We just completed some 16-layer boards and noticed (after wave soldering) that there are some unique, white-ish discolorations. It looks as if someone layed their finger at weird angles on the groundplane areas. It is NOT finger prints...it just looks like the length of a finger. We used no-clean...so it is not residue. Attempts to remove it with cleaning solutions were not successful. I believe it is due to thermal expansion of some sort. Any thoughts out there in SMT Land?