I couldn't agree more. What everyone seems to forget is that the origin of
hailheading was based on drill diameters that were significantly larger than
the microdrills used in today's manufacturing. If you review MIL-STD-275 you
will find that it defined the hole diameters to be no less than 0.002 inch
larger than the lead of the component that is to be inserted into the hole.
Microdrills used for via connection in high density SMD simply did not
exist, yet some stubborn opinions still reign insisting in applying this
stipulation. Also, if you consider the effect of copper weights (1/2 oz
versus 1 oz) the tolerable amount of deformation (ie 150%) is more a
function of the designers choice of copper thickness of inner layer foils
rather than "process control" of the drilling process. (maybe us poor pcb
manufacturers should insist on 4oz inner layer copper so as never to exceed
150% nailheading)


Dave Rooke

___________ reply separator ______________

>Subject: Random nail heading.
>
>As we all know the causes of nail heading are many,  and may be the result of
>drills, drill machines, operational practices, materials of construction,
>construction of laminate and prepreg, cure state, copper foil type, thickness
>and processing.  A detailed list comes up with about 40 contibutors.
>
>Unfortunately, nail heading is what is called a non-defect defect; requests
>for any data on nailheading caused defects in 1989 came up with none.  Only
>responses were "An indiction of poor quality control" and similar.  Many
>smart guys have learned to control the pre-electroless-copper-etch to
>etch-off and eliminate a reportable nailhead defect.
>
>The Industry Spec IPC-RB-276 dropped the nailheading requirement in 1992 and
>the military counterpart, MIL-PRF-31032 also dropped the requirement at its
>issuance in 1995.  MIL-P-55110E still carries the requirement, but I
>understand that its replacement will also not mention the subject.
>
>The nail heading requirement was installed in the early 1960's in the
>military spec by some of us guys who worked for large companies that could
>afford good drill equipment and did not use resharps, mainly. to prevent
>competion from the fabricator who couldn't       
>
>Today many company specs. are a slightly modified copy of the military
>specification and nail heading continues to appear because no one knows how
>to evaluate and many have the belief that its a part of the process control
>system.
>
>So every year many tons of boards are discarded because of this supposed
>defect which has no revelency to board performance.
>
>Phil Hinton 
>Hinton PWB Engineering 
>[log in to unmask]    
>
>
>
D. Rooke
([log in to unmask])