramic nanoparticles. When Sn-3.0 wt % Ag-0.5 wt % Cu was added with ceramic nanoparticles, micro hardness was increased, but compared to the Sn-3.0 wt % Ag-0.5 wt % Cu solder containing no nanoparticles, when La.sub.2O.sub.3 was added hardness was increased by 5%, when SiC was added hardness was increased by 12.01%, and when Cu-CNT was added hardness was increased by 3.72%.

[0096] The grain size (FIG. 2) was identified through the microstructure measured using an electron microscope. The average grain size of the Sn-3.0 wt % Ag-0.5 wt % Cu solder containing no nanoparticles was about 30 μm. In contrast, the La.sub.2O.sub.3-added Sn-3.0 wt % Ag-0.5 wt % Cu solder had an average grain size of about 20 μm. The average grain size was 17.69 μm when SiC was added, and 21.07 μm μm when Cu-CNT was added.

[0097] The thickness of the intermetallic compound of the Sn-3.0 wt % Ag-0.5 wt % Cu solder containing nanopowder was also measured. The approximate measurement of the thickness of the intermetallic compound of the Sn-3.0 wt % Ag-0.5 wt % Cu solder alloys, with or without La.sub.2O.sub.3, SiC, and Cu-CNT, is illustrated in FIG. 3.

[0098] The intermetallic compound (IMC) in the Sn-3.0 wt % Ag-0.5 wt % Cu solder alloy was mostly Ag.sub.3Sn. The average IMC thickness of the Sn-3.0 wt % Ag-0.5 wt % Cu solder containing no nanopowder was 2.97 μm, whereas the IMC thickness of the La.sub.2O.sub.3-added Sn-3.0 wt % Ag-0.5 wt % Cu solder was 2.37 μm. The average IMC thickness was 1.84 μm when SiC was added, and was 2.71 μm when Cu-CNT was added. By adding ceramic nanoparticles, the Sn-3.0 wt % Ag-0.5 wt Cu was decreased in IMC thickness and increased in strength. The alloy containing fine particles further hinders dislocation motion, thus improving the mechanical properties of the alloy.

[0099] -Spreadability:

[0100] The spreadability of the solders manufactured in the present invention is shown in FIG. 4. About 0.03 g of a flux and about 0.3 g of a sample solder alloy were placed at the center of a piece of copper foil. The copper foil was placed in a molten solder bath maintained at 300° C. The sample solder immediately began to melt, and after 30 seconds, the copper foil was taken out of the solder bath and cooled at room temperature, and the spreading ratio was measured.

[0101] The spreading ratio was 77.77% in the Sn-3.0 wt % Ag-0.5 wt % Cu containing no nanopowder, was increased by 13% to 88.19% when La.sub.2O.sub.3 was added, increased by 1% to 78.7% when SiC was added, and was about 76.38% when Cu-CNT was added.

[0102] The spreading ratio was 71% in the Sn-0.7Cu alloy, 73% when La.sub.2O.sub.3 was added, and 74% when SiC was added. The spreading ratio was 75% when Cu-CNT was added to the Sn-0.7Cu alloy.

[0103] The spreading ratio was 72% in the Sn-3.5Ag alloy, 74% when La.sub.2O.sub.3 was added, 74% when SiC was added, and 75% when Cu-CNT was added.

[0104] -Wettability:

[0105] When Sn-3.0 wt % Ag-0.5 wt % Cu was added with 0.01 to 1.0 wt % of La.sub.2O.sub.3 (preferably 0.05 wt % of La.sub.2O.sub.3), the zero cross time was much lower compared to that of the pure Sn-3.0 wt % Ag-0.5 wt % Cu solder, thus exhibiting the best wettability. The wetting properties of the two alloys are shown in FIG. 6.

[0106] Here, FIG. 6 is a graph showing the hardness measurement of Sn-3.0 wt % Ag-0.5 wt % Cu, and Sn-3.0 wt % Ag-0.5 wt % Cu containing La.sub.2O.sub.3, SiC, Cu-CNT, and ZrO.sub.2.

[0107] The wetting test provides information about whether any solder may be wet through stronger bonding to a substrate or PCB in a given time. The important parameter in wettability is the zero cross time (T0). The zero cross time was 1.08 sec in the Sn-3.0 wt % Ag-0.5 wt % Cu containing no nanopowder, and was increased by 0.285 sec when La.sub.2O.sub.3 was added, by 0.25 sec when SiC was added, and by 0.78 sec when Cu-CNT was added.

[0108] -Hardness:

[0109] The hardness was 11.49 VHN in the Sn-3.0 wt % Ag-0.5 wt % Cu containing no nanopowder, 12.05 VHN when La.sub.2O.sub.3 was added to the Sn-3.0 wt % Ag-0.5 wt % Cu, 12.01 VHN when SiC was added, 11.86 VHN when Cu-CNT was added, and 13.3 VHN when ZrO.sub.2 was added, as shown in FIG. 6. The rigid and finely dispersed Ag.sub.3Sn phase is more effectively resistant to pressure and thus hardness is increased, compared to the widely formed Ag.sub.3Sn in the Sn-3.0 wt % Ag-0.5 wt % Cu containing no nanopowder.

[0110] The present invention is used as a soldering material. Specifically, it is used for a solder paste, a solder ball, a solder bar, a solder wire, etc., and in soldering electronic products using them. It is becoming smaller in order to satisfy the requirements of modern electronic devices such as high integration, low power or portability, size, and operating voltage. One critical issue is wettability, spreadability, and strength of the soldering part of the electronic devices. Hence, the demand for a solder having improved wettability and micronized Ag.sub.3Sn is increasing, and the solder of the present invention may be employed to solve such problems.

[0111] FIG. 7 is a block diagram showing the process of preparing a lead-free solder alloy according to the present invention.

[0112] The method of preparing the lead-free solder alloy according to the present invention includes a step of mixing a solder powder (S100), a step of melting the mixed solder powder (S110) and a step of adding an additive (S120).

[0113] In the step of mixing the solder powder (S100), at least one solder powder selected from among Sn—Cu, Sn—Ag, and Sn—Cu—Ag base is mixed.

[0114] In the step of adding the additive (S120), the melted solder powder is added with an additive. The additive may be at least one ceramic powder selected from among La.sub.2O.sub.3, SiC, Cu-coated CNT (Cu-CNT), and ZrO.sub.2. The contents of the ceramic powder are 0.01 wt % to 1.0 wt % of La.sub.203, 0.01 wt % to 1.0 wt % of SiC, and 0.005 wt % to 1.0 wt % of Cu-CNT.

[0115] FIG. 8 is a block diagram showing the process of preparing a lead-free solder paste using the lead-free solder alloy composition of the present invention.

[0116] The method of preparing a lead-free solder paste using the lead-free solder alloy composition of the present invention includes a step of mixing a powder (S200), a step of mixing a flux with the powder (S210), a step of melting the mixed solder (S220), and a step of analyzing experimental properties (S230).

[0117] The step of mixing the powder (S200) is a step of mixing powder in a ball mill, and is performed with SAC (Type 4, a size of 20 μm to 38 μm), La.sub.2O.sub.3 (30 nm) and mixing conditions (200 rpm, 1 hr).

[0118] The step of mixing the flux with the powder (S210) is a step of mixing the flux and the powder at an appropriate ratio, and the mixing ratio of the solder to the flux is 9:1.

[0119] The step of melting the mixed solder (S220) is a step of melting the mixed solder, which is maintained at 500° C. for 30 min and the heating rate is 5° C./min.

[0120] The step of analyzing the experimental properties (S230) includes measuring a fine grain structure of Ag.sub.3Sn or Cu.sub.6Sn.sub.5 (S232), measuring spreadability (S234), measuring wettability (S236), and measuring micro hardness (S238).

[0121] Although the embodiments of the present invention have been disclosed with reference to limited embodiments and the drawings, the present invention is not limited to those embodiments, and for those skilled in the art various modifications and alterations are possible from the above description.

[0122] Therefore, the scope of the present invention should not be confined to the disclosed embodiments, and should be defined by the accompanying claims and equivalents thereto.

INDUSTRIAL APPLICABILITY

[0123] The present invention pertains to a lead-free solder alloy composition and a method of preparing a lead-free solder alloy, wherein the lead-free solder alloy composition of the present invention comprises a ceramic powder added to a lead-free solder of Sn-(0.1 to 2) wt % Cu, Sn-(0.5 to 5) wt % Ag, or Sn-(0.1 to 2) wt % Cu-(0.5 to 5) wt % Ag.

[0124] According to the present invention, a novel lead-free solder alloy, as a replacement for a conventional lead-free solder, can be provided, thus exhibiting superior spreadability, wettability, and mechanical properties than conventional solders.