Effect of Lanthanum Doping on the Microstructure of Tin-Silver Solder Alloys

Abstract

In this study, quantitative microstructure studies were performed on multiple length scales to investigate the effect of lanthanum (La) doping on Sn-Ag leadfree solder materials. Factors considered in this paper include doping amount, aging temperature, and aging time. It was found that La doping reduces the grain size significantly, and the reduced grain size remains stable during thermal aging. The size of the Ag 3 Sn particles is also greatly reduced by La doping, and the particles coarsen during thermal aging, albeit at a much reduced rate than in the undoped alloy. The rate of particle coarsening can be described by a cubic-root law. Another observation is that the interparticle spacing remains unaffected by the doping. Therefore, higher La doping level leads to higher volume fraction of the eutectic region due to the increased total number of Ag 3 Sn particles.

Effect of Lanthanum Doping on the Microstructure Evolution and Intermetallic Compound (IMC) Growth during Thermal Aging of SAC305 Solder Alloy

“…Moreover It was also observed that quantity La5Sn3 IMC depend upon the concentration La doping i.e. with high 0.5wt% of lanthanum bigger snowflakes has be observed as shown in Figure 8 while small isolated snow can be seen with less La doping which is consistent with the results of Min et al [25].…”

“…Moreover It was also observed that quantity La5Sn3 IMC depend upon the concentration La doping i.e. with high 0.5wt% of lanthanum bigger snowflakes has be observed as shown in Figure 8 while small isolated snow can be seen with less La doping which is consistent with the results of Min et al [25].La doping greatly reduces the Ag 3 Sn and Cu 6 Sn 5 particle size particle during thermal aging. Surface area averaged particle size of Cu 6 Sn 5 particles was obtained as per ASM handbook [95].…”

“…Ag 3 Sn, Cu 6 Sn 5 and La 5 Sn 3 . Previous studies [25,57,61,86] found that Lanthanum doped SAC alloy will cause LaSn 3 IMCs while our results shows the presence of La 5 Sn 3 instead of LaSn 3.…”

“…They have been regarded as the vitamin of metals, whereby addition of a small quantity of rare earth elements may dramatically change the performance of metals e.g refining the microstructure . They can more easily agglomerate at the grain/ dendrite boundary and lower the grain/dendrite boundary energy to stabilize the boundaries and restrain the moving or sliding of the boundaries [25]. Some studies consider that RE elements are adsorbed at the grain/dendrite boundaries of the IMC and alter the relative correlation of the growth velocities between the crystalline directions of the polycrystalline IMC, which decrease the size of the IMC particles and distributes the IMCs more uniformly [81][82][83][84][85][86][87].…”

“…However, a thick IMCs layer at the solder/conductor metal interface significantly reduces the reliability of the solder joints because of their inherent brittle nature and their tendency to generate structural defects because physical properties (such as elastic modulus and coefficient of thermal expansion) of IMCs are not compatible with parent metal i.e. SAC alloy [1,10,[19][20][21][22][23][24][25][26]. IMC have much higher strength than the bulk solder material [27,28].…”