Per some of the inputs; I am in agreement with D. Hoover, nailheading has nothing to do with wedge voiding (also called chisel point voids). The circumferential voids that were the point of discussion was the possibility of the direct plate coatings such as the carbon or graphite which must be microetched and undercut and removed from the copper surface interfaces before additional plating; the microetch may etch away a small edge at the extreme of the nailhead and leave a ring of non-conductive surface which may not plate. Several users have said that they have not seen the problem when using this process. The wedge voids are a completely separate cause/condition. Several people have noted that different copper foils such as the Grade 3 (HTE) created more nailheading than the Grade 1, but that the Grade 3 was better for multialyer boards. Many of us have seen this condition and even some brands of HTE are more prone than others. The metallurgist will tell us that the more ductile foils where the slip planes (111) in a large single grain are lined up will give us more metal flow and nailheading. The rolled annealed copper used by the flex fabricators is metallugically perfect for providing the greatest nailhead. It is noted that IPC-RF-245 (1986) specifies nailheading can be up to 2X. John Andresakis in the April issue of Circuit Tree (Page 24) reports that the fine grained foil that he has tested has a much lesser nailheading tendancy than the HTE. He also notes that HTE (Grade 3) nailheaded more than standard electrodeposited (Grade 1). Again the metallurgist will tell us that the fine grained copper foil with unoriented slip planel is less ductile and will flow less than that large grain material and decrease nailheading. I think the best solution is drop all requirements for nailhead as a defect and let the fabricators make their drilling/processing etc. compatible to produce a good board that meets the performance requirements without this limitation that is not function related. If the fabricator so desires, he can set up a "process indicator" that reacts to certain limitations on nailheading that may be material/condition dependent and proceed from there.