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Step 3 Solder Materials
December 31, 1969 |Estimated reading time: 19 minutes
By Jennie S. Hwang
This overview of both leaded and lead-free solder materials focuses on the properties and characteristics important to applications in electronics packaging and assembly.
One general definition of solders explains them as fusible alloys with liquidus temperatures below 400±C (750±F). Solder can be made in various physical forms including bars, ingots, wire, powder, preforms in designated shapes and dimensions, spheres, and pastes. The performance of a solder material is determined by the specific land pattern on which it will be applied and the soldering process used. The soldering process, in turn, is distinguished by the chemistry or cleaning methodology. Three main chemistries rosin, mildly active (RMA) with solvent cleaning; water-soluble with water cleaning; and no-clean chemistry without cleaning are considered state-of-the-art technologies. The intrinsic material properties can be grouped into three categories: physical, metallurgical and mechanical.1
Physical PropertiesFive physical properties that contribute to the overall performance of solder are discussed here. Although other properties can be contributing factors to overall solder performance, these five were chosen for their special importance to current and future packaging and assembly. They are phase transition temperature, electrical conductivity, thermal conductivity, coefficient of thermal expansion (CTE) and surface tension.1
Metallurgical phase transition temperatures have practical implications. In application, the liquidus temperature can be considered to be equivalent to the melting temperature, and the solidus to the softening temperature. At a given composition, the temperature range between liquidus and solidus is referred to as the plastic or "pasty" range.
It is apparent that the solder alloy selected as the interconnecting material must be compatible with the service temperature in the worst-case condition. It is further advisable that the alloy has a liquidus temperature at least two times higher than that of the expected service temperature's upper limit. As the service temperature gets closer to the liquidus temperature, solder generally becomes mechanically and metallurgically "weaker."
Electrical conductivity of solder interconnections contributes to the performance of electrical signal transmission. Solder, a metal, can be viewed as an assemblage of positive ions immersed in a cloud of electrons and metallic crystals held together by the electrostatic attraction between the negatively charged electron cloud and the positively charged ions. By definition, electrical conductivity is the result of the movement of electrons or ions from one location to another under an electrical field. Electron conductivity is predominant in metals; ionic conductivity is responsible for oxides and nonmetallic materials. For solders where electrical conductivity is primarily contributed by electrons, the resistivity (the reciprocal of electrical conductivity) increases with increasing temperature. This is because the reduction of electron mobility is directly proportional to the electron motion's mean free path as temperature increases. The electrical resistivity of solders also can be affected by plastic deformation, i.e., resistivity increases with increasing plastic deformation.
Thermal conductivity of metals normally correlates well with electrical conductivity because of the fact that the electrons carry both thermal and electrical energy and, therefore, are primarily responsible for thermal conductivity as well. For insulators, however, the phonon activity predominates. The thermal conductivity of solders decreases with increasing temperature.
CTE issues are under observation and have been an area of effort by the industry since the inception of surface mount technology in the printed circuit industry. This is due to the large difference in CTEs between materials that are interconnected. A typical assembly consists of an FR-4 board, solder, and leadless or leaded components. Their respective CTEs follow: FR-4, 16.0 °10-6/±C; 63Sn/37Pb, 23.0 °10-6/±C; Cu leads, 16.5 °10-6/±C; Al2O3 leadless components, 6.4 x 10-6/±C. Under temperature fluctuation and power on/offs, this CTE mismatch increases the stress and strain imposed on the solder joint, which may be shortened in service life as a result, and lead to premature failure. Two major material properties dictate the magnitude of CTE: crystal structure and melting point. When materials have similar lattice structure, the thermal expansion of the materials varies inversely with their melting points.
The surface tension of molten solder is a key parameter related to the wetting phenomenon, thus to solderability. The relative strength of the attraction forces acting between molecules of the surface is weaker than that of molecular forces in the interior because of the broken bonds at the surface. Thus, the free surface of a material has higher energy than its interior. Surface tension is a direct measure of the intermolecular forces acting at the surface. A simple but important concept is that wetting/spreading occurs when the free energy of the newly formed system after wetting is lower than that before wetting. In other words, for molten solder to wet the substrate, it must have a higher surface energy than that of the molten solder. In view of this requirement, the lower the surface energy of the molten metal (or the higher the surface energy of the metal substrate), the more favorable the wetting. It should be noted that fluxing is intended to maximize the surface energy of the metal substrate and not lower it, as occasionally is stated in literature. In conjunction with the proper metallurgical reaction, this is how flux and fluxing play a role in wetting.
Metallurgical PropertiesCertain metallurgical phenomena commonly occur in solder materials because of the environment and conditions to which they are exposed during their service life on the circuitry. These conditions include plastic deformation, strain hardening, recovery, recrystallization, solution hardening, precipitation hardening or softening, and superplastic deformation.
When solder is exposed to an applied force, whether the result of mechanical or thermal stress, it deforms irreversibly. Called plastic deformation, it typically is initiated through shearing on numerous parallel planes of its crystal structure. Plastic deformation may proceed globally or locally within the solder joint, depending on the stress level, strain rate, temperature and material characteristics. Continued or cyclic plastic deformation eventually leads to solder joint fracture.
Figure 1. The relative performance of various selected compositions: Tensile stress (s) vs. strain (e) at 300K and 6.56x10-4/second for Sn/Ag/Bi alloys and 63Sn/37Pb (a); Comparison of fatigue life of Sn/Ag/Bi Alloys with 63Sn/37Pb (b); Tensile stress (s) vs. strain (e) at 300K and 6.56x10-4/second for Sn/Ag/Bi/In alloys and 63Sn/37Pb (c); Comparison of fatigue life of Sn/Ag/Bi/In alloys with 63Sn/37Pb (d); Tensile stress (s) vs. strain (e) at 300K and 6.56x10-4/second for Sn/Ag/Cu/Bi alloys and 63Sn/37Pb (e); Comparison of fatigue life of Sn/Ag/Cu/Bi Alloys with 63Sn/37Pb (f); Tensile stress (s) vs. strain (e) at 300K and 6.56x10-4/second for Sn/Ag/Cu/In alloys and 63Sn/37Pb (g); Comparison of fatigue life of Sn/Ag/Cu/In alloys with 63Sn/37Pb (h); Tensile stress (s) vs. strain (e) at 300K and 6.56x10-4/second for Sn/Cu/In/Ga alloys and 63Sn/37Pb (i); Comparison of fatigue life of Sn/Cu/In/Ga alloys with 63Sn/37Pb (j).
Solder also may be hardened (strain hardening), as often observed in the stress vs. strain relationship.
A counteracting phenomenon to strain hardening is the recovery process, which is a softening reaction in which the solder tends to release the stored strain energy. The recovery process is driven by thermodynamics. It begins rapidly and proceeds at a slow rate. During the recovery state, the physical properties that are sensitive to joint defects tend to be restored to their original value. However, this does not impart any detectable change in microstructure.
The recrystallization process is another phenomenon often observed in an Sn/Pb solder joint during its service life. It usually occurs at relatively high temperatures and involves a larger amount of energy release from the strained materials than that of the recovery process. During recrystallization, in addition to the energy release, a new set of essentially strain-free crystal structures is formed, which obviously involves both a nucleation and a growth process.
The effect of solid solution alloying is an increase in yield stress. A typical example of solution hardening is that Sn/Pb compositions are strengthened by an Sb addition. Solution hardening can occur to an even larger extent in the well-designed Pb-free solder alloys. Another strengthening effect can come from a structure having well-distributed fine precipitates (precipitation hardening).
Mechanical PropertiesThree fundamental mechanical properties of solders are stress vs. stress behavior, creep resistance and fatigue resistance. Although stresses can be applied by tension, compression or shear force, most alloys are weaker in shear than in tension or compression. In the case of solders, shear strength is an important property because the majority of solder joints are subjected to shear stress during service.
Creep is a global plastic-deformation process when both temperature and stress (load) are kept constant. This time-dependent deformation theoretically can occur at any temperature above absolute zero. However, only at "active" temperature can creep become significant. (Active temperature is defined as the ratio of the actual temperature to the melting temperature of solder, being larger than 0.5.)
The failure of alloys under alternating stresses is known as fatigue. The stress that an alloy can tolerate under cyclic loading is much less than that under static loading. Therefore the yield strength, which is a measure of the static stress that solders will resist without permanent deformation, often does not correlate in a straightforward manner with the fatigue resistance. The fatigue crack usually starts as small cracks, which grow under repeated applications of stress while the load-carrying cross-section is reduced.
Solders in electronics packaging and assembly applications normally undergo low-cycle fatigue, (defined as a fatigue life less than 10,000 cycles), and are subjected to high stresses. In addition to low-cycle fatigue, which is conducted at a constant temperature, thermomechanical fatigue is another test mode used to characterize the fatigue behavior of solder. This test subjects the material to cyclic temperature extremes (i.e., a thermal fatigue test mode). Either method has its unique features and merits yet both impose strain cycling on solders.2 Overall, solder follows the general behavior of material properties in response to rising temperature.
Solder Joint ReliabilityGenerally, solder joint service life is associated with the creep/fatigue interaction process or intermetallic development as well as microstructural evolution under the service conditions of electronics packaging and assembly.
For Sn/Pb eutectic, it is reasonably well substantiated that the common thermal fatigue failure for solder interconnections is linked with the Pb-rich phase, which cannot be effectively strengthened by Sn solute atoms due to limited solubility and Sn precipitation. At room temper#ature, the limited solubility of Pb in the Sn matrix renders it incapable of improving the plastic-deformati##on slip. Under temperature cycling (thermomechanical fatigue) conditions, this Pb-rich phase tends to coarsen and eventually lead to the solder joint crack. It is, therefore, expected that the absence of a Pb-phase in a properly designed lead-free, tin-based solder joint may impart an improved mechanical behavior, resulting in strengthened solders. The new Pb-free alloys selected and listed in the sidebar reflect an improved performance over 63Sn/37Pb.
In addition to solder-alloy composition, solder joint integrity can be affected by the substrates in contact with the solder, the solder-making process, joint design and configuration, and external environment. Another important factor is system thermal management. Active integrated circuit (IC) packages face increasing challenges in managing heat dissipation. The integrity and service life of the system directly may depend very much on the efficiency of heat dissipation and indirectly on the external temperature. The heat generated in the package during operation must be effectively carried from the die to the package surface and ultimately to the ambient environment. The design and materials in the package and board all contribute to the efficiency of heat dissipation.3
Lead-free TechnologyThis section outlines some design rules as well as the technology of lead-free solder development. For an Sn-matrix, candidates that can serve as viable alloying elements are small in number, practically limited to Ag, Bi, Cu, In and Sb. However, doping elements may extend to a larger group of elements and compounds, such as Ga and Se.4-8 Metallurgical interactions (reactions) and microstructure evolution in relation to temperature increases are the critical scientific basis for developing new lead-free solders.
Binary phase diagrams provide general information about the conditions and extent of metallurgical interactions. Complete phase diagrams beyond the binary system are scarce, nonetheless, binary phase diagrams offer a useful starting point.
After more than a decade of research, it was found that actual test results of the designed multiple-element alloy compositions came very close to the anticipated features in properties and performance between a candidate element and Sn-matrix.9 To illustrate, Se and Te were found to readily embrittle the Sn-based alloys. Sb in an improper amount quickly jeopardizes the alloy's wetting ability. The distribution of In atoms in the Sn host lattice is sensitively reflected in the fatigue performance. The extent of Bi second phase precipitation is closely related to the alloy's mechanical properties. The formation of intermediate phases and intermetallic compounds between Sn and Cu, Ag or Sb, remarkably affect the strength and fatigue life of the alloy, which in turn depends on the concentration of each element as well as on the relative concentration among the elements.
As general performance is as predictable as stated, a high-performance alloy composition, however, demands a stunningly intricate balance of the elemental constituents. In each of the compositional systems, the useful products often are a specific composition or a narrow range of compositions at best.9
New solder alloys must possess characteristics that are compatible with the practical manufacturing techniques and end-use environment. Basic material properties such as liquidus/solidus temperature, electrical/thermal conductivity, and intrinsic wetting ability on surfaces commonly used, mechanical properties and environmental shelf stability must be gauged. Under the current framework, conductivity and shelf stability are not as sensitive to the makeup of a specific system as are intrinsic wetting ability, mechanical performance and phase-transition temperatures. The ability to optimize these properties through in-depth application of materials science and metallurgical phenomena is the key.
Lead-free Material SelectionsFrom the simplest binary system alloy to incrementally complex systems containing more than two elements, lead-free materials have been thoroughly explored, designed and studied.9 Based on more than a decade's study and progressive research and development work, the application of the basic materials fundamentals, coupled with the understanding of practical and process parameters, have led to an array of designs. The following six systems with their corresponding compositions stand out on performance merits. Their strengths and comparison with the established alloy compositions are summarized:
- Sn/Ag/Bi
- Sn/Ag/Cu
- Sn/Ag/Cu/Bi
- Sn/Ag/Bi/In
- Sn/Ag/Cu/In
- Sn/Cu/In/Ga.
Figure 2. Exemplary manufacturing using the Sn/Ag/Bi/Cu composition to successfully produce the Panasonic Panasert Image Acquisition Card.
The discussion of each of these systems will be omitted here. Readers can obtain the detailed illustrations and data from the textbook.9 The selected compositions from each of the systems also are compared with the pertinent known lead-free alloys as well as with 63Sn/37Pb. Figures 1a-j summarize the relative performance of these selected compositions with the established solder alloys. An overall comparison of the six systems, leading to the ranking by melting temperature and fatigue life as well as the final slate of selections are given in Tables 1 and 2.
Figure 2 is a manufacturing example in using a reflow solder of Sn/Ag/Bi/In and a wave solder of Sn/Cu composition to successfully produce the Panasonic Panasert Image Acquisition Card.10 According to Dr. Ken Suetsugu and Tom Baggio of Panasonic-Matsushita, the production enjoyed a relatively low melting point of 210±C while offering the workability, quality and reliability in the finished product similar to that obtained with 63Sn/37Pb (see Sidebar).
The search for a universally adoptable single lead-free solder composition to replace Sn/Pb eutectic appears to be a natural desire. Realistically, however, application-specific and regional adaptation factors will play a role in selecting an alloy composition for the global landscape.
When selecting materials and processes, simplicity always should be the anchorage of principle and practice. (To paraphrase Einstein's statement: "Everything should be done as simple as possible, but not simpler.") In the end, the selection criteria must resort to the following priorities:
- Whether performance meets the application requirements.
- Is cost as low as practicality?
- Other product virtues required.
Performance must come before cost since a failed product always will incur the highest cost to the manufacturer. With the required performance and desirable cost, the value of the product will be enhanced by additional property virtues.
The quick conversion from Pb-bearing to Pb-free across the board is too daunting a task to fulfill. To implement any new technology, a phase-in period is inevitable. Similarly, waiting for a global standard alloy also is too high an expectation to meet. After all, new technology implementation hardly can come with a standard. To quote two colleagues:
- "The idea of a single lead-free alloy that will be globally adaptable by the industry is quixotic. There isn't a 'one-size-fits-all' solution."11
- "Regardless of the ingenuity of the component designer and the brilliance of the architecture, a device still needs to be connected to the outside world. This is invariably achieved by soldering. If the past is any indicator, a resolution will undoubtedly be obtained and the practitioners of printed circuit processing will once again rise to the occasion and create (as coined by Martha Boyle) an even more 'significant trifle.'"12
Challenges to Solder Joint Fatigue Life PredictionsFor both Pb-containing and Pb-free solder joints, it is highly desirable that a model able to predict the service life and reliability of solder joints under a specific set of conditions be available. This is particularly true in the face of IC packages and components that continue to change at a rapid pace while field performance data are hard to come by.
However, deriving such a model is an ever-daunting task. This is primarily because of the complex nature of solder materials in conjunction with the active service conditions. Solder materials impart much more complex behavior in response to temperature, stress and time than high-temperature materials such as steel. Much remains to be understood. The high level of versatility in circuit boards bearing various materials and designs heightens the challenge of the task.
Increased efforts to develop a life-prediction model for solder joints by tailoring the basic life-prediction models established for steels are burgeoning in the electronics industry. Although the result of such efforts may have generated models that predict solder joint life in a comparative sense, a truly working model remains to be developed.
Listed below1 are the areas that either have not been included or are not adequately covered. They are considered to be the reasons contributing to the limitations of the existing models for real-world applications.
- Effect of initial microstructure
- Effect of grain size
- Effect of homogeneity in microstructure
- Change in microstructure vs. external conditions
- Multiaxial creep fatigue
- Identification of presence or absence of crack-free material at the starting point
- Size of existing cracks
- Effect of interfacial metallurgical interaction
- Joint thickness vs. interfacial effect (thinner solder joints impose increased interfacial effects and decreased conventional fatigue creep phenomena)
- Damaged mechanism transgranular or intergranular
- Potential damage mechanism shift (from transgranular or intergranular)
- Presence or absence of grain boundary cavitation
- Effect of fillet geometry
- Effect of free surface condition
- Correlation of accelerated testing conditions and the actual service condition
- Testing condition vs. damage mechanism
- Service conditions to include possible variation in chip power dissipation over time in addition to ambient temperature change and the number of on/off power cycles
- Effect of variation in coplanarity among solder joints.
A universally viable life-prediction model will continue to be a challenge to the science community and the industry.
REFERENCES
- J.S. Hwang, Modern Solder Technology for Competitive Electronics Manufacturing, McGraw Hill, New York 1996, Chapters 3 and 4.
- J.S. Hwang, "Low-cycle Fatigue vs. Thermomehanical Fatigue," Surface Mount Technology, January 1995, p. 20-22.
- J.S. Hwang, Ball Grid Array & Fine Pitch Peripheral Interconnections, Electrochemical Publication Ltd., Great Britain, 1996.
- J.S. Hwang, Z. Guo, "Lead-free Solders for Electronic Packaging and Assembly," Proceedings, SMI Conference, 1993, p. 732.
- J.S. Hwang, "Overview of Lead-free Solders for Electronic Microelectronics," Proceedings, Surface Mount International, 1994, p. 405.
- J.S. Hwang, H. Koenigsmann, "New Developments of Lead-free Solders," Proceedings, Surface Mount International, 1997.
- H-Technologies Group Internal Reports, 1996, 1997 and 1998.
- Modern Solder Technology for Competitive Electronics Manufacturing, (list of 54 references) McGraw Hill, New York, 1996, Chapter 15.
- J.S. Hwang, Environment-Friendly Electronics Lead Free Technology, Electrochemical Publications Ltd., Great Britain, Spring 2001.
- Courtesy of Dr. Ken Suetsugu and Tom Baggio of Panasonic-Matsushita.
- P. Zarrow, "Lead-free: Act, Don't React," Circuits Assembly, June 2000, p. 20.
- H. Hyman, "Lead-free Musings," Advanced Packaging, October 2000, p. 27.
DR. JENNIE S. HWANG, an SMT Editorial Advisory Board member, a member of National Academy of Engineering and an inductee of WITI Hall of Fame, is a highly solicited advisor to the industry. She is the recipient of Distinguished Alumni Awards from her two alma maters and of the Certificate of U.S. Congressional Recognition and Achievement. Dr. Hwang has recently been selected as an R&D-Stars-To-Watch. She has written more than 180 articles and books, including the sole authorship of several SMT-related books. Her new book, Environment-Friendly Electronics Lead-free Technology, will be released in April 2001. Contact her at (440) 349-1968; Fax: (216) 464-5728; E-mail: JSLHWANG@aol.com; Web site: www.smtnet.com/h-tech.
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ABCs of SMTBridge: Errant solder that spans two conductors not intended to be connected and causing an electrical short.Cold solder joint: A solder connection displaying poor appearance (grayish, pourous) owing to inadequate wetting. Causes include insufficient heat, poor cleaning or impurities in the solder.Convection/IR: A solder reflow process in which a combination of convection and infrared radiation is employed to transfer heat.Dendrites: The individual metallic filaments formed when solder begins to melt (solidus) and which stems from electromigration of copper, silver, etc.Dross: Oxides and other entrapped impurities of solder within the molten bath.Eutectic: The alloy of two or more metals of a solder's composition having the lowest melting point.Liquidus: The temperature at which solder reaches its maximum liquidity; the viscous point for best wetting results.No-clean soldering: A process that uses a specially formulated, low-solids solder paste that requires no final cleaning of the residues.Thermal mismatch: CTE differences between bonded materials.
_________________________________
Lead-free RecommendationsThe required performance level of a specific application should determine an optimal composition. A slate of compositions as listed below can be considered:Sn/3.0-3.5Ag/3.0-3.5Bi/0.5-0.7CuSn/3.3-3.5Ag/1.0-3.0Bi/1.7-4.0InSn/3.0-3.5Ag/0.5-0.5Cu/4.0-8.0InSn/0.5-0.7Cu/5.06.0In/0.4-0.6GaSn/3.0-3.5Ag/0.5-1.5CuSn/3.0-3.5Ag/1.0-4.8Bi99.3Sn/0.7Cu96.5Sn/3.5Ag.
- Melting temperature (liquidus) is an important selection criterion.
- A proper reflow profile is able to compensate some extent of the higher melting temperature (higher than 183±C) associated with lead-free alloys.
- For surface mount printed circuit board assembly, the melting temperature of solder alloys below 215±C provides the necessary process window.
- Alloy flow property, thus wetting behavior, of lead-free solder differs from lead-bearing alloy.
- Advancement has been made in increasing creep and fatigue resistance by lead-free research, and alloys are identified for high fatigue-resistant applications.
Overall, the lack of universality, standardization and setup for a quick conversion have been the top three hurdles in accepting or implementing Pb-free products.