Atomic-scale characterization of interfacial Cu-Zn/Al-Zn solid solutions in low-temperature Cu/Al joints soldered with chain-length tailored fluxes via FIB-TEM

A novel no-clean flux enabled direct, plating-free soldering of Cu/Al joints using Sn42Bi58 paste. • Organic acid carbon chain length was tailored to regulate flux activity and interfacial reactions. • A synergistic mechanism between organic acids and fluoroborate salts was elucidated. • The medium-chain succinic acid flux achieved optimal joint strength via interfacial solid solution formation. • The study establishes a clear structure-property relationship from molecular design to joint performance. This study addresses the challenge of low-temperature, flux-based direct soldering for copper‑aluminum dissimilar joints, a critical need in advanced thermal management. A series of novel fluxes were synthesized by incorporating dicarboxylic acids with varying carbon chain lengths—malonic, succinic, and adipic acid—into a triethanolamine-fluoroborate matrix, mixed with Sn42Bi58 powder to form corresponding solder pastes (TL-1 to TL-4). Comprehensive characterization, including wettability tests, shear strength measurements, XRD, SEM-EDS, and FIB-TEM analysis, was conducted. Mechanistically, it is revealed that the carbon chain length dictates the critical balance between flux viscosity (steric hindrance) and carboxylate activation capability. The medium-chain succinic acid in TL-3 achieves an optimal “activation-diffusion-reaction” synergy. It acts concertedly with fluoroborates to thoroughly disrupt the stable native oxides (CuO and Al 2 O 3 ) while simultaneously lowering the diffusion energy barrier for Zn 2+ . This specific interfacial modulation entirely suppresses the conventional formation of brittle intermetallic compounds (IMCs) or non-bonded oxide residues. Consequently, TL-3 facilitates the formation of continuous nanoscale Cu 0.95 Zn 0.049 and Al 0.86 Zn 0.14 solid solution layers at the copper and aluminum interfaces, respectively, achieving robust atomic-level metallurgical bonding. Macroscopically, TL-3 exhibits superior performance with a minimum contact angle of 3.2° on Cu, an exceptional shear strength of 125.86 MPa, and an interfacial nanohardness of 0.826 ± 0.068 GPa. The primary scientific contribution of this work lies in elucidating this novel solid-solution-mediated interfacial strengthening mechanism, offering an innovative atomic-scale engineering pathway for the highly reliable, electroplating-free low-temperature soft soldering of Cu/Al heterogeneous structures. Download: Download high-res image (209KB) Download: Download full-size image