Effects of Surface Roughness on Solder Bump Formation by Direct Droplet Deposition

This paper presents a study of the effects of substrate surface roughness on droplet bouncing during direct solder bumping, a phenomenon that strongly affects the deposition efficiency, as well as the final bump size and shape. A theory was developed to correlate surface roughness to the potential for droplet bouncing. In addition, an experimental study was conducted by depositing Sn microdroplets on Au plated substrates with different surface roughness levels. The population densities and morphologies of the splats collected were analyzed to determine the occurrences of bouncing. The results suggest that a decreass in surface roughness reduces the potential for droplet bouncing.