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High-strength and High-fatigue-resistant Lead-free Solder
December 31, 1969 |Estimated reading time: 9 minutes
Sn/Cu/In/Ga proves to be an interesting solution for high-strength and high-fatigue-resistant electronics applications.
Dr. Jennie S. Hwang
Dr. Zhenfeng Guo
Dr. Holger Koenigsmann
Despite its current widespread use in the electronics industry, Sn/Pb solder alloys face a limited future because of the toxicity of lead and the possibility of a lead ban. Consequently, many worldwide initiatives have been taken to find lead-free alternatives.
Eutectic solder (63Sn/37Pb) is most widely used in electronics assemblies, particularly in surface mount printed circuit boards (PCB). This solder provides a critical physical property - a moderate melting temperature below 210∞C.
A practical soldering process temperature for the surface mount manufacturing environment can be achieved at a temperature approximately 20∞C above the liquidus temperature of the solder alloy - e.g., a solder alloy having a liquidus temperature of 210∞C should be soldered at 230∞C minimum. The melting temperature of solder alloys is critical because a melting temperature that is too high could damage electronic devices and polymer-based PCBs during soldering, while too low of a melting temperature will sacrifice long-term reliability. For PCB manufacturing involving typical polymer-based PCBs, such as FR-4, the process temperature cannot exceed 240∞C. Therefore, a lead-free solder alloy that can replace 63Sn/37Pb and function in the surface mount manufacturing process should have a liquidus temperature below 215∞C, preferably 210∞C.
It is a primary goal of this article to provide a lead-free solder that offers high-strength and high-fatigue resistance with a practical melting temperature (less than 215∞C) and wetting ability. Another objective of this research is to design solder alloys that are as close to a eutectic as practical without sacrificing other performance properties.
Experiment
Designed lead-free solder compositions were prepared by melting - in sequence - pre-prepared master solder alloys and pure metals in a Pyrex glass beaker in a Thermolyne 48,000 electric resistance chamber furnace. The furnace was heated and controlled to approximately 500∞C.
A cast mold made of stainless steel was configured for making mechanical specimens for testing. The cast mold generated standard round tension test specimens with a gauge diameter of 6 mm in accordance with industry standards.
Standard tensile, creep and low-cycle fatigue tests were run in uniaxial tension at room temperature (300∞K) using an electromechanical Instron 4411 material testing system. Load was measured using an Instron static load cell of 5 kN capacity, strain was measured using an Instron extensometer of 1" gauge length, and test data were calculated by operating Instron Merlin series IX software. Specifically, tensile tests were performed at a crosshead speed of 1 mm per minute, corresponding to a strain rate of 6.56 x 10-4 per second. Constant load creep tests were conducted under load control with an accuracy of 0.1 N. Low-cycle fatigue tests were carried out under strain control at a frequency of 0.1 Hz and an R-ratio of 0.8. The fatigue life (Nf) was defined as the number of cycles at which the tensile load on the hysteresis loop (load vs. elongation) was dropped to 50 percent of the maximum load (according to ASTM Standard E606-92).
Basic mechanical properties that were measured in accordance with the ASTM standards as referenced include:
- Yield strength (sy) at 0.2 percent offset plastic strain
- Ultimate tensile strength (sTS) at maximum load
- Plastic strain (ep) at fracture after a drop-in load to 10 percent of the maximum values
- Low-cycle fatigue
- Creep rate.
In addition, wetting characteristics were evaluated on Cu substrates at 240∞C using a no-clean flux vehicle.
Results
Designed lead-free compositions are summarized in Table 1, along with their melting temperatures (Tm), sy, sTS, Young`s modulus (E), ep at fracture and Nf at a total strain of 0.2 percent. All compositions are expressed in weight percent unless otherwise specified. The reference alloys of 63Sn/37Pb and 99.3Sn/0.7Cu are also included.
Phase Transition Temperature
As shown in Table 1, the solder alloy composition in this system has melting temperatures between 209∞ and 219∞C, depending on specific compositions. All alloys designed under this system possess a narrow pasty temperature range well within 5∞C. As expected, the increase of In from 4 to 6 percent reduced the melting temperatures by about 4∞C for the solder alloys containing 0.5 to 0.7 percent Cu. The melting point depression effect of In diminished as Cu dosage approached 1 percent. When comparing the melting temperature of Alloy 719 with that of Alloy 728 in Table 1, the addition of 0.5 percent Ga lowered the melting temperature.
Stress vs. Strain
The monotonic mechanical flow of solder alloys typically consisted of an elastic region, a strain-hardening region, a stress-recovery region and a cracking region. Strain hardening continued until necking occurred at the maximum load or the sTS. Necking was caused by an inhomogeneous plastic deformation somewhere in the gauge length, and was associated with strain localization. Stress-recovery mechanisms are dominant in the region after necking and before abrupt fracture for high-temperature deformation. The designed lead-free solder alloys generally exhibited a lower stress-recovery rate than 63Sn/37Pb solder (Figure 1), i.e., the relatively higher sustaining flow stress after the sTS. This suggests that the designed lead-free solder alloys will have a higher creep resistance than 63Sn/37Pb. The monotonic flow strength and plasticity of the lead-free solders containing about 4 percent In were comparable with those of 63Sn/37Pb. The strength and plasticity of the lead-free solders containing about 6 percent In overall surpassed those of 63Sn/ 37Pb. The sy and sTS of Alloy 719 were higher than that of Sn/Cu eutectic (99.3Sn/ 0.7Cu); the ep at fracture of Alloy 719, although more than adequate, were lower than that of 99.3Sn/0.7Cu.
The effect of In content on sTS is most pronounced for compositions containing 0.5 to 0.7 percent Cu (with 0.5 percent Ga). Increasing In from 4 to 6 percent increased both sTS and ep at fracture (Figure 2). The ep at fracture is most sensitive to In content at 0.7 percent Cu.
Overall, 0.5 to 1 percent Cu had little effect on strength and plasticity, although the plastic strain of the composition containing 4 percent In decreased slightly with the dosage of Cu (Figure 3).
Low-cycle Fatigue
Fatigue life is affected by the dosages of In and Cu. For compositions containing 0.5 to 0.7 percent Cu, an increase of In from 4 to 6 percent significantly increased the number of cycles to failure, as shown in Figure 4. However, when Cu reached 1 percent, the fatigue life decreased with the higher In content.
The fatigue life peaked at about 0.7 percent Cu in the system containing 0.5 percent Ga and 4 to 6 percent In (Figure 5). Alloy 719 (92.8Sn/0.7Cu/6In/0.5Ga) demonstrated the highest low-cycle fatigue resistance in this system.
Alloying with Ga consistently enhanced fatigue life. The fatigue life of Alloy 719 in the Sn/Cu/In/Ga system is 83 percent higher than that of Alloy 711 in the Sn/Cu/In system (Table 1). A small dosage of Ga played a significant role in increasing fatigue resistance.
Wetting and Solderability Phenomena
When proper flux chemistry is used, the Sn/Cu/In/Ga system exhibited a sound wetting and spreading on Cu substrates. The wetting contact angle may appear to be somewhat larger than that of 63Sn/37Pb.
Discussion
In the lead-free quaternary Sn/Cu/In/Ga system, the metallurgical interactions between the minor yet "powerful" elements (Cu, In and Ga), and Sn are considered to be the leading factors in determining solidification mechanisms as well as mechanical properties.
Cu at 0.5 to 1 percent mainly reacts with Sn to form the h intermetallic compound (Cu6Sn5), as indicated by the binary Sn/Cu phase diagram. The addition of other metal elements such as In and Ga further modifies the binary eutectic (99.3Sn/0.7Cu) to a quaternary off-eutectic structure reflected by the presence of a melting temperature range. Any deviation of Cu content, from 0.7 percent at 4 to 6 percent In and 0.5 percent Ga, promoted the generation of pro-eutectic dendrites because the fatigue life peaked at 0.7 percent. In this case, the h intermetallic compound (Cu6Sn5) solidifies as second-phase particles to impart strength and partition a finer Sn-matrix grain structure.
In at 4 to 6 percent mainly enters the Sn-matrix lattice sites as substitute solute atoms, as suggested by the Sn/In binary phase diagram. Like most other strengthening, except grain refining, the solid solution generally increases strength in a nearly linear fashion. However, in Sn/Cu/In/Ga systems, the plasticity also increased with increasing In content (Figure 2). This suggests that there might be an interaction of In with Ga. According to the In/Ga binary phase diagram (Figure 6), Ga can enter into In lattice sites as solute atoms. In addition, the In/Ga solid solution competes with the formation of the Sn/In solution, which solidifies at a higher temperature. Nevertheless, the attraction between In and Ga can render the Ga second phase more uniformly distributed. The more uniform dispersion of Ga second-phase particles can effectively partition a finer grain, accounting for the increased plasticity.
As for the interaction between Ga and Sn, there is no solid solubility of Ga in Sn or vice verse, as indicated by the Sn/Ga binary phase diagram. It is likely that 0.5 percent Ga independently solidifies as a second phase. The size and distribution of Ga second phase cannot be ensured at this time.
Another strengthening mechanism provided by Ga is that Ga second phase is near a molten state at test conditions (room temperature) - its melting temperature is only 29.8∞C. If the morphology is favorable, the molten Ga second phase can relieve the internal stress and absorb the deformation-generated defects, such as vacancies and dislocations, to deliver superior fatigue resistance. However, if the morphology is of large islands or clusters around grain boundaries, the effects of a localized strain concentration may offset the benefits of the molten Ga second phase. In any event, Sn/Cu/In/Ga compositions outperform 63Sn/37Pb and Sn/Cu/In systems, indicating the strengthening effect of Ga in Sn-based systems.
In addition, Ga can combine with Cu to form CuGa2 (an intermetallic compound) as indicated by the Cu/Ga binary phase diagram in Figure 7. If these intermetallics are formed, they exert another strengthening effect.
Conclusion
The lead-free solder Alloy 719 (92.8Sn/ 0.7Cu/6In/0.5Ga) and Alloy 717 (93Sn/ 0.5Cu/6In/0.5Ga) in the Sn/Cu/In/Ga system offer superior strength and fatigue life over 63Sn/37Pb. Fatigue life of Alloy 717 and Alloy 719 is 74 and 196 percent higher than 63Sn/37Pb, respectively. Its melting temperature (209∞ to 214∞C for Alloy 717; 210∞ to 215∞C for Alloy 719) and wetting characteristics are applicable for SMT manufacturing and IC packaging. These compositions are one result of ten years of research and are covered under U.S. patent no. 5,985,212. SMT
REFERENCES
Available from author upon request.
DR. JENNIE S. HWANG, DR. ZHENFENG GUO and DR. HOLGER KOENIGSMANN may be contacted at H-Technologies Group Inc., 5325 Naiman Parkway, Cleveland, OH 44139; (440) 349-1968; Fax: (440) 349-9961.
Figure 1. Tensile stress vs. strain at 300 K and 6.56 x 10-4 per second for Alloy 719: 92.8Sn/0.7Cu/6In/0.5Ga, Alloy 718: 94.8Sn/0.7Cu/4In/0.5Ga, and 63Sn/37Pb.
Figure 2. Tensile strengths (sTS) and plastic strains (ep) at fracture vs. In content.
Figure 3. Tensile strengths (sTS) and plastic strains (ep) at fracture vs. Cu content.
Figure 4. Fatigue life at 0.2 percent strain, 300 K, 0.1 Hz and R=0.8 vs. In content for the Sn/Cu/In/Ga system at 0.5 to 1 weight percent Cu and 0.5 weight percent Ga.
Figure 5. Fatigue life at 0.2 percent strain, 300 K, 0.1 Hz and R=0.8 vs. Cu content for the Sn/Cu/In/Ga system at 4 to 6 weight percent Cu and 0.5 weight percent Ga.
Figure 6. In/Ga binary phase diagram.
Figure 7. Cu/Ga binary phase diagram.