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Further Testing of Hi-rel/Hi-temp Lead-Free Alloy Supports Wide Implementation
December 31, 1969 |Estimated reading time: 6 minutes
A year has passed since Hector Steen, Ph.D., Henkel Corporation and a colleague reported on the development of a novel, patented lead-free solder alloy to meet the high-temperature, high-reliability requirements of automotive applications. 1 Since then, additional testing results for the alloy (SnAg3.8Cu0.7 [SAC387] with Bi, Sb, and Ni additions) have confirmed its robust performance and reliability for high-operating-temperature applications. Steen reports the latest information from additional testing.
Current mainstream solders have been unable to meet the demands of high operating temperature, under-the-hood automotive applications. Traditional tin/lead solders have melting points that are too low for these applications and also face increasing environmental restrictions on their use. While SAC alloys have higher melting points, their reliability in high-temperature environments has been found to be inferior even to that of traditional tin/lead alloys. These considerations led to the development of a six-part lead-free alloy.* A brief history of the formulation work will be reviewed in this article. After promising initial results, manufacturers required additional data to be fully convinced of the new alloy’s viability. New results are presented here.
History of the Requirements and Development Process
Backed by users, materials suppliers, and representatives from the academic community, the alloy development process was in-depth and successful. The team worked together to achieve several goals for the alloy.2 It must be a lead-free material; effective in an operating temperature up to 150°C; it must have solder joints that survive 1,000 cycles at -55°C to +150°C; it must reflow at 230°C or below; and it must meet RoHS standards and maintain cost-competitive pricing.
An innovative, multi-element approach was adopted to realize these objectives and resulted in the successful development of a lead-free alloy which is suitable for high-temperature conditions, while enabling reflow still to occur at conventional SAC alloy temperatures. The six-part alloy specifically addresses the particular requirements of automotive, but its relevance extends beyond this narrow market sector.
Following the initial formulation work, additional analysis was conducted to gain a more complete understanding of the alloy’s capabilities and limitations.
Analysis of the SAC387 + Ni, Bi, Sb Alloy
Though some standard testing was carried out during the original formulation process, more extensive evaluation including thermal shock, vibration testing, drop testing, aging impact and ductility was subsequently conducted during a German cooperative project called ‘LIVE.’ This has created a broader understanding of the alloy’s capabilities. Additionally, specific concerns from some manufacturers regarding impact resistance have been addressed through drop testing analysis.
Figures 1A & B. Cycles to 50% joint strength reduction vs. ΔT in thermal shock, solder alloy comparison, 1206 and 2512 components. The new alloy shows significantly better thermal shock performance than SAC or Sn/Pb alloys.4
Thermal shock and push-off strength. Extended to include a broader range of test conditions, results from further thermal shock analysis were presented at the LIVE project conclusion seminar in Berlin, Germany on October 7, 2008.3 Though the initial development work included some thermal shock testing, the more recent evaluation looked at the impact across four temperature ranges from 80° to 190°C, with dwell times of 30 minutes. The number of cycles required to elicit a 50% reduction in joint strength as a function of the temperature cycle range is shown in Figure 1. The figure indicates that the multi-part alloy outperforms both traditional SAC and SnPb alloys by a significant margin under these conditions.
Figures 2A & B. Solder alloy performance on 1206 components on immersion Sn and Au/Ni PCBs in thermal shock -40°/+125°C; the new alloy outperforms other tested alloys.6
Using the same temperature range as that in the initial development work, additional thermal shock and push-off testing was carried out in the LIVE project to include a wider range of alloy comparisons and significantly more thermal cycles.5 The testing also incorporated two different board finishes: immersion Sn (ImmSn) and gold/nickel (Au/Ni). Again, the new alloy showed markedly better results than all other alloys tested (Figure 2).
Figures 3 A-D. Crack length (% of joint) on various components mounted on immersion Sn and Au/Ni PCBs, thermal shock cycles ranged from -40°/+125°C.8
Crack length is an alternative to joint strength as a measure of the degradation of a solder joint in fatigue and is used in some reliability modeling; this parameter was also used in the LIVE project work.7 Figure 3 illustrates these results.
Vibration testing. The original vibration testing that was performed as part of the alloy formulation program indicated that performance under vibration was similar to that of SAC387, and that if a preliminary 500 thermal shock cycles were added, the new alloy was superior to the SAC formulation. Some in-depth thesis work sponsored by an automotive manufacturer studied the impact of vibration on solder joint reliability, comparing the SAC387-based, multi-part alloy’s performance in a high-frequency fatigue testing rig to that of SAC, SnCu, and SnPb alloys. Results (Figure 4) indicate that the six-part alloy’s vibration reliability is comparable with that of traditional SnPb solder, and superior to that of other Pb-free solders.10
Figure 4. Additional vibration testing upheld the original findings that the new alloy’s vibration performance is consistent with that of tin/lead solder.9 The alloy performs similarly to Sn/Pb and outperforms SAC 305 and Sn/Cu.
Drop testing and drop testing vs. aging time. Because the alloy was developed for applications in which mechanical shock (from being dropped) should not be a significant factor (as it is for handheld devices), drop testing was not part of the original testing regime. However, since the alloy is considerably less ductile than both SnPb and SAC alloys, there was concern over drop test performance; drop test analysis was subsequently conducted. Two PCB types — one with an OSP finish and one with NiAu — were dropped end on through a cylinder from a height of 1.5 m onto a steel anvil. The boards contained a range of BGA components; both were reflowed with a standard SAC solder paste and the new alloy solder paste. The drops to first failure were compared and the multi-part alloy’s performance was consistent with and in some cases slightly better than that of SAC.
Figure 5. Drop test performance as a function of aging time was evaluated. The new alloy showed comparable performance to SAC305.12
Drop testing was also carried out in the LIVE project as a function of aging time, where assembled boards were placed in a 150°C oven for up to 500 hours.11 Figure 5 shows the new alloy’s performance to be comparable to that of SAC, specifically SAC305.
Conclusion
Following the reliability testing performed during its development process, the novel lead-free alloy was subjected to additional analysis to better understand its performance capabilities. Results suggest that the alloy offers a significant advance over SAC and other lead-free solders in temperature cycling and thermal shock reliability. In addition, the alloy showed good performance in vibration testing and similar drop test reliability to that of SAC and other lead-free alloys.
Manufacturers of high-operating-temperature, high-reliability products can incorporate this new lead-free alloy confidently based on the testing documented. The flux vehicle in the solder paste can be selected depending on the particular application and the reflow process used. While the solder alloy is critical for the joint properties after reflow, the paste characteristics will determine how well this alloy can be implemented in an assembly process.
* The alloy, SnAg3.8Cu0.7 (SAC387) with Bi, Sb, and Ni additions, is patented under the name Innolot. This alloy is shared by all patent holders involved in the project.
AcknowledgementsThe author would like to acknowledge valuable data from the German LIVE Project closing Seminar, “Material Verhalten von Loten in Mikrobereichen,” Berlin, October 2008.
REFERENCES:1., 2. H. Steen and B. Toleno, “Development of a Lead-free Alloy for High-Reliability, High-Temperature Applications”, SMT, January 2009, http://smtonline.com. 3., 4. Dr R. Ratchev, Presentation in LIVE Project Seminar, Berlin, October 2008..5-8. Prof J. Albrecht, Presentation in LIVE Project Seminar, Berlin, October 2008.9., 10. N. Barry, “Lead-free Solders for High-Reliability Applications: High-cycle Fatigue Studies”, Thesis, Metallurgy and Materials Department, University of Birmingham, October 2008.11.,12. P. Fruhauf, Presentation in LIVE Project Seminar, Berlin, October 2008.
Hector Steen, Ph.D., research associate, may be contacted at Henkel Ltd., Wood Lane End, Hemel Hempstead, Hertfordshire, HP2 4RQ UK; hector.steen@uk.henkel.com.
SMT, January 2010