Lead-free solder: tin/silver/copper system

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Lead-free solder: tin/silver/copper system

[Source: Surface Mount Technology] [Author: toptouch] [Time: 2005-1-10 10:17:46] [Clicks: 5224]

Lead-free solder: Tin/Silver/Copper system The best chemical composition in the Tin/Silver/Copper system is 95.4Sn/3.1Ag/1.5Cu, which has good strength, fatigue resistance and plasticity. 　　Fundamental properties and phenomena In the Sn/Ag/Cu system, the metallurgical reaction between Sn and the minor elements (Ag and Cu) is the main factor determining the application temperature, solidification mechanism and mechanical properties. According to the binary phase diagram, there are three possible binary eutectic reactions between these three elements. A reaction between Ag and Sn forms a eutectic structure of a Sn matrix phase and an ε intermetallic compound phase (Ag3Sn) at 221°C. Copper reacts with Sn to form a eutectic structure of a Sn matrix phase and an η intermetallic compound phase (Cu6Sn5) at 227°C. Silver can also react with Cu to form a eutectic alloy of Ag-rich α phase and Cu-rich α phase at 779°C. However, in the current study1, no phase transition was found at 779°C for the solidification temperature of the Sn/Ag/Cu ternary compound. This indicates that it is likely that Ag and Cu react directly in the ternary compound. The temperature dynamics are more favorable for Ag or Cu to react with Sn to form Ag3Sn or Cu6Sn5 intermetallic compounds. Therefore, the Sn/Ag/Cu triple reaction can be expected to include a Sn matrix phase, an ε-metal compound phase (Ag3Sn), and an η-metal compound phase (Cu6Sn5). 　　As confirmed for the dual phase Sn/Ag and Sn/Cu systems, the relatively hard Ag3Sn and Cu6Sn5 particles in the Sn/Ag/Cu ternary alloys in the Sn matrix effectively strengthen the alloy by establishing a long term internal stress. These hard particles also effectively block the propagation of fatigue cracks. The formation of Ag3Sn and Cu6Sn5 particles separates the finer Sn matrix particles. The finer the Ag3Sn and Cu6Sn5 particles, the more effectively they separate the Sn matrix particles, resulting in an overall finer microstructure. This facilitates the sliding mechanism at the grain boundaries, thereby extending the fatigue life at elevated temperatures. Although the specific formulation of Ag and Cu in the alloy design is critical to achieving the mechanical properties of the alloy, the melting temperature was found to be insensitive to variations in the content of 0.5-3.0% Cu and 3.0-4.7% Ag. 　　The interrelationships of the mechanical properties to the silver and copper contents are summarized as follows2: When the silver content is about 3.0-3.1%, both the yield strength and the tensile strength increase almost linearly with the copper content up to about 1.5%. Above 1.5% copper, the yield strength decreases, but the tensile strength of the alloy remains stable. The overall alloy plasticity is high for 0.5-1.5% copper and then decreases with further increases in copper. For silver contents (0.5-1.7% copper range), both the yield strength and the tensile strength increase almost linearly with the silver content up to 4.1%, but the plasticity decreases. 　　At 3.0-3.1% silver, fatigue life reaches a maximum at 1.5% copper. It was found that increasing the silver content from 3.0% to higher levels (up to 4.7%) did not improve the mechanical properties. When both copper and silver were formulated at higher levels, plasticity was compromised, such as 96.3Sn/4.7Ag/1.7Cu. 　　The alloy 95.4Sn/3.1Ag/1.5Cu is considered to be the best. Its good properties are the result of fine microstructure formation, which gives high fatigue life and ductility. For solder alloys with 0.5~0.7% copper, any silver content above about 3% will increase the particle volume fraction of Ag3Sn, resulting in higher strength. However, it will not increase fatigue life any more, probably due to the formation of larger Ag3Sn particles. At higher copper contents (1~1.7%Cu), the larger Ag3Sn particles may outweigh the effect of the higher Ag3Sn particle volume fraction, resulting in reduced fatigue life. When copper exceeds 1.5% (3~3.1%Ag), the Cu6Sn5 particle volume fraction will also increase. However, strength and fatigue life will not increase further with copper. In the Sn/Ag/Cu triple system, 1.5% Cu (3~3.1% Ag) is most effective in producing the appropriate number of Cu6Sn5 particles with the finest microstructure size, thereby achieving the highest fatigue life, strength and ductility. 　　Alloy 93.6Sn/4.7Ag/1.7Cu is reported to be a triple eutectic alloy at 217°C3. However, in cooling curve measurements, no precise melting temperature was observed for this alloy composition. Instead, a small temperature range of 216~217°C was obtained. 　　This alloy composition offers the highest tensile strength of the triple alloy compositions studied so far, but its ductility is much lower than that of 63Sn/37Pb. The yield strength of the alloy 95.4Sn/4.1Ag/0.5Cu is lower than that of 95.4Sn/3.1Ag/1.5Cu. The fatigue life of 93.6Sn/4.7Ag/1.7Cu is lower than that of 95.4Sn/3.1Ag/1.5Cu. If the grain boundary sliding mechanism is the main determinant of the eutectic solder alloy, then 95.4Sn/3.1Ag/1.5Cu, rather than 93.6Sn/4.7Ag/1.7Cu, should be closer to the true eutectic behavior. 　　In addition, 95.4Sn/3.1Ag/1.5Cu has economic advantages over 93.6Sn/4.7Ag/1.7Cu and 95.4Sn/4.1Ag/0.5Cu. 　　Compared with 63Sn/37Pb, alloy compositions of 3.0~4.7%Ag and 0.5~1.7%Cu generally have higher tensile strength than 63Sn/37Pb. For example, 95.4Sn/3.1Ag/1.5Cu and 93.6Sn/4.7Ag/1.7Cu are much better than 63Sn/37Pb in strength and fatigue properties. The plasticity of 93.6Sn/4.7Ag/1.7Cu is lower than that of 63Sn/37Pb, while the plasticity of 95.4Sn/3.1Ag/1.5Cu is higher than that of 63Sn/37Pb. 　　Compared with 96.5Sn/35Ag, 95.4Sn/3.1Ag/1.5Cu has a melting temperature of 216~217°C (almost eutectic), which is about 4°C lower than the eutectic 96.5Sn/3.5Ag. When comparing basic mechanical properties with 96.5Sn/3.5Ag, the specific alloy composition in the study performs better in strength and fatigue life. However, alloy compositions containing higher silver and copper, such as 93.6Sn/4.7Ag/1.7Cu, have lower plasticity than 93.6Sn/4.7Ag. 　　Compared with 99.3Sn/0.7Cu, the tin/silver/copper alloy with 3.0~4.7%Ag and 0.5~1.5%Cu has better strength and fatigue properties, but lower plasticity than 99.3Sn/0.7Cu. The best alloy composition recommended in the tin/silver/copper system is 95.4Sn/3.1Ag/1.5Cu, which has good strength, fatigue resistance and plasticity. However, it should be noted that the lowest melting temperature that can be achieved in the tin/silver/copper system is 216~217°C, which is too high to be suitable for circuit board applications under current SMT structures (melting temperatures below 215°C are considered to be a practical standard). 　　In summary, alloy compositions of the Sn/Ag/Cu system containing 0.5~1.5%Cu and 3.0~3.1%Ag have fairly good physical and mechanical properties. Comparatively, 95.4Sn/3.1Ag/1.5Cu costs less than those alloys with higher silver contents, such as 93.6Sn/4.7Ag/1.7Cu and 95.4Sn/4.1Ag/0.5Cu. In some cases, higher silver contents may reduce certain properties. References H-Technoloies Group Inc. Internal Report — Sn/Ag/Cu System, 1998 Jennie S. Hwang, Lead-free solder — Technology & Applications for Environmentally Friendly Manufacturing, Chapter 8, Electrochemical Publications, Great Britain, to be published Fall 2000. Iver E. Anderson, Frederick G. Yost, John F. Smith, Chad M. Miller and Robert L. Terpstra, Iowa State University Research Foundation Inc. and Sandia Corp., "Pb-free Sn/Ag Ternary Eutectic," US patent 5527628, June 18, 1996. Dr. Jennie S. Hwang, may be contacted at (440) 349-1968; Fax: (216) 464-5728; E-mail: JSHwang@aol.com .