Sn基钎料连接柔性石墨膜的结合机理与物理性能研究

Bonding Mechanism and Physical Properties of Flexible Graphite Film Joints Soldered with Sn-Based Solder

摘要:

柔性石墨膜(Flexible Graphite Film, FGF)在柔性电子散热领域应用潜力巨大,但其固有的润湿性差及层状结构制约了可靠连接。本文通过提出超声辅助镀覆-钎焊复合工艺,采用纯Sn与Sn9Zn钎料,实现了FGF的低温连接。系统优化了超声镀覆工艺,并结合第一性原理计算的方法研究了Sn/柔性石墨膜界面结合机制,对比分析了有无超声辅助作用下接头组织与性能。结果表明：超声镀覆可在柔性石墨膜表面形成均匀的镀层,纯Sn钎料超声镀覆最佳时间为45 s、温度为320 ℃、功率为640 W,Sn9Zn钎料超声镀覆最佳时间为35 s、温度为280 ℃、功率为800 W。超声钎焊能显著改善界面结合,促进钎料与柔性石墨膜形成机械互锁结构。第一性原理计算显示,界面处Sn原子失去电子、C原子得到电子,C-Sn键布居数介于0.03~0.06,结合分离功数值,综合分析表明Sn/柔性石墨膜界面结合由范德华力与电荷转移诱导的弱化学键合共同主导,伴随弱共价-离子混合键合；在接头性能方面,纯Sn钎料超声辅助钎焊3s的接头导电性能最佳,且优于Sn9Zn钎料超声辅助钎焊5s的接头,电导率分别为3.49×10₆ ^S/m和2.25×10₆ ^S/m,在弯曲测试中稳定性优于无超声接头；散热方面Sn9Zn焊接的接头整体优于纯Sn接头,因连接过程中二次超声,使不加超声焊接5 s的Sn9Zn接头展现出最优的导热率与热扩散率。

Abstract:

Flexible Graphite Film (FGF) has great application potential in the field of flexible electronic thermal management, but its inherent poor wettability and layered structure restrict reliable joining. In this paper, an ultrasonic-assisted plating–brazing composite process is proposed; using pure Sn and Sn9Zn solders, low-temperature joining of FGF is realized. The ultrasonic plating process is systematically optimized, and the bonding mechanism at the Sn/FGF interface is investigated by first-principles calculations. The microstructure and properties of joints with and without ultrasonic assistance are comparatively analyzed. The results show that ultrasonic plating can form a uniform coating on the FGF surface; for pure Sn solder the optimum ultrasonic plating time is 45 s, temperature 320 °C, power 640 W, whereas for Sn9Zn solder the optimum time is 35 s, temperature 280 °C, power 800 W. Ultrasonic brazing significantly improves interfacial bonding and promotes the formation of a mechanical interlocking structure between solder and FGF. First-principles calculations reveal that Sn atoms lose electrons and C atoms gain electrons at the interface; the C–Sn bond population is between 0.03 and 0.06. Combined with the calculated work of separation, comprehensive analysis indicates that Sn/FGF interfacial bonding is dominated by van der Waals forces and weak chemical bonding induced by charge transfer, accompanied by weak covalent–ionic mixed bonding. In terms of joint performance, joints ultrasonically brazed for 3 s with pure Sn solder exhibit the best electrical conductivity, superior to joints ultrasonically brazed for 5 s with Sn9Zn solder; the electrical conductivities are 3.49×10₆ ^{S/m and 2.25×10₆} S/m, respectively, and the stability in bending tests is better than that of joints without ultrasonic assistance. In heat dissipation, Sn9Zn-brazed joints are overall superior to pure Sn joints; because of secondary ultrasonic application during joining, 5 s Sn9Zn joints brazed without ultrasonic assistance display the optimum thermal conductivity and thermal diffusivity.