Editor's Note: This column originally appeared in the May 2012 issue of SMT Magazine.

Last month’s column outlined the correlation of general mechanical properties and physical properties (specifically melting temperature and liquidus temperature) to the Ag dosage and Cu dosage, respectively, in the SAC solder system. What is the scientific basis behind this correlation as exhibited by the test results? Better yet, how do the scientific fundamentals anticipate the solder’s properties and performance? The information below outlines the key points of the underlying principles and operating phenomena.

In a SnAgCu system, metallurgical reactions between Sn and the minor elements, Ag and Cu, are the primary resources in determining the application temperature and the mechanism of solidification, thus microstructure, which, in turn, controls mechanical properties. The metallurgical reactions referred herein may or may not be an alloying mechanism.

There are three probable binary eutectic reactions among the three components of Sn, Ag, and Cu. A reaction between Ag and Sn forms a eutectic structure of Sn-matrix phase and Σ intermetallic compound phase (Ag₃Sn) at 221°C. Cu reacts with Sn to form a eutectic structure of Sn-matrix phase and η intermetallic compound phase (Cu₆Sn₅) at 227°C. Ag can also react with Cu to form a eutectic structure of Ag-rich α phase and Cu-rich α phase at 779°C.

However, no phase transformation at 779°C was detected in the solidification thermal measurement for the SnAgCu ternary compositions as studied (H-Technologies Group Internal Research Report, 1998). This indicates that it is less likely for Ag and Cu to directly react in the ternary composition range as discussed herein. Instead, it is more thermodynamically favorable and kinetically feasible for Ag or Cu to react with Sn to form Ag₃Sn or Cu₆Sn₅ intermetallic compounds. Therefore, the SnAgCu ternary reaction is expected to consist of Sn-matrix phase, Σ intermetallic compound phase (Ag₃Sn), and η intermetallic compound phase (Cu₆Sn₅).

The relatively hard Ag₃Sn and Cu₆Sn₅ particles in the Sn-matrix of SnAgCu ternary alloys can effectively strengthen the alloy through building a long-range internal stress. These hard particles can also serve as the most effective blocks for fatigue crack propagation. The formation of Ag₃Sn and Cu₆Sn₅ particles can partition finer Sn-matrix grains. The finer the Ag₃Sn and Cu₆Sn₅ particles are, the more effectively they partition Sn-matrix grains, resulting in the overall finer microstructure. This, in turn, facilitates grain boundary gliding mechanisms which accounts for the extended fatigue lifetime under elevated temperatures when the grain boundary gliding is a dominating mechanism barring the presence of other extraneous failure mechanisms.

The correlation of the mechanical properties to Ag content and Cu content, respectively is  summarized as follows: When Ag is around 3.0 to 3.1%, both yield strength and tensile strength increase almost linearly with Cu contents up to approximately 1.5% Cu. Beyond 1.5% Cu, the yield strength decreases with any further increase in Cu, but alloy tensile strength remains steady. Overall the alloy plasticity is high for the compositions having the Cu in the range of 0.5 to 1.5% and then decreases with further increase in Cu. With respect to Ag content (at a range of 0.5 to 1.7% Cu), both yield and tensile strength  increase almost linearly with Ag up to 4.1%; but the plasticity decreases with Ag.