SILVER-FREE, LEAD-FREE SOLDER ALLOYS

Abstract

A silver-free, lead-free solder alloy containing 0.001–0.800% by weight of copper, 0.080–0.120% by weight of bismuth, 0.030–0.050% by weight of nickel, 0.008–0.012% by weight of phosphorus, and tin, along with unavoidable impurities. The solder alloy may be in the form of a bar, stick, solid core or flux-cored wire, foil or strip, or a powder or paste, or solder spheres for use in ball-grid arrangements or chip-scale packages, or other preformed solder parts. The solder alloy can be used to create a solder joint between an electronic component and a pad of an electronic substrate.

Description

SILVER-FREE, LEAD-FREE SOLDER ALLOYS FIELD OF INVENTION The present invention relates to methods of joining electrical or mechanical components, and more particularly to methods of fixing electronic components and associated devices on circuit boards and other electronic substrates. BACKGROUND OF THE INVENTION In 2006, electronic assemblers were required to adopt lead-free solder alloys. An example of an existing lead-free solder alloy can be found in U.S. Patent Application Publication No. 2008 / 0292492 Al, which is incorporated herein by reference in its entirety for any purpose. However, the solder alloy described in this reference contains silver. Most popular lead-free alloys contain up to 4% silver by weight. The market price of silver has steadily increased in recent years, resulting in a substantial increase in soldering costs for many electronic assemblers. Numerous studies have been conducted on the acceptability of low-silver (less than 1% by weight) and silver-free alloys for certain types of electronic assemblies.Furthermore, many assemblers have successfully used silver-free alloys for several years, gaining acceptance within the industry. As a result of the high cost of silver-containing alloys and the industry's growing experience with silver-free alloys, there is increasing market demand for these types of alloys. BRIEF DESCRIPTION OF THE INVENTION One aspect of the present invention relates to a silver-free, lead-free solder alloy comprising 0.001 to 0.800% by weight of copper, 0.080-0.120% by weight of bismuth, 0.030 to 0.050% by weight of nickel, 0.008-0.012% by weight of phosphorus, and the balance of tin, together with unavoidable impurities. The solder alloy may further comprise one or more of the following: 0.10% by weight maximum of silver; 0.05% by weight maximum of lead; 0.05% by weight maximum of antimony; 0.030% by weight maximum of arsenic; 0.001% by weight maximum of cadmium; 0.001% maximum of zinc; 0.020% by weight maximum of iron; 0.001% by weight maximum of aluminum. 0.050% by weight of indium maximum; 0.050% by weight of gold maximum; 0.10% by weight of chromium maximum; and 0.10% by weight of mercury maximum. In another embodiment, copper is 0.600 to 0.800% by weight of the solder alloy.The solder alloy may be in the form of one of a bar, stick, solid core or flux wire, foil or strip, or a powder or paste, or solder spheres for use in ball grid arrangements or chip scale packages, or other preformed solder parts. Welding alloy can be used to manufacture a weld joint. LLfrZLn / LZnZ / E / Yli A method for forming a weld joint is also described. Another aspect of the present invention relates to a silver-free, lead-free solder alloy consisting of 0.001 to 0.800% by weight of copper, 0.001 to 0.050% by weight of nickel, 0.001 to 0.012% by weight of phosphorus, 0.001-0.008% by weight of gallium, and the balance of tin, along with unavoidable impurities. In one embodiment, the copper is 0.600 to 0.800% by weight of the solder alloy. DETAILED DESCRIPTION OF THE INVENTION There are several requirements for a solder alloy to be suitable for use in wave soldering, reflow soldering, hot air leveling processes, ball grid arrangements, and other assembly processes. For example, the alloy must exhibit good wetting characteristics with a variety of substrate materials such as copper, nickel, and nickel-phosphorus (non-electrolytic nickel). Such substrates can be coated to improve wetting, for example, by using tin alloys, gold, or organic plating (OSP). Good wetting also improves the ability of the molten solder to flow into a capillary space and to climb the walls of a hole through plating on a printed circuit board, thus achieving good gap filling. Solder alloys tend to dissolve the substrate and form an intermetallic compound at the interface with the substrate. For example, the tin in the solder alloy can react with the substrate at the interface to form a layer of intermetallic compound. If the substrate is copper, then a CuSns layer will form. Such a layer is typically a fraction of a micrometer to a few micrometers thick. At the interface between this layer and the copper substrate, a CuSn intermetallic compound may be present. Intermetallic interface layers tend to grow during aging, especially when service is performed at higher temperatures, and thicker intermetallic layers, along with any voids that may have proliferated, can further contribute to premature failure of a stressed joint. Other factors include: (i) the presence of intermetallic compounds in the alloy itself, resulting in improved mechanical properties; (ii) oxidation resistance, which is important in solder balls where deterioration during storage or repeated reflows can lead to less than ideal solder performance; (iii) the slag removal rate; and (iv) alloy stability. These latter considerations can be important for applications where the alloy is held in a tank or bath for extended periods. As mentioned previously, many lead-free solder alloys include silver, which adds cost to the solder alloy. The present The invention LLfrZLn / LZnZ / E / Yli aims to address at least some of the problems associated with the prior art and provide an improved solder alloy that reduces or eliminates silver. Accordingly, the present invention provides an alloy suitable for use in a wave soldering process, a reflow soldering process, a hot air leveling process, a ball grid arrangement, and a chip scale pack. The alloy comprises 0.001 to 0.800 wt% copper, 0.080 to 0.120 wt% bismuth, 0.030 to 0.050 wt% nickel, 0.008 to 0.012 wt% phosphorus, and tin in balance. As shown in the table provided for Example 1 below, the solder alloy may also include trace amounts of silver (0.10% by weight maximum), lead (0.05% by weight maximum), antimony (0.05% by weight maximum), arsenic (0.030% by weight maximum), cadmium (0.001% by weight maximum), zinc (0.001% maximum), iron (0.0.20% by weight maximum), aluminum (0.001% by weight maximum), indium (0.050% by weight maximum), gold (0.050% by weight maximum), chromium (0.10% by weight maximum), and mercury (0.10% by weight maximum). The solder alloy preferably comprises 0.001–0.800% by weight of copper. Copper forms a eutectic with tin, lowering the melting point and increasing the alloy's strength. A copper content in the hypereutectic range increases the liquidus temperature but further improves the alloy's strength. Copper also lowers the melting point and improves the wetting properties of the solder to copper and other substrates. The solder alloy preferably comprises 0.080 to 0.120 wt% bismuth. The presence of bismuth strengthens the alloy through its presence in solid solution at low concentrations and as bismuth-rich particles or bismuth-containing intermetallics at higher concentrations. Bismuth lowers the melting point and improves the mechanical properties of the solder alloy for the applications in question, namely wave soldering, reflow soldering, hot air leveling, ball grid arrangements, and chip scale packages. The bismuth content also contributes to reducing the growth rate of copper-tin intermetallics at the interface, leading to improved mechanical properties of solder joints made using the alloy. For this reason, the alloy according to the present invention preferably comprises 0.080–0.120 wt% bismuth. Nickel can act as a modifier of intermetallic compound growth and a grain refiner. For example, although not intended to be bound by theory, metallic nickel is believed to interact with tin and copper substitutes to form a CuNiSn intermetallic. Nickel can also form an intermetallic with bismuth. The presence of nickel in alloying has been found to have an advantageous effect, as it reduces the dissolution rate of thin copper layers on printed circuit boards. In some cases, where large areas of bare copper are to be wetted by the solder, this attribute is useful for maintaining the stability of the solder composition and preventing excessive copper buildup. This is particularly valuable in, for example, hot air solder leveling, as it reduces potential problems caused by changes in the solder pool composition (e.g., an increase in copper level). For these reasons, the alloy according to the present invention preferably comprises at least 0.030% by weight of nickel, for example, 0.030 to 0.050% by weight of nickel. Phosphorus can reduce the volume of slag formed on top of an open welding tank, and is therefore a valuable addition in, for example, wave welding baths. In some embodiments, germanium (Ge) can be replaced by phosphorus. In one embodiment, the welding alloy includes 0.008–0.012% by weight of phosphorus. The alloy will typically comprise at least 90% tin by weight, preferably 94 to 99.6% tin by weight, more preferably 95 to 99% tin by weight, and still more preferably 97 to 99% tin by weight. Accordingly, the present invention further provides an alloy for use in a wave soldering process, a reflow soldering process, a hot air leveling process, a ball grid arrangement, and a chip scale pack. In one embodiment, the solder alloy is particularly suitable for a wave soldering process. In another embodiment, the solder alloy comprises 0.001 to 0.800% by weight of copper, 0.001 to 0.050% by weight of nickel, 0.001 to 0.012% by weight of phosphorus, 0.001–0.008% by weight of gallium, and the balance of tin. As stated above in the previous embodiment, the solder alloy may further include trace amounts of silver (0.10% by weight maximum), lead (0.07% by weight maximum), antimony (0.10% by weight maximum), arsenic (0.030% by weight maximum), cadmium (0.002% by weight maximum), zinc (0.001% maximum), iron (0.020% by weight maximum), aluminum (0.001% by weight maximum), indium (0.050% by weight maximum), and gold (0.050% by weight maximum). The solder alloy preferably comprises 0.001–0.800% by weight of copper. As mentioned previously, copper forms a eutectic with tin, lowering the melting point and increasing the strength of the alloy.Copper further reduces the melting point and improves the wetting properties of solder to copper and other substrates. The welding alloy preferably comprises phosphorus, which acts to reduce the volume of slag formed on top of an open welding tank, and is therefore a valuable addition in, for example, wave welding baths. In some embodiments, germanium (Ge) can be replaced by phosphorus. In one embodiment, the LLfrZLn / LZnZ / E / Yli welding alloy includes 0.001 to 0.002% by weight of phosphorus. The solder alloy also includes gallium, which can be added to improve the overall appearance of a soldered joint. Gallium has a relatively low melting point, for example, approximately 30 °C, and its radius is slightly smaller than that of copper. When creating a solder joint, the rate of wettability propagation of the solder alloy is accelerated, and the joint strength is improved. Additionally, gallium reduces oxidation when wave-welded with the solder alloy. In one embodiment, the solder alloy includes 0.001–0.008 wt% gallium, and more preferably 0.005–0.008 wt% gallium. The alloy will typically comprise at least 90% by weight of tin, preferably 94 to 99.6% by weight of tin, more preferably 95 to 99% by weight of tin, and still more preferably 97 to 99% by weight of tin. Accordingly, the present invention further provides an alloy for use in a wave soldering process, a reflow soldering process, a hot air leveling process, a ball grid arrangement, and a chip scale package. The alloys according to the present invention may consist essentially of the aforementioned elements. Therefore, it will be appreciated that in addition to the mandatory elements (i.e., tin, copper, bismuth, nickel, and phosphorus), other unspecified elements may be present in the composition, provided that the essential characteristics of the composition are not materially affected by their presence. Accordingly, the present invention further provides an alloy for use in wave soldering, reflow soldering, hot air leveling, ball grid arrangement, chip scale assembly, or other procedures used for the assembly of electronic components. The present invention also provides for the use of the solder alloy composition in a ball grid arrangement and chip scale package. The present invention also provides a ball-grid arrangement joint comprising the solder alloy composition described above. The alloys according to the present disclosure are lead-free or essentially lead-free. The alloys offer environmental advantages over conventional lead-containing solder alloys. The alloys according to the present invention will typically be supplied as a bar, stick, or ingot, optionally together with a flux. The alloys may also be provided in the form of a wire, for example, a cored wire, incorporating a flux, sphere, or other pre-form, typically, though not necessarily, made LLfrZLn / LZnZ / E / Yli are produced by cutting or stamping a stick of solder material. These can be single-alloy or coated with a suitable flux as required by the soldering process. The alloys can also be supplied in powder form, or as a powder mixed with a flux to produce a solder paste. The alloys according to the present invention can be used in molten welding baths as a means of welding together two or more substrates and / or for coating a substrate. The alloys according to the present invention can be used to mechanically and electrically bond electronic components to pads on a printed circuit board. It will be appreciated that the alloys according to the present invention may contain unavoidable impurities, although, in total, they are likely not to exceed 1% by weight of the composition. Preferably, the alloys contain unavoidable impurities in an amount of no more than 0.5% by weight of the composition, no more than 0.3% by weight of the composition, and even more preferably no more than 0.1% by weight of the composition. The alloys according to the present invention are particularly suitable for applications involving wave soldering, reflow soldering, air or ball leveling, hot grid arrays, and chip scale packaging. The alloys according to the present invention may also find application in non-electronic applications such as, for example, plumbing and automotive radiators. These solder alloys have demonstrated acceptable soldering performance and reliability in laboratory tests and various field trials. The solder alloy can be sold in a variety of forms, including but not limited to particles, powder, preforms, paste, solid wire, cored wire, and solid rods, pellets, or ingots. The solder alloy can be used in a variety of electronic component soldering assembly processes, including but not limited to reflow soldering, wave soldering, plating, hand soldering, etc. In one embodiment, the welding alloy has the following alloy properties: LLfrZLn / LZnZ / E / Yli liquefaction temperature (°C) 229; solidus temperature (°C) 227; CTE 30-100 °C (pm / m°C) 23.8; CTE 100-180 °C (pm / m°C) 24.3; density (g / cm3) 7.3; impact energy (Joules) 51.2; and hardness (HV 0.2) 9.4. In one form, the welding alloy has the following mechanical properties: tensile strength (MPa) 42.0, tensile strength (MPa) 7.6, yield strength (MPa) 33.4, elongation (%) 33.1 LLfrZLn / LZnZ / E / Yli It should be noted that the silver-free, lead-free solder alloys of the embodiments of the present invention are suitable for use as a replacement for tin-silver-copper (SAC) solder alloys in lead-based solders and other low-silver SAC alloys in wave soldering, selective soldering, lead tinning, and remelting processes. The solder alloys have been designed to minimize copper dissolution compared to silver-containing alloys and also to improve the total cost of ownership. A variant of the solder alloys can be used as a replacement alloy in solder baths with high copper levels. Solder alloys can be used to improve performance characteristics such as reliability, yield, copper erosion, slag generation, and weld bead surface. As a result, solder alloys can achieve performance benefits such as a reduced total cost of ownership due to lower material costs, higher yields and low slag generation, excellent mechanical reliability, improved weldability due to rapid wetting, reduced copper coating erosion during remelting, which improves assembly reliability, greater friendliness and less aggressiveness to the solder crucible material compared to silver-containing alloys, and good performance across different soldering processes.Processes that include the welding alloys of the present invention improve the removal of weld oxides, which can reduce defects such as weld bridging. In some embodiments, in addition to the applications described herein, welding alloys can be used in other joining applications, including wire welding, tape welding, airtight sealing, lid sealing, metal-to-metal bonding, metal-to-glass bonding, general bonding, and bonding to various polymeric materials. In other embodiments, welding alloys according to the invention can find application in a variety of industries, including electronics, consumer electronics, telecommunications, hybrid electric vehicles, solar and wind power generation (including photovoltaic cells), transportation, and industrial applications. The function and advantage of these and other forms of the materials and methods described in the present invention will be better understood from the following examples, which are intended to illustrate the benefits of the described materials and methods, but do not exemplify their full scope. EXAMPLE 1 In one form, a silver-free, lead-free solder alloy includes iviA / a / zuzi / ui 1i The following components: Element Specification Tin (Sn) Balance Copper (Cu) 0.70 + / -0.10% Bismuth (Bi) 0.10 + / -0.02% Nickel (Ni) 0.04 + / -0.01% Phosphorus (P) 0.008-0.012% Silver (Ag) 0.10% maximum Lead (Pb) 0.05% maximum Antimony (Sb) 0.050% maximum Arsenic (As) 0.030% maximum Cadmium (Cd) 0.001% maximum Zinc (Zn) 0.001% maximum Iron (Fe) 0.020% maximum Aluminum (Al) 0.001% maximum Indium (In) 0.050% maximum Gold (Au) 0.050% maximum Chromium (Cr) 0.10% maximum Mercury (Hg) 0.10% maximum. As shown, the constituent parts of the solder alloy are copper (0.60 to 0.80% by weight), bismuth (0.08 to 0.12% by weight), nickel (0.3 to 0.5% by weight), and balance of tin. The solder alloy may also include phosphorus (0.008 to 0.012% by weight). EXAMPLE 2 In another embodiment, a lead-free silver solder alloy was tested to include the following constituent elements: Element Specification Tin (Sn) Balance Copper (Cu) 0.743% Bismuth (B¡) 0.0881% Nickel (Ni) 0.0384% Phosphorus (P) 0.0110% Silver (Ag) <0.0001% 9 Lead (Pb) Antimony (Sb) Arsenic (As) Cadmium (Cd) Zinc (Zn) Iron (Fe) Aluminum (Al) Indium (In) Gold (Au) 0.0320% 0.0110% 0.0156% 0.0003% 0.0010% 0.0040% 0.0006% 0.0023% 0.0002% IVIA / a / ZUZl / vi 11 As shown, the constituent parts of the solder alloy are copper (0.743 wt%), bismuth (0.0881 wt%), nickel (0.0384 wt%), and tin. The solder alloy also includes phosphorus in an amount of 0.0110 wt%. The welding alloy composition in this example is particularly useful for initial filling of a welding bath. In this example, the welding alloy contains copper at a concentration of 0.743% by weight. EXAMPLE 3 In another embodiment, a lead-free silver solder alloy was tested to include the following constituent elements: Element Specification Tin (Sn) Copper (Cu) Bismuth (Bi) Nickel (Ni) Phosphorus (P) Silver (Ag) Lead (Pb) Antimony (Sb) Arsenic (As) Cadmium (Cd) Zinc (Zn) Iron (Fe) Aluminum (Al) Indium (In) Gold (Au) Antioxidant Balance 0.0267% 0.119% 0.0379% 0.0092% 0.00083% 0.0329% 0.0126% 0.0112% 0.00015% 0.00057% 0.00429% <0.00005% 0.0019% 0.00012% 0.0092% As shown, the constituent parts of the solder alloy are copper (0.0267 wt%), bismuth (0.119 wt%), nickel (0.0379 wt%), and tin. The solder alloy also includes phosphorus in an amount of 0.0092 wt%. The solder alloy formulations in this example are particularly useful for replenishing a solder pool that has been subjected to copper erosion or dissolution. The solder alloy contains copper at a concentration of 0.0267% by weight. In this example, copper is not added to the solder alloy. EXAMPLE 4 In another form, a silver-free, lead-free solder alloy includes the LLfrZLn / LZnZ / E / Yli The following components: Element Specification Tin (Sn) Balance Copper (Cu) 0.70 + / - 0.10% Nickel (Ni) 0.04 + / - 0.01% Phosphorus (P) 0.002 - 0.004% Gallium (Ga) 0.005 - 0.008% Silver (Ag) 0.10% maximum Bismuth (Bi) 0.10% maximum Lead (Pb) 0.05% maximum Antimony (Sb) 0.050% maximum Arsenic (As) 0.030% maximum Cadmium (Cd) 0.001% maximum Zinc (Zn) 0.001% maximum Iron (Fe) 0.020% maximum Aluminum (Al) 0.001% maximum Indium (In) 0.050% maximum Gold (Au) 0.050% maximum Chromium (Cr) 0.10% maximum Mercury (Hg) 0.10% maximum As shown, the constituent parts of the solder alloy are copper (0.60 to 0.80% by weight), nickel (0.3 to 0.5% by weight), gallium (0.005-0.008% by weight) and tin balance. The solder alloy may also include phosphorus (0.002 to 0.04% by weight). This alloy composition is suitable for high-temperature applications, especially if the operating temperature exceeds 360°C. It is applicable in soldering processes such as hot air leveling, tinning, and wave soldering. It should be noted that the modalities of the compositions and methods described herein are not limited in their application to the construction details and arrangement set forth herein. The compositions and methods are capable of being applied in other modalities and practiced or carried out in various ways. Examples of specific implementations are provided herein for illustrative purposes only and are not intended to be limiting. In particular, the acts, elements, and characteristics described in relation to one or more modalities are not intended to be excluded from a similar role in any other modality. Furthermore, the phraseology and terminology used here is for descriptive purposes and should not be considered limiting. The use here of includes, comprises, has, contains, implies, and variations thereof is intended to encompass the elements listed below and their equivalents, as well as additional items. Having described above several aspects of at least one embodiment, it should be appreciated that various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to form part of this description and are intended to be within the scope of the present invention. Accordingly, the foregoing description and drawings are merely illustrative, and their true scope is included in the following claims. LLfrZLn / LZnZ / E / Yli NOVELTY OF THE INVENTION Having described the present invention as above, it is considered novel and, therefore, the contents contained in the following are claimed as property:

Claims

1. A silver-free, lead-free solder alloy comprising: LLfrZLn / LZnZ / E / Yli 0.001 to 0.800% by weight copper; 0.001 to 0.050% by weight nickel; 0.001 to 0.012% by weight phosphorus; 0.001 to 0.008% by weight gallium; and a balance of tin, together with unavoidable impurities 2. The welding alloy according to claim 1, wherein the copper is 0.600 to 0.800% by weight of the welding alloy.