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Alternative processing for forming solder joints with lead-free solder alloys
December 31, 1969 |Estimated reading time: 10 minutes
This article describes the results of recently completed research that indicates liquid-phase enhanced sintering of solder paste may be an alternative processing technique for developing lead-free solder joints. By Mark A. Palmer
It is likely that lead-containing alloys will be banned from electronics in the near future. Japan has introduced a consumer take-back program that will virtually require that lead-free solders be used in consumer electronics by 2001. Europe is likely to follow suit with the Waste from Electrical and Electronic Equipment (WEEE) directive to become effective in 2004. Progress has been made and there are consumer electronics that have been produced using lead-free alloys.
Figure 1. Necks have formed in Sn/Pb solder paste after being processed at 160°C for 30 minutes. Necking is the first stage of sintering.
The IPC Association Connecting Electronics Industries developed a roadmap to remove Sn/Pb solder from consumer electronics in the next three years. There has been preliminary work in the United States to develop lead-free solder alloys. Most notable are the projects coordinated by the National Center for Manufacturing Sciences. The first of these (the lead-free solder project) recommended three possible replacements for eutectic tin/lead solder: eutectic Sn/Bi for low-temperature consumer applications, eutectic Sn/Ag, and Sn/Ag/Bi for higher temperature applications.1 The final report did note that "there was no drop-in replacement alloy." Eutectic Sn/Bi has been considered as a low-temperature alternative.2 In addition to environmental concerns, alternatives to eutectic tin/lead are being considered for higher temperature applications by the automotive and avionics industries.3 Both Sn/Ag and Sn/Ag/Bi melt at temperatures significantly higher than eutectic tin/lead solder. Sn/Ag/Bi alloys have been characterized and shown to possess sufficient wettability and mechanical properties for use in electronic assemblies.4,5 Concerns about the ternary Sn/Pb/Bi eutectic that occurs at 98°C, have prompted the consideration of Sn/Ag/Cu alloys.6,7 Marconi Electronics have used these alloys in consumer products.8 These three alloys, and variations of them, melt at temperatures higher than 210°C. This means that reflow temperatures are likely to reach 260°C or higher. As the industry moves toward placing lead-free solders in electronic assemblies, low-temperature alternatives should be considered. Sn/Zn and Sn/Zn/Bi systems are being considered because they melt at temperatures slightly below 200°C. Using Zn instead of Ag is economically attractive.9 However, zinc oxidation is a major concern and some experimental techniques have been attempted, but not yet scaled to the shop floor.10 Using small amounts of Bi to prevent oxidation has been suggested.11
Figure 2. Adding small amounts of Sn/Bi (Tm = 138?C) to the paste causes a small amount of liquid to form between the solid particles. The elimination of intergranular pores such as these is the second stage of sintering.
Whichever replacement alloy, or series of replacement alloys, is chosen, implementing a changeover will entail developing new components, board finishes, board and packaging materials, fluxes, and soldermasks.1 This will not be easy or inexpensive. Low-temperature alternative processing techniques for these higher temperature materials are attractive because they minimize the impact on the manufacturing infrastructure.Environmental consciousness has driven the development of lead-free solders. The electronics industry has responded to a similar challenge: the ban on chlorofluorocarbon (CFC) cleaners. Until 1992, the removal of flux residue was done by using the industry's "solvent of choice," Freon TMS or 111-trichloroethane. At that time, the Montreal Protocol and subsequent Amsterdam regulations banned manufacturers from using ozone depleting chemicals effective January 1, 1996.12,13 As a result, white residue formation has increased. Efforts have been focused on finding alternative "drop-in replacement" cleaners for CFCs, alternative fluxes that leave less residue, or a combination of the two. The driving force behind these efforts is minimizing capital investment costs for replacement machinery.14
Proposed Alternative Process
Similar environmental and economic issues must be addressed when developing lead-free electronics manufacturing techniques. One possible answer may be found in the physical and chemical makeup of solder paste. Solder paste is a collection of pre-alloyed metal powders, flux and binder. Powder materials are frequently joined through a process known as sintering, as opposed to melting and solidification (reflow).
Sintering is the solid-state bonding of powder particles at high-homologous temperatures.15,16 The driving force for this process is the reduction of excess surface area (and surface energy) that occurs as particles join. Sintering is used to manufacture products with high-melting temperature materials (super-alloys, ceramics and refractory metals). Because solder paste is a collection of flux, binder and metal powder, it may be possible to sinter solder paste. Examples of sintering are shown in Figures 1 and 2.
Figure 3. Interface between copper and eutectic Sn/Ag solder paste doped with 3 v/o eutectic Sn/Bi powder, sintered for 10 minutes at 210?C.
There is actually a third stage of sintering grain growth. In this stage, any remaining pores are internal to the grains. Prior research demonstrated that it was possible to prepare solder joints that have mechanical integrity from Sn/Pb solder paste processed 23°C below the melting point by adding small amounts of eutectic Sn/Bi (Tm = 138°C) powder to the mixture.17,18 The addition of small amounts of low-temperature powder increases the rate of sintering as it allows liquid-phase enhanced sintering to occur. The liquid flows around the powder particles, increasing the rate of diffusion. The material may be absorbed into the bulk solder alloy if its presence is small. This technique has recently been applied to form joints from eutectic Sn/Ag (Tm = 221°C) solder paste, identified as one of three possible replacements for Sn/Pb solder that had shear strengths comparable to that of eutectic Sn/Pb solder.1,19,20 This technique was also used to prepare solder joints from alloys with wide thermal ranges (the difference between the solidus and liquidus temperatures or the temperature range through which the material is part liquid and part solid) that had mechanical integrity.20,21
Results of Current Work
Recent studies where solder joints have been prepared using liquid-phase sintering are promising. Solder joints have been prepared by adding up to 3 v/o Sn/Bi to eutectic Sn/Ag solder paste. Sintering at 210°C for as little as 10 minutes has produced joints with shear strengths exceeding 25 MPa. This time refers to how long a joint is in a convection oven, stabilized at 210°C; after 10 minutes of processing, an effective joint is formed. By substituting approximately 1 v/o nano-Bi powder for the 3 v/o Sn/Bi, the processing temperature could be reduced to 200°C. Cold solder joints with mechanical integrity have been prepared by sintering at temperatures just above solidus.
Sintering Sn/Ag Alloys Figure 5. Interface between copper and eutectic Sn/Ag paste doped with nano-Bi powder and sintered for 15 minutes at 200?C.
Based on the results of the study where eutectic Sn/Bi was added to eutectic Sn/Pb solder paste to develop sinterable solder joints with mechanical integrity, a similar technique was attempted to produce a eutectic Sn/Ag joint.18 Lap shear joints were prepared by placing solder paste between two copper strips. These were then placed in a convection oven that had stabilized at the desired temperature for the required amount of time. The shear stress as a function of sintering time and temperature is shown in the Table 1. Note that while temperature is the primary factor, shear strength is also time dependent. However, at 210°C, the shear strength at 10 and 15 minutes are equal within experimental error.Figure 3 is a micrograph of a solder joint formed by sintering eutectic Sn/Ag solder paste doped with 3 v/o eutectic Sn/Bi powder for 10 minutes at 210°C. Figure 4 shows the corresponding stress strain curve. Note the shear stress-strain curve differs from the joint that was prepared for metallographic examination and then subjected to mechanical testing.
Figure 4. Representative shear-stress/shear-strain curve for the solder joint described in Figure 3.
Because of their small size, nanoparticles may disperse better through solder paste. Because of the high curvature of these particles, they melt at lower temperatures and more readily diffuse into the solder paste. A soldered compact of eutectic Sn/Ag doped with 3 v/o nano-Bi particles that was processed at 200°C for 15 minutes is shown in Figure 5. Note the sintered microstructure is similar to that of eutectic Sn/Ag doped with 3 v/o Sn/Bi and processed at 210°C for 15 minutes. It appears that nanoparticles may be useful in developing sinterable pastes.
Cold Solder Joints Figure 6. Interface between copper and Alloy 206 sintered at 210°C for 15 minutes.
The use of materials with large thermal ranges was avoided because of the "cold solder joint problem" a sensitivity to vibration during cooling of the board from the soldering temperature that may lead to defective solder joints.1 The Metals Handbook recommends that when using solder alloys with high thermal ranges, avoid any kind of movement during the solidification phase to prevent hot tearing in solders with a wide freezing range.22 Although solidification is a rapid phenomenon, its duration cannot be neglected during practical soldering. During the solidification period, liquid and solid solder are present simultaneously. Fluidity strongly decreases as solidification progresses. Consequently, any motion imparted on the soldered joint during solidification is liable to cause cracks in the soldered mass that will not be filled again, resulting in an unreliable joint.22 Hot tearing is introduced because the solid is typically less dense than the liquid and the contraction induces stress on the liquid. This problem only manifests itself when a solder joint that is formed as molten solder solidifies. If the joint were formed in the solid state, this problem could be avoided. This will greatly increase the number of alternative materials.
Solder joints were prepared in the same manner as above from two alloys with wide freezing ranges: Indium Alloy 206 (60Pb/40In, 197° to 231°C) and Indium Alloy 97 (43Pb/43Sn/14Bi, 144° to 163°C). The alloys were sintered at temperatures slightly above the solidus (Table 2). A micrograph of a cold solder joint and the corresponding stress strain diagram are shown in Figures 6 and 7.
Figure 7. Representative shear-stress/shear-strain curve for the solder joint described in Figure 6.
When examining compacts of sintered solder paste, it was noted that large amounts of golden liquid frequently surrounded the compact.20 This liquid is likely unreacted or partially reacted flux. When examining these compacts under a scanning electron microscope, significant charging was observed, indicating the presence of an organic material on the surface. A collaborative study between Rensselaer Polytechnic Institute and Boeing Aircraft Corp. supported by EMPF and the Naval Air Warfare Center, attempted to identify how formation of residue could be avoided by modifying the manufacturing process.23 The results indicate that if the fluxing reaction can be controlled, the likelihood of residue formation is reduced.24 Because the amount of flux required to form an effective joint is reduced if liquid-phase enhanced sintering is used, cleaning is minimal.
Based on these observations, it may be possible to form effective joints with significant reductions in the amount of flux present in the solder paste.
Potential Impact of Using Sinterable Solder Pastes
Although the work described in this article is preliminary, the results indicate that future work is warranted. The following may be possible:
- Replacing the manufacturing infrastructure would be unnecessary, with the possible exception of developing new fluxes, board finishes and soldermasks.
- Sn/Zn alloys may be feasible, because the oxidation of Zn in the solid state will be much slower than in the liquid. Dross will not be a concern.
- The reduction in the amount of required flux prevents the formation of flux residue and reduces the need to clean.
- Alloys with high thermal ranges may be considered because the "cold solder joint problem" will no longer be an issue.
- The solder joints will probably have superior mechanical properties as the grain structure will be finer.
- Hierarchical soldering will become a non-issue because it is unlikely that one will over-sinter a previously sintered joint.
- Concentration variations in the solder joint will no longer be an issue because there is no solidification.
- The spreading of the solder across a printed circuit assembly will not be significant because the metal never melts.
Future Work
While it is not proposed that sintering be established as the method of choice for producing electronic assemblies from lead-free solder alloys, based on the results, the issues raised at IPCWorks
ACKNOWLEDGMENTS
The contributions of the following are gratefully acknowledged: the Provost's Office of Virginia Commonwealth University; the Dean of the School of Engineering; Prof. Jennifer S. Wayne; Prof. M. Samy El-Shall; the White Oak Semiconductor Failure Analysis Team; Heraeus-Cermalloy, Indium Corp. and Alpha Metals; the National Center for Manufacturing Sciences Lead-free and High-temperature Fatigue Resistant Solder Projects; James F. Maguire and Sarah Keough of Boeing during the residue study; Christy Alexander and Brian Nguyen; and Nicole Erdman, David McCall, Sid Ketchum and Jamie Schmidt.
REFERENCES
Available from the author upon request.
MARK A. PALMER is an assistant professor of mechanical engineering at Virginia Commonwealth University, 601 West Main Street, P.O. Box 843015, Richmond, VA 23284-3015; (804) 827-6275; Fax: (804) 828-4269; E-mail: mapalmer@saturn.vcu.edu; Web site: saturn.vcu.edu/~mapalmer/smtsint.htm.