Effect of addition of manganese and antimony on viscoplastic properties and cyclic mechanical durability of low silver Sn-Ag-Cu solder

The amount of silver (Ag) in SAC alloys has undergone progressive reduction due to cost and concerns related to mechanical properties and toxicity of Ag. Micro-alloying via addition of various amounts of minor/micro dopants, such as manganese (Mn), antimony (Sb), nickel (Ni), cobalt (Co) or rare earth elements, such as lanthanum (La) or cerium (Ce), is currently being explored in the electronic packaging industry as a means for producing fine-grained, stable microstructures in SAC solders, to improve reliability under different types of loading conditions such as thermal cycling, mechanical cycling, vibration, drop, and shock testing. These solders are often termed SAC-X solders, X being the micro-alloying element(s). However, the mechanical properties of these new alloys are not yet well characterized. The current study investigates time-dependent viscoplastic response and isothermal cyclic durability of two relatively new SAC105-X solders - Sn1.0Ag0.5Cu (SAC105) doped with 0.05 wt-percent Mn (termed SAC105-05Mn in this paper) and with 0.55 wt-percent Sb (termed SAC105-55Sb in this paper). The above two compositions are chosen based on their improved drop durability response reported in the literature. Morphology and distribution of intermetallic compounds in as-solidified microstructures were investigated in this work. Results show that the addition of Mn or Sb increases the creep resistance of SAC105 solder by one to two orders of magnitude. The addition of Mn was found to be more effective than that of Sb in improving the creep resistance of SAC105 solders at stress levels lower than 10MPa. However, at higher stress levels (more than 10MPa) the effect of Mn and Sb addition becomes indistinguishable. Cyclic isothermal mechanical durability tests were also conducted at room temperature at a constant strain rate of approx 5.5E-2/sec. It was found that addition of trace elements does not significantly change the cyclic mechanical durability of SAC105 solder. Cyclic- isothermal mechanical durability of the above materials was also studied as a function of varying aging treatments.