PCB nail heading
Nailheading is the distortion of the copper inner layer at the hole wall. this distortion takes the form of a “Nailhead”. There are several types of nailheads. the most common type seen on small hole diameters is a thermomechanical nailhead. The thermo-mechanical nailhead produced by excessive heat and the drill bit pushing. and pulling the copper in each direction the drill bit travels. Drill sizes above 0.0292” diameter are susceptible to both thermomechanical. and mechanical nailheading. Mechanical nailheading only characterized by a onedirectional nailhead. This nailheading is usually accompanied by a heavy resin smear over the copper.
Nail Heading Evaluation
Nail Heading is a phenomenon that occurs in the drilling of multilayer PWB's. The normal copper inner layers are drawn out or extruded by the drill which forms a nail head like shape on the hole wall.The inner hole wall is inspected by cross-section, the dopper layer is measured where the nail head is formed. The nail heading value is expressed as a percentage according to the thickness of the inner layer copper as shown in the figure below.If the evaluation using stack drilling has been done, all the panels are measured.The maximum nailheading points are measured around final hit hit counts.
The inner layer copper thickness is measured from the average position of the "roughed" surface of the copper layer.Nail Head[%]= b/a x 100
a: Copper layer thickness [µm]
b: Nail head width [µm]
nail heading Causes reason
Exposed copper of inner layers on through holes. formed the shape of nail heading during drilling. Such a huge burden to hole brings non-uniform surface of through holes. and may cause conductivity failure of plating.
1. Incorrect infeed.
2. Incorrect spindle RPM.
3. Incorrect retract rate.
4. Undercured resin.
5. Drill bit clogging.
6. High temperature elongation
copper / Number of copper layers.
7. Entry or backer material.
nail heading PCB solution
1. Determine “type” of nailhead,
single sided nailheading or bidirectional nailheading.
2. For Single sided nailheading:
Reduce infeed rate, or increase
RPM. For Bi-directional
nailheading, reduce RPM or
increase infeed rate.
3. On small diameter drills, retract
rates may reduced.
4. Undercured “B” stage will soften
and not provide mechanical
support for the copper, and will
5. Check the drill bit for clogging.
Clogging of the flutes can
be caused by drill bit design,
aluminum entry sticking to the drill
flutes, under cured resin, backer
material or poor vacuum removal
of drill debris.
6. High temperature elongation
copper foils will increase
nailheading, the higher the tensile
elongation of the copper, the
more severe the nailheading will
be under conditions that would
7. Some types of entry and backer
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