Wettability effects of immersion tin final finishes with lead free solder

Surface conditions on Printed Circuit Board (PCB) final finishes have an important impact on the wetting behaviour with lead-free solder. The focus of this work is the tin oxide characteristics and their influence on solderability. A good wetting result can be only achieved if the oxides are completely removed. Due to this fact different flux systems are used to remove the oxide layers and other contaminations from the surface. In order to supply sufficient flux for complete removal of tin oxides, knowledge about the oxidation state of tin oxides on new and on aged immersion tin final finishes were obtained. The oxidation process of the immersion tin surface on PCBs was investigated under different reflow cycles with lead-free temperature profiles and different soldering atmospheres. The characterisation of the thin oxide layers were studied by using high resolution analysis methods as XPS, TEM, and SERA. The results proved a thickness of immersion tin oxide layers at approximately 7 nm by TEM and XPS analysis after plating. Furthermore, an amorphous mixture of SnO and SnO₂ were initially found. Additionally, the examined immersion tin surface on SMD 1206 resistor pads did not show a remelting after the first reflow cycle although the pads experienced a peak-temperature of 250degC and the melting point of tin amounts to 231.8degC. A further solder test presented a considerable decrease in the wetting behaviour after reflow ageing. In this work is shown that this behaviour results from thicker oxides, which were formed by the influence of high temperatures and oxygen atmosphere. Furthermore, a TEM examination indicated slight semi-crystalline tin oxide regions after two reflow cycles under nitrogen atmosphere.