Abstract
Sn-Bi solder with different Bi content can realize a low-to-medium-to-high soldering process. To obtain the effect of Bi content in Sn-Bi solder on the microstructure of solder, interfacial behaviors in solder joints with Cu and the joints strength, five Sn-Bi solders including Sn-5Bi and Sn-15Bi solid solution, Sn-30Bi and Sn-45Bi hypoeutectic and Sn-58Bi eutectic were selected in this work. The microstructure, interfacial reaction under soldering and subsequent aging and the shear properties of Sn-Bi solder joints were studied. Bi content in Sn-Bi solder had an obvious effect on the microstructure and the distribution of Bi phases. Solid solution Sn-Bi solder was composed of the β-Sn phases embedded with fine Bi particles, while hypoeutectic Sn-Bi solder was composed of the primary β-Sn phases and Sn-Bi eutectic structure from networked Sn and Bi phases, and eutectic Sn-Bi solder was mainly composed of a eutectic structure from short striped Sn and Bi phases. During soldering with Cu, the increase on Bi content in Sn-Bi solder slightly increased the interfacial Cu6Sn5 intermetallic compound (IMC)thickness, gradually flattened the IMC morphology, and promoted the accumulation of more Bi atoms to interfacial Cu6Sn5 IMC. During the subsequent aging, the growth rate of the IMC layer at the interface of Sn-Bi solder/Cu rapidly increased from solid solution Sn-Bi solder to hypoeutectic Sn-Bi solder, and then slightly decreased for Sn-58Bi solder joints. The accumulation of Bi atoms at the interface promoted the rapid growth of interfacial Cu6Sn5 IMC layer in hypoeutectic or eutectic Sn-Bi solder through blocking the formation of Cu6Sn5 in solder matrix and the transition from Cu6Sn5 to Cu3Sn. Ball shear tests on Sn-Bi as-soldered joints showed that the increase of Bi content in Sn-Bi deteriorated the shear strength of solder joints. The addition of Bi into Sn solder was also inclined to produce brittle morphology with interfacial fracture, which suggests that the addition of Bi increased the shear resistance strength of Sn-Bi solder.
Highlights
Sn-Ag-Cu Pb-free solder alloy is commonly employed as the interconnected material to replace traditional Sn-Pb solder due to its superior wettability, reliability and mechanical properties [1].The melting temperature (MP) for Sn-Ag-Cu Pb-free solder ranges from 217 ◦ C to 221 ◦ C and is far higher than 183 ◦ C for Sn-37Pb eutectic solder, which means that a higher temperature will be required during assembling the components with boards
21 wt % occurring at eutectic temperature, theresolder is almost no solution for Snin inFigure
In Sn-30Bi or Sn-45Bi solder growing on interfacial Cu6Sn5 intermetallic compound (IMC), more Bi atoms were accumulated at the interface between joint, as shown in Figure 8b, Bi particles were precipitated in the grooves of the scallop-type Cu6 Sn5 grains during soldering, which was not observed in Figure 8a because Bi atoms were mainly dissolved in β-Sn phases to produce the solid solution phases for Sn-5Bi or Sn-15Bi
Summary
Sn-Ag-Cu Pb-free solder alloy is commonly employed as the interconnected material to replace traditional Sn-Pb solder due to its superior wettability, reliability and mechanical properties [1]. With continued shrinkage in size and thickness of electronic packaging, higher soldering temperature from Sn-Ag-Cu solder inevitably results in the thermal warpage on the thin or ultra-thin components [2]. Bi distribution from different Bi addition in Sn-Bi solder, the interfacial characteristics at the interface and the resulting joint strength were systematically studied. 920addition in Sn-Bi solder, the interfacial characteristics at the interface and the resulting from different joint strength were systematically studied. This paper selects Sn-Bi solders with different Bi content different Bi content ranging from 5–58 wt %, and the effect of Bi content on the microstructure of ranging from 5–58 wt %, and the effect of Bi content on the microstructure of solders, the interfacial solders, the interfacial behaviors between solder and Cu during soldering and the following aging behaviors between solder and Cu during soldering and the following aging conditions, and the shear conditions, and the shear strength on ball joints are studied.
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