Influence of Lanthanum Additions on the Microstructure and Microhardness of Sn-3.5Ag Solder

Abstract

This study evaluates the effects of different amounts of lanthanum (La) additions on the microstructure and microhardness of Sn-3.5Ag solders. Sn-3.5Ag-xLa ternary solders were prepared by adding 0 wt.% to 1.0 wt.% La to Sn-3.5Ag alloy. Copper substrates were then dipped in the molten solders and these samples aged at 150°C for up to 625 h. The microstructure and microhardness of the as-solidified solder and the aged solder/copper samples were investigated. The Sn-3.5Ag-xLa solders comprised b-Sn, Ag 3 Sn, and LaSn 3 phases, and their microstructure was refined by La additions. As-cast, the addition of La increased the microhardness of the Sn-Ag solder due to the refining effect of Ag 3 Sn particles and increased formation of LaSn 3 compounds. As the aging time was increased, the microhardness of the solders decreased and the Ag 3 Sn compounds coarsened. However, the coarsening of Ag 3 Sn compounds was retarded by La, and the size and amount of LaSn 3 compounds did not change perceptibly with aging time. Therefore, La additions can improve the microhardness and thermal resistance of solder joints.

Mechanical testing and characterization of tin–zinc–antimony–lanthanum lead-free solders produced using mechanical alloying and furnace melting techniques

“…The average hardness of Composition 1 was 17.8 HV and that of Composition 2 was 19.4 HV. Lee and Chen (2009) reported that the addition of lanthanum to Pb-free alloys increased their hardnesses. Zulai et al (2021) discovered that doping the alloy with lanthanum, a rare earth element, increased the hardness, and this was also found in the work reported here.…”

Effect of La addition on adhesive strength and fracture behavior of Sn–3.5Ag solder joints

“…In a previous study [9], it was shown that the refined microstructure and large number of LaSn 3 compounds in La-containing solders give rise to a hardening effect in the as-cast state. In this study, the relationship between the hardness and the adhesive strength of the solder joint is further explored.…”

“…It was also shown that tiny amounts of RE addition yielded no more than a small improvement in the ultimate tensile strength, but increased the elongation under tensile testing by up to 30%. Many researchers have shown that the addition of RE elements to solder systems results in a refinement of the microstructure [5][6][7][8][9]. In addition, it has been shown that RE addition suppresses the growth of the intermetallic compound (IMC) layer in solder joints, thereby improving the mechanical strength [9][10][11].…”

“…Lee and Chen [9] reported that the addition of La to Sn-3.5Ag solder has a microstructure refining effect and suppresses the growth of the Cu 6 Sn 5 IMC layer. Furthermore, it was shown that LaSn 3 compounds are formed in the Sn matrix for La additions of more than 0.1 wt% and increase in both size and number as the amount of La addition increases.…”

Interfacial IMC layer growth and tensile properties of low‐silver Cu/SACBE/Cu solder joints

“…For example, Li and Shi (2006) and Wang et al (2011) reported that the addition of a particular amount of Bi in a Sn-Ag-Cu solder alloy could refine the grain size of the IMC and inhibit excessive IMC growth at the interface. Additionally, some studies (Wu et al, 2004;Lu et al, 2007;Dong et al, 2008;Lee and Chen 2009) on the use of rare earth elements (RE) in the solder indicated that RE doping of the solder alloy reduced the IMC layer thickness and the interdiffusion rate of interfacial reaction in a ball grid array package.…”