Properties of lead-free solder SnAgCu containing minute amounts of rare earth

Abstract

Because of excellent wetting and mechanical properties, SnAgCu solder alloys have been regarded as the most promising Pb-free substitutes for the SnPb solder. The Sn-3.8Ag-0.7Cu solder has garnered attention because of its creep resistance. However, under the drives of increasingly finer pitch design and severe service conditions, novel lead-free solders with higher creep performance may be needed. Adding a surface-active element to an alloy is an effective way to improve the high-temperature performance of the solder. The present work focuses on the effect of rare earth (RE) on the physical properties, spreading property, and mechanical properties of SnAgCu solder. Results show that the creep-rupture life of SnAgCu solder joints at room temperature could be notably increased by adding a minute amount of RE, up to 7 times more than that of SnAgCu solder joints when containing 1.0wt.%RE. The differential scanning calorimetry (DSC) curves indicated that the melting temperature of SnAgCu solder with RE increased a little, and no lower melting-temperature, eutectic endothermal peak appears on the DSC curve. The electrical conductivity of the solder decreased slightly, but it is still superior to the SnPb eutectic solder. Compared to that of SnPb solder, the coefficient of thermal expansion (CTE) of SnAgCu (RE) is closer to copper, which usually serves as the substrate of printed circuit boards (PCBs). It is assumed that this will comparably reduce the thermal stress derived from thermal mismatch between the solder and the PCBs. The RE had no apparent effect on the spreading property, but when RE added up to 1.0 wt.%, the spreading area of the solder on the copper substrate decreased, obviously, because of mass oxide. The RE improved the ultimate tensile strength little, but it increased the elongation up to 30%. However, as the content of the RE increases, the elongation of the solder gradually decreased to the level of SnAgCu when the RE exceeds 0.25 wt.%. Additionally, RE made the elastic modulus of SnAgCu solder increase, so the resistance to elastic deformation of the solder is enhanced. The microstructure of SnAgCuRE led to a refining trend as the RE content increased. The RE compounds appeared in the solder when RE was 0.1 wt.%. This deteriorates the mechanical properties of the solder. The fractography of the tensile specimen containing 0.1 wt.% indicated a superior ductility to Sn-3.8Ag-0.7Cu bulk solder. However, as RE is increased to 1.0 wt.%, the fractography shows less ductile characteristics, which is believed to serve as the reason that the elongation of solder degrades as RE increases. Summarily, the most suitable content of RE is within 0.05-0.5 wt.% and is inadvisable beyond 1.0 wt.%.

Impact of Thermal Aging on the Microstructure Evolution and Mechanical Properties of Lanthanum-Doped Tin-Silver-Copper Lead-Free Solders

“…This form is dispersed thoroughly in the bulk alloy. 5,8 Due to the small size of these oxides, they have not been determined exactly.…”

“…This refinement is due to the particular effect of lanthanum adsorption at different planes in the SAC alloys. 8,13,25 Mechanical Properties Figure 10a shows the yield stress and tensile strength for the as-cast samples at room temperature. A significant increase in both yield stress and tensile strength is observed.…”

“…This may be due to an increase in the quantity of hard, RE-bearing particles. 8,13 In Ref. 26, it is shown that lanthanum results in the formation of LaSn 3 compounds.…”

“…7 In Ref. 8, addition of small traces of RE elements reduced the size of Ag 3 Sn and Cu 6 Sn 5 IMCs, while in Refs. 6,11 the impact of lanthanum on the microstructure evolution due to isothermal aging was studied for eutectic Sn-3.5Ag alloy.…”

“…[5][6][7][8][9][10] They are widely used in Sn-Zn, Sn-Ag, and Ag-Cu alloys. [6][7][8] Lanthanum (La) is considered as a good additive due to its lower cost, wide availability, and low melting point 4 as compared with other RE elements.…”