Tin fog effect analysis during fluxless soldering under reflow with formic acid: Methodology, mechanism, and reliability impact

Abstract

Fluxless tin soldering eliminates flux residues but introduces tin fog, which affects the reliability of electronic packaging. The influence of tin fog was first analyzed through the shear strength of the Al wires bonded on DBC substrates. After aging at 300 °C for 4 h, shear strength increased by 4.5 % on pure copper surfaces but decreased by 22.6 % on tin fog-covered surfaces. Further SEM revealed tin fog created interfacial porosity, reducing strength. Secondly, the impact of tin fog formation during the formic acid reflow process was studied under different formic acid concentrations and different vacuum frequencies during the reflow's preheating stage. Through SEM, EDS, Micro-IR, and XPS tests, the principal components of tin fog were identified as elemental tin, tin dioxide, and organic compounds such as amides and esters. It is found that tin fog forms through a process wherein tin formate produced during formic acid reflow adheres to the substrate surface, which is already adsorbed with organic substances such as esters and amides, and subsequently decomposes at high temperatures to generate derivative products creating the fog. Therefore, to mitigate tin fog formation, tin fog was effectively controlled under a tripled vacuum frequency, resulting in reduced organic residues on substrates. Under such a mitigation strategy, further reliability tests showed that the shear strength of Al wire bonding after aging at 300 °C was comparable to that on clean DBC substrates. This research provides a valuable reference for enhancing the reliability of fluxless soldering in electronic packaging.

Keywords

• Fluxless solder paste
• Formic acid
• Tin fog
• Reliability
• Mechanism
• Mitigation