Effect of temperature and substrate surface roughness on wetting behavior and interfacial structure between Sn–35Bi–1Ag solder and Cu substrate

The effects of peak temperature (483 K and 533 K) and substrate surface roughness on wetting characteristics and interfacial structure between Sn–35Bi–1Ag solder and Cu substrates were studied. Results revealed that the wettability of Sn–35Bi–1Ag solder was enhanced with increasing surface roughness of Cu substrates and temperature. However, the surface topography characteristics (i.e., anisotropic or isotropic) had little effect on wettability of Sn–35Bi–1Ag solder. The liquid solder had a preferential spreading direction, which caused an elliptical triple-line pattern when Sn–35Bi–1Ag solder spread over an anisotropic surface, while the solder spread as almost a perfect circle when the surface of Cu substrate was isotropic. Besides, the surface roughness had little effect on the interfacial structure, while the interfacial IMCs increased with increasing the temperature. Moreover, the spreading process of Sn–35Bi–1Ag solder spreading on Cu substrates with different surface roughness was similar at 483 K and 553 K. The whole spreading process was featured with four distinguishable stages: (i) initial stage, (ii) rapid spreading stage, (iii) limited spreading stage and (iv) steady stage. Wetting kinetics of the rapid spreading stage and limited spreading stage were described by the Rⁿ ~ t model and the values of n of the rapid spreading stage and limited spreading stage decreased with increasing surface roughness of Cu substrates, which indicated increasing surface roughness of Cu substrates increased slightly the spreading rate. However, the temperature and surface topography characteristics had little effect on the spreading rate. Meanwhile, the dynamic contact angle and time t could be characterized by an equation, i.e., cosθ_F – cosθ = (cosθ_F – cosθ₀) e−^kt. The value of k decreased with increasing surface roughness of Cu substrates, while the surface topography characteristics and the temperature had little effect on the value of k.