Abstract
Low-temperature lead (Pb)-free solders are demanding in the electronic packaging industry, because it would open the door for various economic choices of polymeric materials as substrates and also revives the lower cost processes. Here, we proposed a tin–bismuth–indium–gallium (Sn-52.5Bi-2.68In-1Ga, SBIG (in wt.%)) quaternary low-temperature solder, designed based on systematic CALPHAD (CALculation of PHAse Diagram)-type thermodynamic calculations and corresponding key experiments. The solidification behavior of SBIG was carefully elaborated based on the computations using the lever rule and the Scheil model, and the experiments in terms of thermal analyses and microstructures of sample produced with step-quenching and various cooling rates. The mechanical properties of as-cast and 80 °C-annealed SBIG as well as their microstructures and fracture surfaces were evaluated in the tensile tests. The proposed SBIG solder is with a low liquidus temperature of 141.9 °C and is typically composed of the primary (Sn) phase, the (Sn) + (Bi) eutectic structure and a small amount of (Ga) phase. Air cooling has been identified as a satisfactory cooling rate, which would not lead to the formation of the brittle BiIn intermetallic compound. The as-cast SBIG solder exhibited high yield strength (YS) of 43.7 MPa, high ultimate tensile strength (UTS) of 53.3 MPa and an extremely large elongation of 97.3% as comparing to the conventional eutectic Sn-58Bi solder (YS: 43.1 MPa, UTS: 49.5 MPa, and elongation: 37.5%). However, the proposed SBIG solder does not possess qualified thermal stability, that significant degradation in both strength and elongation were observed after being subjected to extensive thermal ageing at 80 °C for 504 h.
Highlights
Soldering is one of the most important joining technologies for various levels of electronic packaging [1,2]
We performed a virtual experiment by considering the alloy compositions below 80 °C
We performed a virtual experiment by considering the alloy compositions examined in the literature [16], in which the content of solidified near the (Sn) was fixed at 42 wt.%, while the content examined in the literature [16], in which the content of Sn was fixed at 42 wt.%, while the content of of InInwas fromthe the based
Summary
Soldering is one of the most important joining technologies for various levels of electronic packaging [1,2]. In order to avoid thermal damages in step-soldering processes and to eliminate the coefficient of thermal expansion (CTE) mismatches of materials in packaging modules, low-temperature Pb-free solders with low cost and high reliability are demanding. The Sn-58 wt.% Bi (Sn-58Bi) eutectic alloy with high mechanical properties, good wettability and low melting temperature at 139 ◦ C has drawn great attentions in the industry [13,14]. Designing proper alloying elements for improving the mechanical properties of the Sn-58Bi solder is necessary for low-temperature soldering applications. The goal of alloy design is to improve the elongation of Sn-Bi-based solders by doping, while keeping their low melting temperatures
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