Just incase you were wondering, here are the wetting results you can expect when soldering Pb-free spheres to different surface finishes. The chart shown here is the average result of about 15 different ball attach fluxes from across the industry. This was just some left-over data from testing that will appear in the October issue of Advanced Packaging.
The x and y axis are not labeled due to space constraints, but x is obviously surface finish. The y axis is (less obviously) the increase in solder diameter after reflow. Here’s the formula:
Sø = (Ds/Di)100-100
Where: Sø = Increase in sphere diameter,
Ds = Diameter of post-reflow solder, and
Di = Initial diameter of sphere