Effect of Surface Roughness and Droplet Size on Solder Wetting Angles

Reactive wetting is a process ubiquitous in modern electronicsolderingprocesses. Nonetheless, the dependencies of reactive wetting angleson surface roughness and droplet size are not well understood butbecome 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 aforementioneddependencies are evaluated. Several surface roughnesses and a largearray of droplet sizes are measured. Clear size dependencies are observedfor both smooth and rough substrates, yet interestingly, the trendsare inverted. This inversion of the size dependence of the wettingangle is discussed on the basis of the triple line pinning phenomenon,Wenzel's wetting model, and the consumption of Sn by a solderreaction. Quantitative models are proposed, with which size dependencieson rough and smooth surfaces can be better understood. Triple linepinning barrier energies are calculated. Larger pinning barriers aredetermined for rougher substrates.