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

The effect of the concentration gradient lies in the solder on the interfacial reaction was investigated in this paper. Sn-9Zn/Sn double layers solders were reacted with Cu at 250°C for 1-5min. The concentration gradient and its direction affect the composition and the morphology of the reaction products. When the reflow time was less than 3min, the reaction products were Cu5Zn8 and CuZn5, only Cu5Zn8 was found when the reflow time was 5min. Solder pieces with different thicknesses were reacted at 250°C for 5min. Only Cu5Zn8 was found at the interface and the size and the amount of the reaction product changed as the thickness varied. When the thickness of the solder piece was 300μm, bulk type Cu5Zn8 with large faceted surfaces was observed in the fine grains Cu5Zn8 phase next to the Cu substrate.

http://ieeexplore.ieee.org/iel7/6471288/6474554/06474642.pdf

Interfacial reaction between Sn-9Zn/Sn double layers solder and Cu

Three kinds of Sn-Bi-In-(Ag) solder alloys were calculated by the cluster-plus-glue-atom (CPGA) theory. The main purpose of adding In and Ag elements to Sn-Bi solders is to reduce the brittleness of Sn-Bi solders and to promote the mechanical reliability. The addition of In element decreased the melting point and expanded the melting range of the solders. However, the excellent wettability of Sn-Bi-In counteracted the adverse effects of the increase in melting range. Moreover, In promoted the interfacial reaction and transformed the interfacial intermetallic compound (IMC) from scalloped Cu6Sn5 to prismatic Cu6(Sn,In)5. The shear displacement of Sn-46Bi-3.3In solder joint increased by about 16.4 % from 0.85 mm to 0.99 mm. The addition of Ag further enhanced the toughness of the solders. Considering the performance and cost, Sn-46Bi-3.3In-0.5Ag was the optimal low-temperature solder.

Composition design and mechanical properties of low-temperature Sn-Bi-In-(Ag) solders with low Bi content

Sn–Zn–x(Al,Ag) near-eutectic solders, namely Sn–8.3Zn–0.73Ag, Sn–8.4Zn–0.44Al and Sn–7.4Zn–0.26Al–0.68Ag (in wt%) with melting points of 200.74, 198.00 and 197.32 °C, respectively, as well as the Sn–9Zn eutectic solder, were used to join Al and Cu substrates. The addition of Ag led to the formation of dendritic AgZn3 phases, while the addition of Al obviously refined the microstructure of Sn–Zn eutectic, as well as the AgZn3 phases. The Sn–Zn–Al solder possessed the best wettability on both Cu and Al substrates among the four solders. Al4.2Cu3.2Zn0.7 intermetallic compound (IMC) layers formed at the Sn–Zn–x(Al,Ag)/Cu interfaces while Al-rich (Zn) solid solutions at the Sn–Zn–x(Al,Ag)/Al interfaces of all the as-soldered joints. The shear strength of the Al/Sn–Zn–Al/Cu solder joints was the highest among the four solder joints. The declining degree of the shear strength of the Sn–Zn–x(Al,Ag) solder joints in 3.5 % NaCl solution was in agreement with the corrosion-resistance order of the bulk solders. The Al/Sn–Zn–Ag/Cu joint thus owned the best corrosion resistance.

Microstructure and interfacial reaction of Sn–Zn–x(Al,Ag) near-eutectic solders on Al and Cu substrates

Solder Size Effect on Early Stage Interfacial Intermetallic Compound Evolution in Wetting Reaction of Sn3.0Ag0.5Cu/ENEPIG Joints

This study focused on the effect of solder volume on the interfacial reaction between Sn3.5Ag0.75Cu solder balls and Cu pads on PCB after various reflow soldering times. The diameters of the solder balls were 200, 300, 400 and 500 µm, respectively, and the opening diameter of the Cu pads was 250 µm. The solder volume ratio for the 200, 300, 400 and 500 µm balls was approximately 1:4:8:16. The interfacial intermetallic compound (IMC) was common scallop-type Cu 6 Sn 5 . The thickness of the interfacial Cu 6 Sn 5 IMC layer was obviously thicker for the smaller solder ball after the same reflowing times from 1 to 5. After the first reflow, the thickness of the interfacial Cu 6 Sn 5 IMC layer for the 200 µm solder ball was 4.39 µm compared to that of 3.15 µm for the 500 µm solder ball. The average diameter of the interfacial Cu 6 Sn 5 grains for the smaller solder ball was significantly larger than that for the bigger one after the same reflowing times from 1 to 5. After the first reflow, the average diameter of the interfacial Cu 6 Sn 5 grains for the 200 µm solder ball was 5.23 µm compared to that of 4.22 µm for the 500 µm solder ball. Both the thickness of the Cu 6 Sn 5 IMC layer and the average diameter of the interfacial Cu 6 Sn 5 grains increased with the increasing reflow time. The ratios of the average diameter of Cu 6 Sn 5 grains to the thickness of the interfacial Cu 6 Sn 5 IMC layer for all the four kinds of solder balls varied from 1 to 1.5, which showed a vivid growth of the interfacial Cu 6 Sn 5 grains from the perspective of 3 dimensions. Based on the data from the experiment, the dissolution kinetics of the Cu pad can be simulated.

Effect of solder volume on interfacial reactions between Sn3.5Ag0.75Cu solder balls and cu pad

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