To solder metal parts efficiently, layers of oxides on the surfaces need to be removed. The use of formic acid vapour as a reducing agent is a valid alternative to liquid fluxes, i.e. organic or inorganic based. Fluxless soldering is growing in its applicability, especially in microelectronics packaging. Nevertheless, some residues are produced. This study focuses on the characterization of these residues, which are obtained by flowing nitrogen gas enriched with formic acid at 2008C on Sn3Ag0.5Cu (SAC305) metal alloy, a widely used soldering material. The formation of microcrystals is observed and attributed to heat and mass transfer processes of the formic acid vapor. While there is a wealth of reported studies on soldering results under a formic acid atmosphere, there is a lack of information with regards to a specific understanding of the chemical compositions of these undesired residues. The second purpose of this study was to confirm the recently introduced method of producing Sn-based crystals under a formic acid atmosphere. X-ray diffraction (XRD), energy-dispersive X-ray (EDX) and infrared spectroscopy (IR spectroscopy) investigations allow us to assign the composition of the microstructures. Crystals of Sn formate, Sn oxide, and bimetallic Cu6Sn5 were detected and a mechanism of formation of these crystals was suggested.