The defect called "nailheading" was a carryover from the original
multilayer military specification MIL-P-55640 that was initiated
in the late 1950s.  It was placed in this specification as a
penalty for those fabricators whose drilling techniques were
questionable.  The military insisted that this requirement be
added so that suppliers could be blocked from furnishing
defective product.

Unfortunately, all the subsequent military specifications, and
many industry documents, continued to carry
this requirement.

With the advent of smaller holes, high ductility copper foils,
and thinner internal foils (1/2 and 1/4 oz. instead of the
original mandatory 2 oz. foil), printed boards were rejected
using the 150% rule.  

When the IPC-RB-276 task group first explored this 
issue, none of its members was aware of any functional defect associated 
with "nailheading," and
some believed it to be helpful in adding more surface area to the
interconnect.  It was also reasoned that if "nailheading" caused
an interconnect or laminate defect, that the defect would be
detected during normal microsection analysis.  It was, therefore,
considered a cosmetic defect that in no way affected the
performance of the printed multilayer board, and was
intentionally deleted from IPC-RB-276.