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Lead Free Solders for Surface Mount Technology Applications (Part 1)Legal and technological factors are pressing for alternatives to lead-based solders.
By Mulugeta Abtew, SCSI Systems Inc., San Jose, Calif., and Dr. Guna Selvaduray, San Jose State University
Surface Mount Technology (SMT) is a board level packaging method that primarily uses eutectic tin-lead solders, either the 63Sn-37Pb or the near eutectic 60Sn-40Pb compositions.
With a melting temperature of 183°C, the tin-lead binary system allows soldering conditions that are compatible with most substrate materials and SMT devices. As one of the primary components of eutectic solders, lead imparts many technical advantages to tin-lead solders, including the following:
Knowledge BaseA relatively well-established knowledge base about the physical metallurgy, mechanical properties, flux chemistries, manufacturing processes and reliability of eutectic tin-lead solders exists.
SMT assembly and soldering equipment is almost exclusively engineered with tin-lead solder in mind. A good understanding of the behavior of tin-lead solders has enabled current SMT technology to assemble and create small geometry solder joints (approaching 75 microns in size), in high volume and at a competitive cost. Nevertheless, there are legal and technological factors that are pressing for alternative soldering materials and processing approaches.
Among these factors is legislation that taxes, restricts or eliminates the use of lead. Additionally, the continued trend towards packaging and interconnect miniaturization in SMT is stretching the physical capability of tin-lead solder to provide sound and reliable solder joints.
The natural radius of curvature of molten solder, R, as determined by surface tension, (R = g/rg)1/2 = 2.2mm)3 is already larger than the sizes of the solder joints of SMT devices with less than 0.5mm pitch. This forces the solder to form joints with a smaller radius of curvature which may cause it to flow away from the desired locations, due to high liquid pressure.
Alternative SoldersThere are strict performance requirements for alternative solder alloys used in SMT manufacturing. In addition to providing expected levels of electrical and mechanical performance, the solders must have the desired melting temperature and adequately wet common PC board lands. They must also form inspectable solder joints, allow high volume soldering and rework of defective joints, provide reliable solder joints under service conditions and must not significantly increase assembly cost.
There are several lead-free solders, such as Sn-Au, Sn-In, Sn-Ag, Sn-Bi, that have been employed in the electronics industry for special applications. Major solder paste vendors have research programs targeted at developing new alloys that can be "drop-in" replacements for eutectic tin-lead.
These new alloys tend to be ternaries and quaternaries, rather than binaries, and information regarding the characteristics of these alloys is strictly proprietary; there is little or no information found in the available literature.
This paper presents a review of the existing literature on the metallurgy of lead-free solders for SMT applications, with the primary focus, due to space constraints, on binary systems with a liquid temperature between 120°C and 232°C.
This review is based primarily on a high-volume SMT perspective. Accordingly, we will discuss the physical, electrical and mechanical properties, as well as the physical metallurgy of alternative solders. The corrosion behavior of lead-free solders as applicable to SMT, is also described.
First, a brief description of the legislative and regulatory actions advocated to restrict the use of lead is provided.
Adverse Effects of LeadThe Environmental Protection Agency (EPA) has cited lead and its compounds as one of the top 17 chemicals which poses the greatest threat to human life and the environment.4
When lead accumulates in the body over time, it can have adverse health effects. Lead binds strongly to proteins in the body and inhibits normal processing and functions. Nervous and reproductive system disorders, delays in neurological and physical development, cognitive and behavioral changes, as well as reduced production of hemoglobin resulting in anemia and hypertension5 are some of the adverse effects of lead on human health.
When the level of lead in the blood exceeds 25 mg/dl of blood, lead poisoning is considered to have occurred.6 Recent studies have found that a lead level even well below the established official threshold can be hazardous to a child's neurological and physical development.
The concern about the use of lead in the electronics industry stems from occupational exposure, lead waste derived from the manufacturing process and the disposal of electronics assemblies.
Usage MinimalAlthough the use of lead by the electronics industry appears to be minimal,5 the potential for lead exposure cannot be ignored. The soldering process is a potential source of occupational exposure in electronics, especially the wave soldering operation. Studies have shown that there is little danger of exposure to lead in hand soldering and tinning operations because lead is relatively non-volatile at normal soldering temperatures.7 However, inhalation of lead vapors or lead-bearing dust generated by dross during the wave soldering operation is a possible danger to workers.
Wave soldering generates dross due to surface oxidation at the surface of the molten solder. About 90 percent of the dross formed during wave soldering can be refined to pure metal for reuse,8 but the remainder is a waste product. The Resource Conservation and Recovery Act has classified this waste as hazardous to human health, requiring special handling and disposal.
The Occupational Safety and Health Administration (OSHA) mandates that workers have no more han 50 mg/dl of lead in their blood.9
This exposure limit is related to maintaining blood-lead levels at or below 40 mg/dl, believed to prevent adverse health effects from exposure to lead throughout a working lifetime. For workers planning to have children, OSHA recommends maintaining blood-lead levels below 30 mg/dl.
The EnvironmentLead and lead-containing compounds are considered environmental concerns because of lead's toxicity.10 In the electronics industry, the lead generated by the disposal of electronic assemblies is considered hazardous to the environment.
The world markets for the assembly and soldering of PC boards are shown in Table 1. Japan and the U.S. are the major suppliers and users of PC board assemblies.11 This market is expected to double within the next ten years.11,12 At the end of the useful life of lead-bearing electronic products, they are typically disposed of in solid waste landfills. There is no scientific data or study that clearly describes the mechanism by which lead from discarded electronic products enters the ground-water stream or the animal or human food chain. The only available data that appears to be relevant comes from unrelated studies that have characterized the breakdown of PbO to PbCO3 in the presence of Cl- and CO2.12
In 1986, a review of the use of lead in electrical and electronics applications revealed a dramatic decrease,13 while the use of lead in batteries appeared to be constant. The use of lead for electronic soldering, however, is about 40-50 percent of the total for all soldering uses. It is likely that industry may be increasingly required to recycle lead. The use of recycled lead for electronics applications, however, can be severely limited since recycled lead emits alpha particles, which can have detrimental effects on the performance of integrated circuits.14
Table 1. World markets for PC board assemblies 1992.
Legislation to limit the use of lead has been introduced in both the Senate and House of Representatives. The Legislation includex H.R. 2922, the Lead-based Paint Hazard Abatement Act of 1991, which would impose a tax on lead; S.391, the Lead Exposure Reduction Act of 1991; and H.R. 3554, the Lead Exposure Act of 1992.16
These bills have not passed yet, but it is highly likely that some form of legislation will be passed. Recent proposals suggest that the use of lead will be restricted and requirements such as notification to EPA for new products containing lead may be imposed.
Solder CharacteristicsSoldering is a well-known metallurgical joining method using a filler metal (the solder) with a melting point below 425#&176;C.18 To form a proper metallurgical bond between two metals, wetting must takes place. This means that a specific interaction must take place between liquid solder and the solid surface of the parts to be soldered. Solders can be classified as soft and hard solders.
Soft solders are characterized by melting temperatures below about 190°C, while hard solders melt between 190°C and 425°C. The distinction between the two classes of solders is important.
Most alloys have a tendency to recrystallize at homologous temperatures of 50 to 60 percent of their melting point in °K. At or above room temperature, soft solders tend to undergo recrysallization (annealing) spontaneously. In the proper metallurgical sense, soft solders cannot work hardened; rather they remain ductile under all conditions.
A lead-free alternative solder that could be a drop-in replacement for eutectic tin-lead solder has yet to be found. Low melting point metals are the obvious starting points in the search for lead-free solders. Of these metals, cadmium, mercury, lead, and thallium have intrinsic toxicity. Despite their low melting point, the alkali metals tend to form water-soluble oxides and are extremely reactive. Therefore, this group of elements must be excluded from consideration.
Gallium is a relatively rare element with an insufficient world supply.19 Magnesium is a very reactive element and Sn-Mg alloys are known to tarnish readily in air.20 Nevertheless, there are alternative lead-free solders that have the performance potential to be replacements for tin-lead solder. The performance characteristics that are of importance include the melting temperature, the microstructure, surface tension, coefficient of thermal expansion, electrical resistivity, oxidation behavior, corrosion behavior, elastic modulus, yield strength, shear strength, fatigue behavior and creep behavior.
Melting TemperatureOne of the fundamental performance characteristics of solder for SMT applications is melting temperature. For these applications, the melting temperature of the solder determines the maximum allowable temperature a product can be exposed to in service and the maximum processing temperature that devices and substrates can withstand during soldering.
The solidus and liquidus temperatures of various lead-free solders is provided in Table 2. Some of the alloys are ternaries and quaternaries. Many of the lead-free alloys shown in the table have not been studied extensively. For some of the alloys, virtually no data other than the melting temperature is available.
MicrostructureThe useful properties of materials are strongly dependent on their microstructure, 20 which describes the grain structure and the combination of phases present in a material, as well as its defects, morphology and distribution.21 Generally, for a material of a given chemical composition, the microstructure is not constant and varies greatly, depending on processing and service conditions. In surface mount assembly, the time-and-temperature-dependent soldering profile affects the microstructure of the solder joints, including the intermetallic layer thickness and the number of intermetallic phases present in the solder joint.
The most critical soldering parameter that affects the initial microstructure in surface mount assembly is the cooling rate. One of the desirable properties of potential lead-free solders for SMT applications is having eutectic behavior, where both phases solidify concurrently at a single temperature, rather than over a range of temperatures.
The nature and distribution of the phase mixture of the eutectic composition that is established during solidification is strongly affected by the cooling rate. Slow cooling, where the solidification reaction proceeds more slowly, results in wider interlamellar spacing. At a sufficiently fast cooling rate, the eutectic structure looses its lamellar character, and a microstructure with fine, uniform dispersion of phases within small eutectic colonies or grain sizes is formed.
The grain size also varies with cooling rate. Faster cooling enhances the number of colony nuclei formed,21 and therefore colony size increases with a slower cooling rate. This microstructural variation, due to cooling rate, can drastically affect the fatigue life of the solder joint. Because room temperature is a high homologous temperature for solder, the initial eutectic microstructure evolves over time.
The lamellar structures of the eutectic become coarser to attain an energetically more favorable morphology. There is energy tied up at any grain boundary; a solder joint with a coarse grain structure has a lower energy content than one consisting of fine grain size because of surface area and free energy relations. This energy difference is the driving force for the grain coarsening, which proceeds through the mechanism of solid-state diffusion.22
Table 2. Pb-free solders with liquidus (T1), solidus (Ts) and eutectic (Te) temperatures.
Intermetallic CompoundsThe intermetallic compounds that are formed at the solder substrate interface continue to grow over time. This growth is a result of a solid-state reaction driven by an energy differential. The solder substrate reaction is exothermic, which means that the intermetallic compounds that are formed have a lower energy content than the reacting metals by themselves.23 Like all reactions, the kinetics of grain coarsening and intermetallic-layer growth increase with rising temperatures.
In the following section, what is known about the microstructures of the most relevant lead-free solders for SMT applications is reviewed. It is highly likely that a new lead-free solder alloy will be tin- based. Therefore, a brief overview of the property of this metal is provided.
TinBecause of its ability to wet and spread on a wide range of substrates using mild fluxes, tin has become the principal component of most solder alloys used for electronic applications. Tin exists in two different forms with two different crystal structures. White or b tin has a bodycentered tetragonal cystal structure and is stable at room temperature. Gray tin or a tin, which has a diamond cubic crystal structure, is thermodynamically stable below 13°C. The transformation of b to a tine, also called "tin pest," takes place when the temperature falls below 13°C, and results in a large increase in volume which can induce cracking in the tin structure. Consequently, tin pest can be a problem for applications that operate at extremely low temperature.
Tin is also prone to whisker growth. Whiskers can be defined as a single-crystal growth resembling fine wire that can extend up to 0.64 mm high.24
The whiskers are tetragonal b-tin that may grow in response to tinernal stress in the material or external loads. Rapid whisker growth in tin occurs at about 51°C and is influenced by plating conditions and substrate property. Whiskers do not affect solderability nor do they cause deterioration of the tin coatings. However, longer whiskers may cause electrical shorts in PC board assemblies. Elements such as lead suppress whisker growth in tin, and virtually no whisker growth is encountered in eutectic tin-lead solder.24
Tin-ZincThe Sn-9wt.%Zn appears to be an attractive alternative to eutectic tin-lead with a relatively close melting temperature of 198°C. Its eutectic structure consists of two phases: a body centered tetragonal b Sn matrix phase and a secondary phase of hexagonal Zn containing less than 1% tin in solid solution.25
The solidified microstructure exhibits large grains with a fine, uniform, two-phase eutectic colony. The microstructure of this high Sn alloy, essentially comprising a small volume fraction of intermetallics distributed throughout a Sn-rich solid solution, differs significantly from the microstructure of eutectic Sn-Pb solders.
Tin-CopperThe Sn-Cu binary alloy has a eutectic composition at Sn-0.7Cu. The solidification reaction consists of Cu precipitated in the form of hollow rods of the intermetallic Cu6Sn5. Data that describes the property of this alloy hardly exists. Because of the high concentration of tin in this alloy, it may be prone to whisker growth or transformation to gray tin may occur.
Further, dissolution of the substrate in the molten solder, the oxidation of the flux, the soldering environment, etc. affect the surface tension of solder. Studies have shown that the surface tension value of solder varies with temperature,36 flux composition37 and the extent of solder substrate interactions.38 For these reasons the surface tensions of solder and most other liquid metals are not precisely known.39 There are very few surface tension data available for lead- free solders. Vincent, et al.40 measured the surface tension of a range of binary Pb-free alloys in both air and nitrogen with <20 ppm O2, at 50°C over their liquidus temperature. The data are shown in Table 3. Generally, surface tension values tend to be lower in air than in an inert atmosphere, since oxidation lowers the free energy of the liquid surface. The data for eutectic Sn-Zn and Sn-Cu do not fit this trend. Further, the surface tension of a liquid decreases linearly with increasing temperature and continues to decrease as the temperature approaches the boiling point. Further work is needed in this area.
Tin-BismuthThe Sn-Bi alloy has a eutectic composition of 42Sn-58Bi. The equilibrium phases are Bi and Sn with about 4% Bi in solid solution.26 Since tin has very low solubility in Bi at the eutectic solidification temperature of 130°C, the Bi phase is primarily pure Bi. However, the maximum solubility of Bi in Sn is about 21 wt %.27 As the alloy cools, Bi precipitates in the Sn phase.
At moderate cooling rates, the eutectic Sn-Bi microstructure is lamellar, with degenerate material at the boundaries of the eutectic grains. This microstructure is similar to the one theoretically predicted by Croker28 for relatively slow cooling rates. Wild29 observed cracks on slowly cooled eutectic Sn-Bi solder joints. Slow cooling resulted in the formation of large grains. Tin precipitated from the solder matrix along the boundaries of these large grains through which cracking occurred. Cracking was not observed during rapid cooling. Cooling rates, however, were not specified in the literature.
Tin-SilverThe eutectic composition for the Sn-Ag binary system occurs at Sn-3.5Ag. The microstructure consists of Sn and the intermetallic Ag3Sn in the form of thin platelets.30 McCormack31 described the solidified microstructure of the binary eutectic Sn-3.5%Ag as consisting of a ß-Sn phase with dendritic globules and inter-dendritic regions with a eutectic dispersion of Ag3Sn precipitates within a ß-Sn matrix. The addition of 1% Zn has been shown to improve the solidification microstructure of this alloy by eliminating the large b-Sn dendritic globules and introducing a finer, more uniform, two-phase distribution throughout the alloy.30
The addition of Zn suppresses the formation of b-Sn dendrites and results in a uniform dispersion of Ag3Sn.30 Similar to Sn-.07Cu alloy, this solder may likely be prone to whisker growth due to its high tin composition. However, there is no information available in the literature with regard to Sn-Ag whisker growth.
Tin-IndiumDue to their substantially lower melting temperature and a much lower tendency to scavenge gold compared to tin-lead solders, In-Sn solders have been used for SMT applications. The indium-based solder, with the composition of 52In-48Sn, is the one that is commonly used for SMT. The eutectic composition is 50.9In-49.1Sn. The two phases that form are intermetallic phases: an In-rich, pseudo-body-centered tetragonal phase, b, which is 44.8%Sn, and a hexagonal Sn-rich phase, g, with 77.6%Sn.31
Mei and Morris32 described the microstructure of In-48Sn solder on a Cu substrate as having lamellar features. The Sn-rich phase is composed of equiaxed grains. The In-rich phase contains Sn precipitates.
A similar structure with less irregularity was observed by Freer and Morris on a Ni substrate33 and significant microstructural coarsening was observed by Seyyedi34 after prolonged aging of the solder joints made on a Cu substrate.
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