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The majority of studies on high-reliability solder alloys have focused on the six-element system of Sn, Ag, Cu, Bi, Sb, and Ni. However, there is limited research on the influence of Co element within this system. In this study, a Sn 3.0Ag 0.5Cu Bi Sb Ni xCo (x = 0 wt%, 0.02 wt%, 0.05 wt%, 0.08 wt%, 0.1 wt%) alloy (referred to as SACBSN-xCo alloy) was prepared using a melting method. The mechanical properties of SACBSN-xCo alloy solder joints were evaluated through ultimate shear strength testing. The composition analysis of the alloy, phase composition examination, intermetallic compound (IMC) investigation and interfacial layer morphology analysis were conducted using ICP, XRD, SEM and EDS techniques respectively. Furthermore, the evolution process of solder structure and solder joint interface layer under different aging times was observed in detail. Results indicate that with the addition of Co element in the alloy solder system two heat release peaks appear during the solidification process; specifically when adding 0.05 wt% Co element content to the mixture it reduces supercooling degree by 15.17 °C to only 1.03 °C; Moreover wettability improvement can be achieved to some extent when adding either 0.02 wt% or 0.05 wt% Co content. The addition of trace Co can inhibit the excessive growth of IMC in the solder alloy matrix and refine the alloy structure. It can promote the growth of Cu 6 Sn 5 -based IMCs and inhibit the growth of Cu 3 Sn layer in intermetallic compound layer (IMCs). In addition, the mechanical properties and thermal fatigue stability of the solder joints are steadily improved by Co element. After adding Co element, the shear strength of the alloy solder joint is increased by about 14.84 %. After aging at 150 °C for 25 days, the shear strength of SACBSN-xCo alloy solder joints is increased by about 20.4 %, which significantly improves the thermal fatigue stability of the solder joints after high temperature aging treatment. The results show that when Co content is 0.05 wt%, the alloy solder has better comprehensive properties. • The addition of Co element can increase the nucleation point in the solder matrix, refine the structure, reduce the formation of large size intermetallic compounds and inhibit their overgrowth during high temperature treatment. • The addition of Co element can increase the nucleation density of IMC, and promote the growth of Cu6Sn5-based IMCs. Thicker Cu6Sn5-based IMCs act as a good diffusion barrier, preventing the mutual diffusion of Sn and Cu atoms. Thus impeding the growth of the Cu3Sn layer. • The addition of Co can effectively improve the shear resistance of the solder joint, and make the shear resistance of the solder joint show excellent stability in the process of high temperature aging treatment. [ABSTRACT FROM AUTHOR]

Based on the alloy Sn1.0Ag0.5Cu, Sb powder is added at a certain mass percentage to form the solder alloy Sn1.0Ag0.5CuxSb (x = 0 wt%, 0.5 wt%, 1.5 wt%). The melting point of the solder was determined by differential scanning calorimetry, the alloy interface morphology was observed by scanning electron microscopy, and energy-dispersive X-ray spectroscopy was performed to analyze phase compositions. Thermo-Calc software was used to calculate the variation in phase content, the solidification curve, Gibbs energy, the driving force, liquid/solid phase lines, the phase diagram, and the nucleation radius of Sn1.0Ag0.5CuxSb(wt%). In addition, the calculation results were used to explain the interface growth mechanism. According to the results, the precipitation/transition temperature of each phase and the content of the newly formed phase SnSb gradually increased with the increase in Sb content. After the addition of Sb, both temperatures of the liquid-phase line and the solid-phase line slightly increased, and the calculated values showed the same trend as the measured values. Two heat absorption peaks were observed during the melting process. The Gibbs energy and nucleation radius of the Cu 6 Sn 5 phase increased with the increase in Sb content, indicating that Sb can inhibit the growth of the Cu 6 Sn 5 phase. The addition of Sb rendered the Sn-rich phase denser and finer and reduced the thickness of the intermetallic compound layer and the generation of cracks; these changes intensified with increasing aging time. Comprehensive analysis revealed optimal reliability of the solder for an Sb content of 0.5 wt%. • With the increase of Sb content, the precipitation/transition temperature of each phase gradually increased, and the new phase SnSb phase gradually increased. • The Gibbs energy and nucleation radius of Cu 6 Sn 5 phase increased with the increase of Sb content. • Sb can obviously reduce the thickness of IMC layer and the crack generation. [ABSTRACT FROM AUTHOR]

Recently multi-alloying has been used for optimization of the SnAgCu (SAC) solder. To comprehensively reveal the effects of multi-alloying on mechanical property of the SAC solder, in this study the Sb, Bi, Ni, In and Ce elements were added into the Sn-3.8Ag-0.7Cu (wt%) solder, then the deformation and fracture behaviors of the multi-alloyed SAC solder (MAS) and MAS/Cu solder joint were in-situ observed and compared with the SAC. The results reveal that more intermetallic compounds (IMCs) and some Bi particles are formed in the MAS, while the (Cu,Ni) 6 Sn 5 IMC in the MAS is refined. Due to the solid solution strengthening and precipitation strengthening, yield strength of the MAS increases by about 25 % compared with the SAC. Under tensile stress, severe slip and grain rotation occurs in the SAC solder, while plastic deformation of the MAS is restrained to a high extent by the dispersed particles within it. The MAS fractures in a tearing mechanism without obvious necking, but still shows a relative high elongation (~35 %), and the MAS has superior creep resistance since the dislocation motion and grain rotation are restrained. The deformation mismatch between the MAS and the Cu substrate is less serious, and the IMC at the MAS/Cu interface are less likely to fracture, resulting in a higher tensile strength (~160 MPa). Multi-alloying is recommended to improve the strength and creep resistance of the SAC. • The deformation behaviors of the multi-alloyed SAC solder (MAS) are investigated. • In-situ observation was employed to show the dynamic deformation process. • The grain rotation and strain localization are restrained in the MAS. • The MAS fractures at a strain of 35 % without necking. • The MAS shows much higher creep resistance than the SAC. [ABSTRACT FROM AUTHOR]