Effect of Surface Roughness and Droplet Size on Solder Wetting Angles

Reactive wetting is a process ubiquitous in modern electronic soldering processes. Nonetheless, the dependencies of reactive wetting angles on surface roughness and droplet size are not well understood but become increasingly important for reliable miniaturization of devices. In this study, advancing contact angles of the Sn^(l)/Cu^(s) reactive solder system are investigated and the aforementioned dependencies are evaluated. Several surface roughnesses and a large array of droplet sizes are measured. Clear size dependencies are observed for both smooth and rough substrates, yet interestingly, the trends are inverted. This inversion of the size dependence of the wetting angle is discussed on the basis of the triple line pinning phenomenon, Wenzel's wetting model, and the consumption of Sn by a solder reaction. Quantitative models are proposed, with which size dependencies on rough and smooth surfaces can be better understood. Triple line pinning barrier energies are calculated. Larger pinning barriers are determined for rougher substrates.