Effects of intermetallic compounds on properties of Sn–Ag–Cu lead-free soldered joints

Due to the inherent toxicity of lead (Pb), environmental regulations around the world have been targeted to eliminate the usage of Pb-bearing solders in electronic assemblies. This has prompted the development of “Pb-free” solders, and has enhanced the research activities in this field. In order to become a successful solder material, Pb-free alloys need to be reliable over long term use. Although many of these alloys possess higher strength than the traditional Sn–Pb ones, there still exist reliability problems such as electromigration and creep. Also, the solderability of many Pb-free alloys is inferior to that of Sn–Pb and any improvement or replacement will be welcomed by industry. In order to develop new Pb-free solders with better properties, trace amounts of rare earth (RE) elements were selected by some researchers as alloying additions into Sn-based solders. These solder alloys are mainly Sn–Ag, Sn–Cu, Sn–Zn and Sn–Ag–Cu. In general, the resulting RE-doped solders are found to have better performances than their original ones. The improvements include better wettability, creep strength and tensile strength. In particular, the increase in creep resistance in some RE-doped alloys gives creep rupture time increases by over four times for Sn–Ag and seven times for Sn–Cu and Sn–Ag–Cu. Like other Sn-based alloys, their creep rates are controlled by dislocation pipe diffusion in the Sn matrix. Also, it was found that the creep rate of these Sn-based alloys can be represented by a single empirical equation. With the addition of RE elements, solders for bonding on difficult substrates such as on semiconductors, diamond, and optical materials have also been developed. This report summarizes the effect of RE elements on the microstructure, mechanical properties, wetting behavior of certain Pb-free solder alloys. As an illustration of the advantage of RE doping, interfacial studies were carried out for electronic interconnections with RE-doped Pb-free alloys. It was found that the intermetallic compound (IMC) layer thickness and the amount of interfacial reaction were reduced in a Ball Grid Array (BGA) package. These results indicate that RE elements would play an important role in providing better electronic interconnections.

Properties of lead-free solder alloys with rare earth element additions

The characterization of lead-free solders, especially after isothermal aging, is very important in order to accurately predict the reliability of solder joints. However, due to lack of experimental testing standards and the high homologous temperature of solder alloys (Th > 0.5Tm even at room temperature), there are very large discrepancies in both the tensile and creep properties provided in current databases for both lead-free and Sn–Pb solder alloys. Some recent researches show that the room temperature aging has significant effects on mechanical properties of solders. This paper is intended to review all available data in the field and give rise to the possible factors including room temperature effects which causes the large discrepancies of data. This review of the research literatures has documented the dramatic changes that occur in the constitutive and failure behavior of solder materials and solder joint interfaces during isothermal aging. However, these effects have been largely ignored in most previous studies involving solder material characterization or finite element predictions of solder joint reliability during thermal cycling. It is widely acknowledged that the large discrepancies in measured solder mechanical properties from one study to another arise due to differences in the microstructures of the tested samples. This problem is exacerbated by the aging issue, as it is clear that the microstructure and material behavior of the samples used in even a single investigation are moving targets that change rapidly even at room temperature. Furthermore, the effects of aging on solder behavior must be better understood so that more accurate viscoplastic constitutive equations can be developed for SnPb and SAC solders. Without such well-defined relationship, it is doubtful that finite element reliability predictions can ever reach their full potential.

A review of mechanical properties of lead-free solders for electronic packaging

Effects of fourth alloying additive on microstructures and tensile properties of Sn–Ag–Cu alloy and joints with Cu

During the solidification of solder joints composed of near-eutectic Sn–Ag–Cu alloys, the Sn phase grows rapidly with a dendritic growth morphology, characterized by copious branching. Notwithstanding the complicated Sn growth topology, the Sn phase demonstrates single crystallographic orientations over large regions. Typical solder ball grid array joints, 900 μm in diameter, are composed of 1 to perhaps 12 different Sn crystallographic domains (Sn grains). When such solder joints are submitted to cyclic thermomechanical strains, the solder joint fatigue process is characterized by the recrystallization of the Sn phase in the higher deformation regions with the production of a much smaller grain size. Grain boundary sliding and diffusion in these recrystallized regions then leads to extensive grain boundary damage and results in fatigue crack initiation and growth along the recrystallized Sn grain boundaries.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400093729

The microstructure of Sn in near-eutectic Sn-Ag-Cu alloy solder joints and its role in thermomechanical fatigue

The microstructures and tensile properties of three typical Sn–Ag–Cu alloys, Sn–30wt%Ag–05wt%Cu, Sn–35wt%Ag–07wt%Cu and Sn–39wt%Ag–06wt%Cu, prepared under three different cooling conditions were evaluated after casting The microstructures of all rapidly cooled specimens consisted of the eutectic phase of β-Sn with fine fibrous Ag3Sn dispersion surrounding primary β-Sn grains The slowly cooled Sn–35Ag–07Cu and Sn–39Ag–06Cu alloys exhibited additional large primary Ag3Sn platelets, while the Sn–30Ag–05Cu did not For all alloys, both ultimate tensile strength and 02% proof stress increased with increasing strain-rates in tensile tests Lowering cooling speed decreased tensile strength Elongation increased with an increasing strain rate from 10−5 to 10−2 s−1, and decreased slightly at 10−1 s−1 for the rapidly cooled specimens Elongation remarkably decreased for the slowly cooled Sn–35Ag–07Cu and Sn–39Ag–06Cu alloys, a degradation attributable to the formation of large primary Ag3Sn platelets

S.H Huh

Papers

Effects of cooling speed on microstructure and tensile properties of Sn–Ag–Cu alloys