Shukla Vishnu, Ahmed Omar, Su Peng, Jiang Tengfei
Department of Materials Science and Engineering, Advanced Materials Processing and Analysis Center, University of Central Florida, Orlando, FL 32816, USA.
Juniper Networks, Sunnyvale, CA 94089, USA.
Materials (Basel). 2024 Jul 21;17(14):3602. doi: 10.3390/ma17143602.
Bi-doped Sn-Ag-Cu (SAC) microelectronic solder is gaining attention for its utility as a material for solder joints that connect substrates to printed circuit boards (PCB) in future advanced packages, as Bi-doped SAC is reported to have a lower melting temperature, higher strength, higher wettability on conducting pads, and lower intermetallic compound (IMC) formation at the solder-pad interface. As solder joints are subjected to aging during their service life, an investigation of aging-induced changes in the microstructure and mechanical properties of the solder alloy is needed before its wider acceptance in advanced packages. This study focuses on the effects of 1 to 3 wt.% Bi doping in an Sn-3.0Ag-0.5Cu (SAC305) solder alloy on aging-induced changes in hardness and creep resistance for samples prepared by high cooling rates (>5 °C/s). The specimens were aged at ambient and elevated temperatures for up to 90 days and subjected to quasistatic nanoindentation to determine hardness and nanoscale dynamic nanoindentation to determine creep behavior. The microstructural evolution was investigated with a scanning electron microscope in tandem with energy-dispersive spectroscopy to correlate with aging-induced property changes. The hardness and creep strength of the samples were found to increase as the Bi content increased. Moreover, the hardness and creep strength of the 0-1 wt.% Bi-doped SAC305 was significantly reduced with aging, while that of the 2-3 wt.% Bi-doped SAC305 increased with aging. The changes in these properties with aging were correlated to the interplay of multiple hardening and softening mechanisms. In particular, for 2-3 wt.% Bi, the enhanced performance was attributed to the potential formation of additional AgSn IMCs with aging due to non-equilibrium solidification and the more uniform distribution of Bi precipitates. The observations that 2-3 wt.% Bi enhances the hardness and creep strength of the SAC305 alloy with isothermal aging to mitigate reliability risks is relevant for solder samples prepared using high cooling rates.
铋掺杂的Sn-Ag-Cu（SAC）微电子焊料作为一种用于未来先进封装中连接基板与印刷电路板（PCB）焊点的材料而受到关注，因为据报道铋掺杂的SAC具有较低的熔化温度、较高的强度、在导电焊盘上具有较高的润湿性以及在焊盘界面处较低的金属间化合物（IMC）形成。由于焊点在其使用寿命期间会经历老化，因此在其被更广泛地应用于先进封装之前，需要对焊料合金的微观结构和力学性能的老化诱导变化进行研究。本研究聚焦于在通过高冷却速率（>5℃/s）制备的样品中，1至3 wt.%的铋掺杂对Sn-3.0Ag-0.5Cu（SAC305）焊料合金的硬度和抗蠕变性的老化诱导变化的影响。将试样在环境温度和高温下老化长达90天，并进行准静态纳米压痕以确定硬度，进行纳米尺度动态纳米压痕以确定蠕变行为。使用扫描电子显微镜与能量色散光谱仪联用研究微观结构演变，以关联老化诱导的性能变化。发现随着铋含量的增加，样品的硬度和蠕变强度增加。此外，0-1 wt.%铋掺杂的SAC305的硬度和蠕变强度随着老化而显著降低，而2-3 wt.%铋掺杂的SAC305的硬度和蠕变强度随着老化而增加。这些性能随老化的变化与多种硬化和软化机制的相互作用相关。特别是对于2-3 wt.%的铋，性能的增强归因于由于非平衡凝固导致老化时可能形成额外的AgSn IMC以及铋沉淀物更均匀的分布。2-3 wt.%的铋通过等温老化提高SAC305合金的硬度和蠕变强度以降低可靠性风险的观察结果，对于使用高冷却速率制备的焊料样品具有重要意义。
