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

A review of lead-free solders for electronics applications

Composite lead-free solders, containing micro and nano particles, have been widely studied. Due to grain boundary drag or Zener drag, these particles can refrain the solder microstructure from coarsening in services, especially for Cu6Sn5, Ag3Sn intermetallic compounds and the β-Sn phases. Due to dispersion hardening or dislocation drag, the mechanical properties of the composite solder alloys were enhanced significantly. Moreover, these particles can influence the rate of interfacial reactions, and some particles can transform into a layer of intermetallic compound. Wettability, creep resistance, and hardness properties were affected by these particles. A systematic review of the development of these lead-free composite solders is given here, which hopefully may find applications in microbumps to be used in the future 3D IC technology.

Structure and properties of lead-free solders bearing micro and nano particles

Microstructure and mechanical properties of Sn–Bi, Sn–Ag and Sn–Zn lead-free solder alloys

During soldering and service, intermetallic compounds (IMCs) have an important impact on the performance and reliability of electronic products. A thin and continuous intermetallic layer facilitates the formation of reliable solder joints and improves the creep and fatigue resistance of solder joints. However, if the IMCs overgrow, the coarse IMC becomes brittle and tends to crack under stress, leading to a decrease in solder joint reliability. Based on the latest developments in the field of lead-free solders at home and abroad, this paper comprehensively reviews the interfacial reaction between SnAgCu Pb-free solders and different substrates and the growth behavior of IMCs and clarifies the growth mechanism of interfacial IMCs. The effects of the modification measures of lead-free solder on the IMCs and reliability of SnAgCu/substrate interface are analyzed, which provide a theoretical basis for the development and application of new lead-free solder.

Interface reaction and intermetallic compound growth behavior of Sn-Ag-Cu lead-free solder joints on different substrates in electronic packaging

Synchrotron radiation real-time imaging technology was conducted to in situ investigate the liquid-solid thermomigration behavior in Cu/Sn/Ni solder joint. Asymmetrical growth of the intermetallic compounds (IMCs) at the two ends (interfaces) was clearly observed in the Cu/Sn/Ni solder joint during reflow on a hot plate. The thickness of the IMC layer at the cold end increased linearly with reflow time, while that at the hot end changed little, resulting in increasing difference in IMC thickness between the two ends with reflow time. Due to the thermomigration of Ni atoms toward the cold end under temperature gradient, the (Cu,Ni) 6 Sn 5 IMCs grew quite fast at the cold end. The changes in the morphology of the (Cu,Ni) 6 Sn 5 grains at cold end associated with the increasing Ni content. Cu atoms from the cold end could migrated across the liquid solder layer to the hot end against temperature gradient, resulting in the formation of a smooth (Cu,Ni) 6 Sn 5 layer at the hot end.

Synchrotron radiation in situ study on liquid-solid thermomigration in Cu/Sn/Ni solder joint

In this paper, by using the synchrotron radiation real-time imaging, the growth of gas bubbles on solid-liquid interface during the soldering process was investigated. It was proved that the formation and growth of the bubbles was a process of heterogeneous nucleation. During the soldering heating insulation stage, the bubbles volume increased first and remained unchanged afterwards. When there were more than one bubbles, the annexation process was happened among the adjacent bubbles, the larger bubbles were pulled to the smaller ones. The bubbles volume remained unchanged during the cooling stage. At the same time, the dissolution of the Cu substrate and the growth of the IMCs were influenced by the bubbles.

In-situ study on the formation and evolution behavior of voids at the interface during soldering process by synchrotron radiation real-time imaging technology

The diffusion behavior of Cu and Ni atoms undergoing liquid-solid thermomigration in Cu/Sn/Cu and Cu/Sn/Ni solder joints was investigated under the temperature gradients of 136.5 °C/cm and 154.0 °C/cm at 250 °C, respectively. Asymmetrical growth of the interfacial IMCs was observed in the Cu/Sn/Cu solder joints, i.e., the IMCs at the cold end were much thicker than those at the hot end. The Cu-Ni cross-solder interaction occurred in the Cu/Sn/Ni solder joints during reflow on hot plate. Cu and Ni atoms arrived to the opposite sides under the concentration gradients, resulting in the formation of (Cu x ,Ni 1−x ) 6 Sn 5 IMCs at both interfaces. The interfacial reaction was accelerated by the combined effect of Cu concentration gradient and thermomigration. While in Cu/Sn/Cu solder joints, the thermomigration process was weakened by Cu concentration gradient. As a result, the IMCs layers at both cold and hot ends in Cu/Sn/Ni solder joints were thicker than those in Cu/Sn/Cu solder joints after the same reflow time.

Interfacial reactions in Cu/Sn/Cu(Ni) systems during soldering under temperature gradient

The microstructure and mechanical properties of Sn-Ag-Cu/Sn-Pb hybrid solder joints with different Pb contents were studied during isothermal aging. Two hybrid solder joints: complete hybrid and partial hybrid solder joints were prepared at the peak temperatures of 220 °C and 240 °C, respectively. A continuous scallop-type Cu6Sn5 formed in three solder joints during reflow, and the thickness of interface intermetallic compound (IMC) increased with the prolongation of aging time. Compared with partial hybrid and Sn-37Pb solder joints, the complete hybrid solder joint had the slowest growth rate of interface IMC and the highest shear strength under aging process. Compared with Sn-37Pb solder joints, complete hybrid and partial hybrid solder joints both exhibited a mixed fracture of ductile and brittle but not ductile fracture at the IMC/solder interface after aging for 1000 h.

Microstructure and mechanical properties of Sn-Ag-Cu/Sn-Pb hybrid solder joints with different Pb contents

In order to improve the mechanical properties of Sn-Au-Cu solder, Indium was added into Sn-Au-Cu solder and five kinds of Sn-Au-Cu-In solders were designed based on cluster-plus-glue-atom (CPGA) model and the melting properties, microstructure, wettability, interfacial reaction and mechanical properties were studied systematically. The DSC results showed that the addition of In reduced the melting point of the Sn-Au-Cu-In solder. The microstructure of Sn-2.85Au-2.34Cu-0.65In solder consisted of white AuSn4 phase, little Cu6Sn5 phase and gray β-Sn matrix, while the other 4 solders consisted of white Au(Sn,In)4 phase, little Cu6Sn5 phase and gray β-Sn matrix. The wettability of five solders on Cu substrate were better than those on Ni substrate. The wettability of Sn-2.81Au-0.89Cu-3.21In solder on Cu substrate was the best; while that of Sn-2.85Au-2.34Cu-0.65In solder on Ni substrate was the best. The shear strength of Sn-2.85Au-2.34Cu-0.65In solder joint was 72 MPa, which was the highest among the five solders.

Effect of In addition on the properties of Sn-Au-Cu lead-free solder

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