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Quaternary Pb-free solder composition incorporating Sn-Ag-Cu-In

Imported: 10 Mar '17 | Published: 27 Nov '08

Jong-Hyun Lee, Chang-Woo Lee, Jeong-Han Kim

USPTO - Utility Patents

Abstract

Provided is a quaternary Pb-free solder composition incorporating SnAgCuIn, which can prevent a cost increase and sufficiently ensure proccessability and mechanical property as a solder material. To this end, indium (In) with appropriate amount is added into the Pb-free solder composition, and the addition amount of Ag is optimized, thus preventing a decrease in wettability caused by a decrease in the amount of Ag and improving resistance to a thermal cycling and a mechanical impact. The quaternary Pb-free solder composition includes silver (Ag) of about 0.3 wt. % or more, and less than about 2.5 wt. %, copper (Cu) of about 0.2 wt. % or more, and less than about 2.0 wt. %, indium (In) of about 0.2 wt. % or more, and less than about 1.0 wt. % or less, and a balance of tin (Sn).

Description

CROSS-REFERENCE(S) TO RELATED APPLICATIONS

The present invention claims priority of Korean patent application number 10-2007-0050905, filed on May 25, 2007, which is incorporated by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention relates to a lead (Pb)-free solder composition, and more particularly, to a quaternary Pb-free solder composition incorporating tin-silver-copper-indium (SnAgCuIn), which can reduce the amount of silver by using indium.

A SnAgCu-based composition is most popularly used as a Pb-free solder composition at present, and its representative composition may be expressed as Sn-3.0Ag-0.5Cu. To improve anti-oxidation properties of such a Pb-free solder composition, phosphor (P), germanium (Ge), gallium (Ga), aluminum (Al), silicon (Si), or the like may be added at a concentration of several tens to several thousands of ppm. Furthermore, to enhance mechanical properties and interfacial reaction properties, nickel (Ni), cobalt (Co), iron (Fe), bismuth (Bi), gold (Au), platinum (Pt), lead (Pb), manganese (Mn), vanadium (V), titanium (Ti), chromium (Cr), niobium (Nb), palladium (Pd), antimony (Sb), magnesium (Mg), tantalum (Ta), cadmium (Cd), rare earth metal, or the like, of which each concentration is in the range of several tens to several thousands of ppm, may be added into the Pb-free composition.

However, as there are ongoing demands and efforts to reduce fabrication cost in packaging electronic devices, several attempts are being made to reduce the amount of silver (Ag) because Ag is the most expensive among additive elements. For example, a Sn-2.5Ag-0.5Cu or Sn-1.0Ag-0.5Cu composition is applied to the Pb-free solder. Further, a Sn-0.3Ag-0.5Cu composition has been recently suggested and its properties are being analyzed whether it is suitable for the Pb-free solder.

Variations in metallurgical and mechanical properties of a SnAgCu based solder according to the amount of Ag are summarized as followings.

1) As the addition amount of Ag decreases, a difference between a liquidus temperature and a solidus temperature increases, resulting in an increase of a pasty range or mush zone.

2) As the addition amount of Ag decreases, wettability decreases due to the increase of the pasty range or mush zone.

3) As the addition amount of Ag decreases, the strength and creep resistance of the alloy decrease.

4) As the amount of Ag decreases, the fracture speed of a solder joint according to a thermal cycling test increases because the strength and creep resistance of the alloy decrease.

5) As the amount of Ag decreases, the elongation of an alloy increases, and the fracture speed of the solder joint according to a mechanical impact test decreases.

Herein, the case 1) represents a variation in metallurgical properties depending on the amount of Ag in the solder. Even in the case of reducing the amount of Ag, therefore, the appropriate addition amount of Ag should be determined. Moreover, although the amount of Ag is reduced, the solder composition should have wettability similar to that of the typical Sn-3.0Ag-0.5Cu composition in order for this composition to be used as a solder material with low fabrication cost.

The cases 4) and 5) represent opposite characteristics according as the amount of Ag decreases in the Pb-free solder. Hence, the appropriate addition amount of Ag should be determined in consideration of these opposite characteristics. In addition to the appropriate amount of Ag, mechanical properties of the Pb-free solder composition should also be improved for example, by adding alloy metals, which allows the solder composition to have resistance to both a thermal cycling and a mechanical impact. Accordingly, an ideal solder composition with high reliability can be achieved and further fabrication cost can be reduced through a decrease in the addition amount of Ag.

However, such a Pb-free solder composition satisfying the above conditions has not been developed yet.

SUMMARY OF THE INVENTION

Embodiments of the present invention are directed to providing a quaternary lead (Pb)-free solder composition incorporating tin-silver-copper-indium (SnAgCuIn), which can prevent a cost increase and sufficiently ensure proccessability and mechanical property as a solder material. To this end, indium (In) with appropriate amount is added into the Pb-free solder composition, and the addition amount of Ag is optimized, thus preventing a decrease in wettability caused by a decrease in the amount of Ag and improving resistance to a thermal cycling and a mechanical impact.

In accordance with an aspect of the present invention, there is provided a quaternary Pb-free solder composition incorporating tin-silver-copper-indium, including: silver (Ag) of about 0.3 wt. % or more, and less than about 2.5 wt. %; copper (Cu) of about 0.2 wt. % or more, and less than about 2.0 wt. %; indium (In) of about 0.2 wt. % or more, and less than about 1.0 wt. %; and a balance of tin (Sn).

In the Pb-free solder composition of the present invention, the amount of Ag is reduced to save fabrication cost. Therefore, to improve reliability on a thermal cycling and a mechanical impact and also prevent a decrease in wettability caused by the decrease in the addition amount of Ag, indium (In) is added into the Pb-free solder composition. Accordingly, it is possible to provide a high-quality Pb-free solder composition with low price.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Hereinafter, a quaternary lead (Pb)-free composition incorporating tin-silver-copper-indium (SnAgCuIn) in accordance with the present invention will be described in detail with reference to the accompanying drawings. Herebelow, specific descriptions for a related well-known function or construction will be omitted when it is deemed that they make the gist of the present invention vague unnecessarily.

In a Pb-free solder composition of the present invention, the weight percent of Ag is less than about 2.5 wt. % but not less than about 0.3 wt. %. If the weight percent of Ag is less than 0.3 wt. %, a liquidus temperature hardly drops, leading to an increase in a melting point of a solder and a packaging process temperature. On the contrary, if the weight percent of Ag is 2.5 wt. % or more, the fabrication cost increases unfavorably. Consequently, the weight percent of Ag should be less than about 2.5 wt. % but not less than about 0.3 wt. %, preferably about 1.2 wt. %.

The Pb-free solder composition of the present invention includes Cu of which weight percent is less than about 2.0 wt. % but not less than about 0.2 wt. %. If the weight percent of Cu is less than 0.2 wt. %, the liquidus temperature drops little and a fraction of Cu6Sn5 phase is extremely small, which reduces the strength of a solder alloy excessively. In contrast, if the weight percent of Cu is 2.0 wt. % or more, a difference between the liquidus temperature and solidus temperature increases and thus a pasty range or mush zone increases. This leads to an increase in a fraction of Cu6Sn5 phase, thus intensifying the mechanical properties of the solder alloy in excess and increasing a growth rate of an interfacial reaction layer. Consequently, the weight percent of Cu should be less than about 2.0 wt. % but not less than about 0.2 wt. %, preferably about 0.5 wt. %.

The Pb-free solder composition of the present invention further includes In of which weight percent is less than about 1.0 wt. % but not less than about 0.2 wt. %. If the weight percent of In is less than 0.2 wt. %, wettability and mechanical properties are not enhanced substantially. If the weight percent of In is 1.0 wt. % or more, wettability and mechanical properties are not enhanced in proportion to the addition amount of In, but a price of the solder alloy increases drastically. Therefore, the weight percent of In should be less than about 1.0 wt. % but not be less than about 0.2 wt. %, preferably about 0.4 wt. %.

In accordance with a desirable ratio of each additive element, most preferable Pb-free solder composition is Sn-1.2Ag-0.5Cu-0.4In. The most preferable composition of Sn-1.2Ag-0.5Cu-0.4In, other research compositions and conventional compositions of Sn-3.0Ag-0.5Cu, Sn-1.0Ag-0.5Cu and Sn-1.2Ag-0.5Cu-0.05Ni are respectively tested under the same conditions and then evaluated, of which results are shown in FIGS. 1 to 11.

FIGS. 1 and 2 are graphs illustrating endothermic peaks of solder compositions in a heating state. Specifically, FIGS. 1 and 2 show endothermic peaks observed by the use of a differential scanning calorimeter (DSC) when a solder composition (about 8 mg) is heated up at a heating rate of 10 C./min and at a nitrogen flow rate of 50 ml/min. As shown in FIG. 1, the Sn-3.0Ag-0.5Cu composition has an endothermic peak at about 217 C. to about 218 C., which is substantially equal to a melting point of this alloy. On the contrary, the Sn-1.0Ag-0.5Cu composition has a first endothermic peak at about 218 C. to about 219 C. and a second endothermic peak at about 226 C., which are respectively observed as a liquidus temperature and a solidus temperature. Therefore, it can be observed that a pasty range or mush zone significantly increases. The Sn-1.2Ag-0.5Cu-0.05Ni composition has a first endothermic peak at about 219 C. to about 220 C. and a second endothermic peak at about 225 C. to about 226 C., which are respectively observed as a liquidus temperature and a solidus temperature. Therefore, it can be observed that a pasty range or a mush zone also significantly increases.

As shown in FIG. 2, the Sn-1.0Ag-0.5Cu-1.0In composition has a first endothermic peak at about 216 C. and a second endothermic peak at about 224 C. to about 225 C., which are respectively observed as a liquidus temperature and a solidus temperature. In this case, it can be observed that a pasty range or mush zone also increases considerably but the liquidus temperature and the solidus temperature are relatively lowered in totality. Such a transition of the liquidus and solidus lines to a low temperature provides an excellent effect in solder wettability at a low temperature. The Sn-1.0Ag-0.5Cu-0.5In composition has a first endothermic peak at about 217 C. and a second endothermic peak at about 225 C., which are respectively observed as a liquidus temperature and a solidus temperature. In this case, it can be observed that a pasty range or a mush zone also increases considerably but the liquidus temperature and the solidus temperature are relatively lowered in totality.

The Sn-1.2Ag-0.5Cu-0.80.4In composition has a first endothermic peak at about 217 C. to about 218 C. and a second endothermic peak at about 224 C. to about 225 C., which are respectively observed as a liquidus temperature and a solidus temperature. In this case, it can be observed that a pasty range or a mush zone also increases considerably but the liquidus temperature and the solidus temperature are relatively lowered in totality. The Sn-1.2Ag-0.5Cu-0.2In composition has a first endothermic peak at about 219 C. to about 220 C. and a second endothermic peak at about 226 C., which are respectively observed as a liquidus temperature and a solidus temperature. In this case, it can be observed that a pasty range or a mush zone also increases considerably but the liquidus temperature and the solidus temperature are not lowered in comparison with the Sn-1.0Ag-0.5Cu composition. From this result, it can be appreciated that the solder wettability of the Sn-1.2Ag-0.5Cu-0.2In composition in a low temperature range is not much improved compared to that of the Sn-1.0Ag-0.5Cu composition.

FIGS. 3 and 4 are graphs illustrating first exothermic peaks of solder compositions in a cooling state after being melted. Specifically, FIGS. 3 and 4 show exothermic peaks observed through a DSC when a solder composition (about 8 mg) is cooled down after it is heated up to 250 C. at a heating rate of 10 C./min and at a nitrogen flow rate of 50 ml/min. As shown in FIG. 3, the Sn-3.0Ag-0.5Cu composition has a first exothermic peak at about 194 C., which means an actual solidification temperature of this alloy. Metallurgically, a difference between a melting temperature and an actual solidification temperature of an alloy, that is, a temperature difference of about 23 C. to 24 C. in this case, is called undercooling or supercooling. The degree of undercooling increases according to the amount of Ag in the alloy. For instance, the Sn-1.0Ag-0.5Cu composition has a first exothermic peak at about 188 C., which proves that the degree of undercooling is increased. In contrast, the Sn-1.2Ag-0.5Cu-0.05Ni composition has a first exothermic peak at about 206 C. to about 207 C. From this result, it can be appreciated that small addition amount of Ni reduces the degree of undercooling drastically.

The results obtained by adding In into compositions are shown in FIG. 4. From FIG. 4, it can be observed that the Sn-1.0Ag-0.5Cu-1.0In composition has a first exothermic peak at about 200 C. and the Sn-1.0Ag-0.5Cu-0.5In composition has a first exothermic peak at about 190 C. to about 191 C. Therefore, the addition of In also reduces the degree of undercooling greatly. Further, it can be observed that the Sn-1.2Ag-0.5Cu-0.8In composition has a first exothermic peak at about 192 C. to about 193 C., the Sn-1.2Ag-0.5Cu-0.6In composition has a first exothermic peak at about 197 C. to about 198 C., the Sn-1.2Ag-0.5Cu-0.4In composition has a first exothermic peak at about 200 C. to about 201 C., and the Sn-1.2Ag-0.5Cu-0.2In composition has a first exothermic peak at about 202 C. to about 203 C.

FIGS. 5 and 6 are graphs illustrating a zero cross time value versus a soldering temperature. In one-time wettability test, a zero cross time value, a wetting force at 2 seconds and a final wetting force are measured at once, and following results are mean values obtained from test results of 10 or more times. A specimen used in the wettability test was a Cu piece having 3 mm in width and 10 mm in length. A water-soluble type flux of SENJU Company was coated on a surface of the Cu piece and it was then charged into a melted solder, wherein a charging depth was 2 mm. Each of a charging speed and a separating speed of the Cu piece was 5 mm/sec. As shown in FIG. 5, the Sn-1.2Ag-0.5Cu-0.05Ni composition and the Sn-1.0Ag-0.5Cu composition are much greater in zero cross time value than the Sn-3.0Ag-0.5Cu composition. In particular, it can be observed that the zero cross time value is more increased in a low temperature range of about 230-240 C. On the contrary, if In is added into the composition as illustrated in FIG. 6, it can be observed that the zero cross time value is significantly reduced and the zero cross time value is more effectively reduced in a low temperature range of 230-240 C. Especially, the representative composition of the present invention, i.e., the Sn-1.2Ag-0.5Cu-0.4In composition, has the zero cross time value similar to or better than that of the Sn-3.0Ag-0.5Cu composition. Therefore, it can be confirmed that the Sn-1.2Ag-0.5Cu-0.4In composition of the present invention has very excellent wettability as a solder material.

FIGS. 7 and 8 are graphs illustrating a wetting force at 2 seconds versus a soldering temperature. As shown in FIG. 7, the Sn-1.0Ag-0.5Cu composition and the Sn-1.2Ag-0.5Cu-0.05Ni composition have wetting forces at 2 seconds, which are lower than that of the Sn-3.0Ag-0.5Cu composition. Particularly, the wetting forces at 2 seconds of the Sn-1.0Ag-0.5Cu composition and the Sn-1.2Ag-0.5Cu-0.05Ni composition are significantly decreased in a low temperature range of 230-240 C. In contrast, if In is added into compositions as shown in FIG. 8, the wetting force at 2 seconds is remarkably increased, and the wetting force at 2 seconds in a low temperature of 230-240 C. is more effectively increased. In especial, it can be confirmed that the representative composition of the present invention, i.e., the Sn-1.2Ag-0.5Cu-0.4In composition, has the wetting force at 2 seconds which is similar to or better than that of the Sn-3.0Ag-0.5Cu composition.

From the results above, the composition of the present invention has excellent wettability in spite of very low price, and thus it is very suitable for a soldering material. Accordingly, the Pb-free solder composition of the present invention is used in fabricating a solder paste, a solder ball, a solder bar, a solder wire, a solder bump, a solder foil, a solder powder, and a solder perform. Herein, the solder perform may include a solder pellet, a solder granule, a solder ribbon, a solder washer, a solder ring and a solder disk.

FIGS. 9 and 10 are graphs illustrating a final wetting force versus a soldering temperature. As shown in FIG. 9, the Sn-1.0Ag-0.5Cu composition and the Sn-1.2Ag-0.5Cu-0.05Ni composition have final wetting forces which are lower than that of the Sn-3.0Ag-0.5Cu composition. Particularly, the final wetting force is significantly decreased in a low temperature range of 230-240 C. In contrast, the addition of In brings interesting results as shown in FIG. 10. That is, when the addition amount of In is high, e.g., 0.8 wt. %, in the Sn-1.2Ag-0.5Cu-xIn composition, the final wetting force is enhanced little due to a low surface tension of melted indium; and when the addition amount of In is low, e.g., 0.2 wt. %, wettability is improved little so that the final wetting force is not improved. In contrast, it can be confirmed that the representative composition of the present invention, i.e., Sn-1.2Ag-0.5Cu-0.4In composition has a final wetting force that is similar to or somewhat worse than that of the Sn-3.0Ag-0.5Cu composition. Particularly, it can be observed that the Sn-1.2Ag-0.5Cu-0.4In composition has a more excellent final wetting force in a low temperature range of 230-240 C. than other compositions with small amount of Ag.

FIG. 11 is a graph illustrating test results obtained from tensile specimens having conventional solder compositions of Sn-3.0Ag-0.5Cu, Sn-1.0Ag-0.5Cu and Sn-1.2Ag-0.5Cu-0.05Ni. The tensile specimen was prepared as a proportional specimen based on the Korean Standard (KS) 13A, of which a thickness was 2 mm and a length is 27 mm. A tensile test is performed in a room temperature at a tensile test speed of 7.8 mm/min. As shown in FIG. 11, the Sn-3.0Ag-0.5Cu composition has a high strength but a low elongation. From this, when the Sn-3.0Ag-0.5Cu composition is used as a solder joint material, it is expected that the composition is excellent in resistance to a thermal cycling but poor in resistance to a mechanical impact. On the contrary, the elongation of the Sn-1.0Ag-0.5Cu composition is increased slightly but its strength is too small. Therefore, the Sn-1.0Ag-0.5Cu composition is expected to be better in resistance to a mechanical impact but poorer in resistance to a thermal cycling than the Sn-3.0Ag-0.5Cu composition. The Sn-1.2Ag-0.5Cu-0.05Ni composition exhibits medium characteristics between the Sn-3.0Ag-0.5Cu composition and the Sn-1.0Ag-0.5Cu composition.

FIG. 12 is a graph illustrating test results obtained from tensile specimens having solder compositions of Sn-1.2Ag-0.5Cu-0.4In, Sn-1.2Ag-0.5Cu-0.2In, Sn-1.2Ag-0.5Cu-0.6In, Sn-1.2Ag-0.5Cu-0.8In and Sn-1.0Ag-0.5Cu-1.0In in accordance with the present invention. From FIG. 12, it can be observed that the Sn-1.2Ag-0.5Cu-0.4In composition has a greater strength and a higher elongation than a similar composition, e.g., Sn-1.0Ag-0.5Cu composition. That is, the toughness of the Sn-1.2Ag-0.5Cu-0.4In composition is improved in comparison with the Sn-1.0Ag-0.5Cu composition. From this result, it is expected that the Sn-1.2Ag-0.5Cu-0.4In composition has the most excellent resistance to a mechanical impact and also has good resistance to a thermal cycling. Therefore, the Sn-1.2Ag-0.5Cu-0.4In composition is suitably used as a bonding material for electronics inside automobiles and mobile products which are subject to a mechanical shock or vibration. In the Sn-1.2Ag-0.5Cu-0.2In composition, the intensification of a metal due to the addition of In decreases so that its strength is lowered. In the Sn-1.2Ag-0.5Cu-0.6In composition and the Sn-1.2Ag-0.5Cu-0.8In composition, the elongation gradually decreases as the addition amount of In increases. The Sn-1.0Ag-0.5Cu-1.0In composition, however, does not exhibit good strength in spite of great addition amount of In.

Another quaternary Pb-free solder composition of the present invention, i.e., Sn-0.3Ag-0.7Cu-0.2In composition, is compared with the conventional Sn-3.0Ag-0.5Cu composition, Sn-1.0Ag-0.5Cu composition and Sn-0.3Ag-0.7Cu composition through the same experimental procedure as described above. Results of wetting properties are illustrated in FIGS. 13 to 15.

FIG. 13 is a graph illustrating a zero cross time value versus a soldering temperature, FIG. 14 is a graph illustrating a wetting force at 2 seconds versus a soldering temperature, and FIG. 15 is a graph illustrating a final wetting force versus a soldering temperature. As shown in FIGS. 13 to 15, the Sn-0.3Ag-0.7Cu-0.2In composition with small amount of In is excellent in the above-described wetting properties at a temperature above 240 C., compared to the Sn-0.3Ag-0.7Cu composition. That is, the Sn-0.3Ag-0.7Cu-0.2In composition exhibits similar wetting properties to those of the Sn-1.0Ag-0.5Cu composition. From this result, it can be also confirmed that the Pb-free solder composition with an appropriate amount of indium in accordance with the present invention can minimize cost increase and prevent a decrease in wettability caused by a reduction in Ag content.

To improve anti-oxidation properties of the quaternary Pb-free solder composition incorporating SnAgCuIn, one or more elements selected from phosphor (P), germanium (Ge), gallium (Ga), aluminum (Al) and silicon (Si) may be added into the quaternary Pb-free solder composition in a weight percent range of about 0.001 wt. % to about 1 wt. %.

In addition, one or more elements selected from zinc (Zn) and bismuth (Bi) may be added into the quaternary Pb-free solder composition in a weight percent range of about 0.001 wt. % to about 2 wt. % to improve interfacial reaction properties and drop a melting point of the quaternary Pb-free solder composition incorporating SnAgCuIn.

Further, to improve mechanical properties and interfacial reaction properties of the quaternary Pb-free solder composition incorporating SnAgCuIn, one or more elements, which are selected from nickel (Ni), cobalt (Co), gold (Au), platinum (Pt), lead (Pb), manganese (Mn), vanadium (V), titanium (Ti), chromium (Cr), niobium (Nb), palladium (Pd), antimony (Sb), magnesium (Mg), tantalum (Ta), cadmium (Cd) and rare earth metals, may be added into the quaternary Pb-free solder composition in a weight percent range of about 0.001 wt. % to about 1 wt. %.

The reason that additional remarks as above are made is to clarify that the quaternary Pb-free solder composition incorporating SnAgCuIn with other element(s) added, which are intended to avoid the patent of the quaternary Pb-free solder composition in accordance with the present invention, also falls within the technical idea of the present invention

As described above, in accordance with the present invention, by reducing the amount of Ag but adding In, it is possible to complement the wettability due to a decrease in amount of Ag and improve resistance to both a thermal cycling and a mechanical impact. Therefore, it is possible to provide a high-quality Pb-free solder composition with low price.

While the present invention has been described with respect to the specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.

Claims

1. A quaternary lead (Pb)-free solder composition incorporating tin-silver-copper-indium, comprising:
silver (Ag) of about 0.3 wt. % or more, and less than about 2.5 wt. %;
copper (Cu) of about 0.2 wt. % or more, and less than about 2.0 wt. %;
indium (In) of about 0.2 wt. % or more, and less than about 1.0 wt. %; and
a balance of tin (Sn).
silver (Ag) of about 0.3 wt. % or more, and less than about 2.5 wt. %;
copper (Cu) of about 0.2 wt. % or more, and less than about 2.0 wt. %;
indium (In) of about 0.2 wt. % or more, and less than about 1.0 wt. %; and
a balance of tin (Sn).
2. The composition as recited in claim 1, wherein one or more elements selected from phosphor (P), germanium (Ge), gallium (Ga), aluminum (Al) and silicon (Si) are added into the quaternary Pb-free solder composition in a weight percent range of about 0.001 wt. % to about 1 wt. % to improve anti-oxidation properties of the Pb-free solder composition.
3. The composition as recited in claim 1, wherein one or more elements selected from zinc (Zn) and bismuth (Bi) are added into the quaternary Pb-free solder composition in a weight percent range of about 0.001 wt. % to about 2 wt. % to improve interfacial reaction properties and drop a melting point of the Pb-free solder composition.
4. The composition as recited in claim 1, wherein one or more elements, which are selected from nickel (Ni), cobalt (Co), gold (Au), platinum (Pt), lead (Pb), manganese (Mn), vanadium (V), titanium (Ti), chromium (Cr), niobium (Nb), palladium (Pd), antimony (Sb), magnesium (Mg), tantalum (Ta), cadmium (Cd) and rare earth metals, are added into the quaternary Pb-free solder composition in a weight percent range of about 0.001 wt. % to about 1 wt. %.