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Step 3: Soldering Materials
December 31, 1969 |Estimated reading time: 11 minutes
By Jennie S. Hwang, Ph.D.
The continued convergence of computing, communication and entertainment products as well as the relentless growth of wireless, portable, handheld digital electronics and optoelectronics drive the end-use market. From these market demands have come technological innovation and an ever-shortening product lifecycle. The demand for environment-friendly manufacturing of end-use products has prompted a focus on solder materials, specifically in the properties and characteristics of lead-free solders, and to their applications in electronics packaging and assembly.
Serving as interconnections in packaging and assembly, solder joints typically entail a simultaneous exposure to more than one hostile environment, such as temperature and stress. Most solder materials, even under ambient temperature (298°K), reach homologous temperatures (T/Tm) well beyond 0.5. Under these service conditions, both creep and fatigue processes may exist and operate interactively, situations that would be equivalent to creep under cyclic loading or fatigue at high temperature.
Figure 1. A SEM micrograph of an as-reflowed 63Sn/37Pb solder joint (a) on a copper substrate. A solder joint of the same alloy (b) displays fatigue via a coarsening near the interface.
Solder Creep/Fatigue Interaction Overview Figure 1a is an example of a scanning electron microscope's (SEM) view of a 63Sn/37Pb solder joint on a Cu substrate. Figure 1b represents a fatigued solder joint of the same composition exhibiting a coarsened band near the joint interface area.1,2 Whether a "wear-out" phenomenon should be viewed as creep-aggravated fatigue or fatigue-accelerated creep depends on several factors. Generally, when the cyclic stress (or strain) amplitude is small in comparison to the mean stress (or strain), or the applied frequency is low and/or the temperature is high, the phenomenon can be treated as creep perturbed by fatigue. In contrast, when the cyclic stress amplitude is large (or the applied frequency is high and/or the temperature is low), the degradation phenomenon should be considered due to fatigue accelerated by creep behavior.
Figure 2. Tensile stress vs. strain at 300°K and 6.56 x 10-4/second for Sn/Ag/Bi alloys and 63Sn/37Pb.
Solder material for interconnections may undergo changes through one of these two interactive behaviors involving both creep and fatigue. The readily measurable engineering properties for obtaining maximum creep resistance often differ from those for maximum fatigue resistance. For these reasons, the development of improved materials should aim at enhancing both creep and fatigue resistance.
Figure 3. Comparison of fatigue life of Sn/Ag/Bi alloys with 63Sn/37Pb.
Conventional vs. Lead-free Solder There are two primary driving forces for lead-free solders: performance demands and environmental/health concerns.1,3 It is reasonably well substantiated that thermal-fatigue failures of solder interconnections are linked with the Pb-rich phase, which cannot be strengthened effectively by Sn solute atoms because of limited solubility and Sn precipitation. At room temperature, the limited solubility of Pb in the Sn matrix renders it incapable of improving the plastic deformation slip. Under temperature cycling (thermomechanical fatigue) conditions, this Pb-rich phase tends to coarsen and eventually leads to solder joint crack. Therefore, it is expected that the absence of the Pb-phase in a properly designed, lead-free (tin-based) solder may impart improved mechanical behavior and result in strengthened solders.
Figure 4. Tensile stress vs. strain at 300°K and 6.56 x 10-4/second for Sn/Ag/Bi/In alloys and 63Sn/37Pb.
Criteria for Lead-free Alloy SelectionGenerally, alloy selection is based on the following criteria:
- Alloy melting range in relation to service temperature
- Mechanical properties in relation to service conditions
- Metallurgical compatibility (consideration of leaching phenomenon and the potential formation of intermetallic compounds)
- Rate of intermetallic formation vs. service temperature
- Other service compatibility (consideration: such as silver migration)
- Wettability on specified substrate
- Eutectic vs. non-eutectic compositions
- Ambient environment stability.
Figure 5. Comparison of fatigue life of Sn/Ag/Bi/In alloys with 63Sn/37Pb.
Lead-free Fundamental Technology.1 Any viable lead-free solders in lieu of Sn/Pb eutectic or near-eutectic compositions could not escape from being the Sn-based system (i.e., a minimum of 60 wt percent of tin). This was concluded based on both fundamental materials science and practical perspectives. Fundamentals include metallurgical bonding capability on commonly used substrates, dynamic wetting ability under practical reflow conditions, and metallurgical "interactions" or alloying phenomena between elements. "Practical" factors cover the availability of natural resources, manufacturability, toxicity and cost. In selecting any elements, it is alloying ability with Sn and property in melting point depression (while alloying with tin) that are the two crucial material characteristics together with the specific dosages.
Based on their metallurgy, elements such as In, Bi, Mg, Ag, Cu, Al, Ga and Zn are the candidates that can lower the melting temperature of Sn to create the alloys that possess the required properties for packaging and assembly. Table 1 tabulates the speculated melting point depressions with Sn at the selected temperature ranges for the candidate elements.1
Figure 6. Tensile stress vs. strain at 300°K and 6.56 x 10-4/second for Sn/Ag/Cu/Bi alloys and 63Sn/37Pb.
Alloy DesignFor an Sn-matrix, elements that can serve as viable alloying candidates are few, being practically limited to Ag, Bi, Cu, In and Sb. However, doping elements may extend possibilities to a larger group of elements and compounds, such as Ga and Se. Metallurgical interactions (reactions) and microstructure evolution in relation to increasing temperatures are the critical scientific bases for developing new lead-free solders.
Figure 7. Comparison of fatigue life of Sn/Ag/Cu/Bi alloys with 63Sn/37Pb.
Binary-phase diagrams provide general information on the conditions and extent of metallurgical interactions, albeit complete phase diagrams beyond the binary system are scarce. Nonetheless, binary-phase diagrams offer a useful starting point.
Figure 8. Tensile stress vs. strain at 300°K and 6.56 x 10-4/second for Sn/Ag/Cu/In alloys and 63Sn/37Pb.
After a decade of research, it was found that the actual test results of the designed multiple-element alloy compositions came close to the anticipated features in properties and performance between a candidate element and the Sn-matrix.1 For example, Se and Te were found to readily embrittle the Sn-based alloys. And Sb in an improper amount quickly jeopardizes an alloy's wetting ability; the distribution of In atoms in the Sn host lattice is reflected in fatigue performance; the level of Bi second-phase precipitation is associated closely with the mechanical properties of the Bi-bearing alloys; and 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.
Figure 9. Comparison of fatigue life of Sn/Ag/Cu/In alloys with 63Sn/37Pb.
However, because general performance is as predictable as stated, a high-performance alloy demands a stunningly intricate balance of the elemental constituents. In each compositional system, the useful products often are a specific composition or a narrow range of compositions at best.12,13,14 New solder alloys must possess the characteristics that are compatible with practical manufacturing techniques and end-use environments. Basic material properties such as liquidus/solidus temperature, electrical/thermal conductivity, intrinsic wetting ability on commonly used surfaces, 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 key is the ability to optimize these properties through in-depth application of materials science and metallurgical phenomena.
Figure 10. Tensile stress vs. strain at 300°K and 6.56 x 10-4/second for Sn/Cu/In/Ga alloys and 63Sn/37Pb.
Selection MenuFrom the simplest alloy that is a binary system to incrementally complex systems containing more than two elements, lead-free materials have been thoroughly explored, designed and studied.1,2,3,12,13,14 Six systems with their corresponding compositions stand out on their performance merits: Sn/Ag/Bi, Sn/Ag/Cu, Sn/Ag/Cu/Bi, Sn/Ag/Bi/In, Sn/Ag/Cu/In and Sn/Cu/In/Ga. Their strengths and comparisons with the established alloy compositions are summarized.
Some compositions are covered under the patents.4-14 Those selected from each of the systems also are compared to the pertinent known lead-free alloys as well as with 63Sn/37Pb. Figures 2 through 11 summarize the relative performance of the selected compositions with the established solder alloys. An overall comparison also is provided, leading to ranking by melting temperature (Table 2), fatigue life (Table 3) and the final slate of selections.
Recommendations1. An optimal composition should be determined by the requirements of the performance level for a specific application. Tables 2 and 3 provide the relative performance of the selected alloys that show the most promise. The following compositions as can be considered:
2. Melting temperature (liquidus) is an important selection criterion.
3. A proper reflow profile can compensate to some extent for the effects of the higher melting temperatures (>183°C) associated with lead-free alloys.
4. For surface mount PCB assembly, the melting temperature of solder alloys below 215°C provides the necessary process window.
5. Alloy flow property (i.e., wetting behavior of lead-free solder) differs from that of lead-bearing alloys.
6. Overall, technological advancements via lead-free materials research have enhanced creep and fatigue resistance and the viable alloys are identified for high fatigue-resistant applications.
Figure 11. Comparison of fatigue life of Sn/Cu/In/Ga alloys with 63Sn/37Pb.
Modeling Solder Joint Life Predictions It is well recognized that solder joint reliability relies not only on intrinsic material properties, but also on design, component type, the process that forms the solder connections and the long-term service conditions. 2,16,17 As integrated circuit (IC) packages and components continue evolving rapidly, it is highly desirable to have a model able to predict solder joint service life and reliability under a specific set of conditions. However, to derive such a model is an ever-daunting task, primarily because of the complex nature of solder materials in conjunction with "active" service conditions. Solder materials impart more complex behavior in response to temperature, stress and time than high-temperature materials such as steel. Much remains to be understood. The challenges are further complicated by the high-level of versatility in today's PCBs characterized by the use of various materials and designs.
For a given solder composition and design, the main physical factors affecting performance are seven: temperature, ambient environment, strain range, strain rate, loading wave form, intrinsic microscopic structure and solder joint surface condition. It generally is accepted that, under cyclic strain conditions, the creep-fatigue process essentially accounts for any solder joint degradation, assuming that the interfacial problems, such as those caused by excessive intermetallics or poor wetting, are not the determining factors for failure. Thus, most studies have been conducted under the creep-fatigue testing mode, the goals for which are:
1. To understand material behavior under cyclic strains, which are inevitable encounters during solder joint service life in electronic assembly.
2. To develop or improve degradation resistance under cyclic strains by taking a system approach.
3. To predict the fatigue life of solder joints so that performance reliability at a given set of service conditions can be designed and assured.
Numerous fatigue-life prediction methods have been proposed. They include the frequency modified Coffin-Manson (C-M) method, strain-range partitioning, fracture mechanics and finite element analysis (FEA). The methods are borrowed largely from the established fatigue and creep phenomena of steels as a result of extensive studies coupled with the field data obtained over a longer period of time. However, both the frequency modified C-M and fracture mechanics-based methods are incapable of handling complex loading waveform.
Similarly, strain-range partitioning can deal with the strains in any wave form, yet separating the total inelastic strain range per cycle into creep strain and plastic strain is difficult. And lastly, FEA lacks the capability of including complex waveform.
Increased efforts to tailor the basic life-predication models established for steels are burgeoning in the electronics industry. Although the result of efforts may have generated the models that predict solder joint fatigue life in a comparative sense, a true working model remains to be found.
Single Model LimitationsService conditions under which solder joints must perform in component packages and assemblies often involve random multiaxial stresses that expose them to creep ranges as well as cyclic strains. At this time, sufficient and integrated data of solder joint behavior under such conditions and corresponding damage evolution are lacking. Consequently, some important areas and conditions are grossly ignored in the modeling scheme.
Listed are the areas that either have not been included or are not covered adequately. They are, in turn, considered to be the reasons that contribute to the limitations of a single model for wider applications:
1. Effect of initial microstructure
2. Effect of grain size
3. Effect of microstructure that is not homogeneous
4. Change in microstructure vs. external conditions
5. Multiaxial creep-fatigue
6. Identification of the presence or absence of crack-free materials at the starting point
7. Size of existing cracks, if present
8. Effect of interfacial metallurgical interaction
9. Joint thickness vs. interfacial effect
10. Damage mechanism (transgranular or intergranular)
11. Potential damage mechanism shift (from transgranular or intergranular)
12. Presence or absence of grain boundary cavitation
13. Effect of fillet geometry
14. Effect of free surface condition
15. Correlation of accelerated testing conditions vs. those of actual service
16. Testing condition vs. damage mechanism
17. Service conditioned to include possible variation in chip-power dissipation over time
18. Ambient temperature change
19. The number of on/off power cycles
20. Effect of variations in coplanarity among solder joints.
Including these areas in modeling is not only overwhelmingly time consuming but also information demanding it is a daunting task. Nonetheless, their inclusion is necessary to achieve a model's ultimate utility for predicting solder joint service life for specific applications. However, before a universal model is validated, combining knowledge and data in conjunction with experimentation can produce practically sound systems.
Dr. Jennie S. Hwang is an SMT editorial advisory board member and president of H-Technologies Group Inc. For a list of references cited in this article e-mail the author at JSLHwang@aol.com.