Residue-Free Depaneling Of PCBs - OnBoard Technology

PCBFABRICATION
Residue-Free Depaneling
Of PCBs
Electronic circuit boards are becoming
smaller and smaller and can now
only be produced cost-effectively in
multiple panels. But more and more
applications are also making high demands
on the subsequent depaneling
process: exact dimensions, clean
edges and stress-free processing. The
benefits of depanelization with a laser
beam, such as high area utilisation,
higher precision, dry and stress-free
processing, and the possibility of producing
PCBs in any shape with clean
edges, should open up this technology
to cost-effective widespread use.
However, it is not enough to just
choose a laser as your cutting tool. To
achieve a high yield, precise machine
technology, a laser wavelength suited
to the material and appropriately adjusted
feed are also necessary.
component-fitting
near the edge
of a board, especially
with taller
components.
To solve these
problems, Fico
offers PCB singulation
equipment
based on laser
beams. These laser
cutters from
the Bright Line
series cut with extreme
precision,
produce a clean
cutting edge and
do not stress the
material in the
slightest.
by Henk Wensink,
Fico
Figure 1 - A panelized PCB layout optimised for laser
cutting. In this particular case the usable area doubles
from 40 to 80 percent
Production facilities for PCBs yield
the highest throughput with large formats.
Small electronic circuit boards
are therefore produced in panelized
form, with several being combined to
form one large-scale board on which
they are processed together. With
only one lot of set-up costs plus collective
tinning, solder-resist application
and component-legend printing,
this is a very cost-effective process.
Depending on the outer contours of
the individual PCBs, they are singulated
by scoring followed by rotaryblade
cutting, sawing, punching or
milling. For example, a final milling
or routing step removes the material
between contour-milled panelized
PCBs, as shown in Figure 1 (left).
However, such singulation procedures
apply mechanical force on the
PCBs, may produce milling dust, and
the accuracy and edge quality are in
fact quite limited. In addition, the
milling tools, or their holders, require
a certain amount of free space above
the material to be cut, which limits
High area utilisation
Because of the laser cut width of just
0.2mm, which is precisely controlled
to ± 50 µm, a large board can be much
better utilised for panelization. If the
contour milling of the board in Figure
1 (left) is cut by laser, the area utilisation
doubles from 40 to 80% (Figure
1, right). An additional benefit is the
stability of the closed structure of the
large-format PCB which warps less
during reflow soldering and therefore
results in higher yields.
Clean cut edges
Generally speaking, the cutting edge
of a PCB singulated by laser beam is
not straight but has a bevel. This reduces
the usable surface of the board,
a result which has an ever greater impact
as board sizes continue to shrink.
Using a special angle compensation
procedure, however, the developers
at Fico have managed to achieve perpendicular
edges with the laser cut,
an additional factor to help alleviate
space problems in small devices such
as cellphones.
Figure 2 reveals a further problem
of conventional laser-cutting methods:
the cutting surface is contaminated
by a carbon coating, which is
electrically conductive and thus can
significantly interfere with the functioning
of the product. This residue
arising from the cutting process is
incidentally one of the main reasons
why laser cutting for depanelization
of PCBs has not really caught on so
far. But through close cooperation
with laser experts, a new laser cutting
process has been developed, which
delivers perfectly clean edges with no
carbonisation (Figure 2, right).
Suitable laser beam sources
The Fico Bright Line (FBL) laser
cutters can be equipped with different
laser types and thus be optimally
adapted to the cutting task at hand.
When cutting with a laser, its radiant
energy must be absorbed by the material.
If this light absorption is high
enough, the local temperature in-
OnBoard Technology June 2007 - page 20
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Figure 2 - Depanelization with the Fico laser process guarantees straight edges
and residue-free cutting surfaces (right) compared with conventional lasercutting
procedures (left)
creases to a level where the material
“evaporates”. The extent to which the
light is reflected or absorbed depends
on the properties of the material and
the wavelength of the light. CO2
and YAG lasers in particular provide
enough energy in different spectral
ranges to make them usable for depanelization.
The CO 2
laser is one of the oldest laser
types and has a wavelength in the
infrared range of 10,600 nm. This laser
technology is commonly available
and provides high laser power at reasonable
costs. In the FBL this laser is
largely used to cut through plain PCB
material and can reach cutting speeds
of up to 500 mm/s for thin boards (0.2
mm-thick FR4). However, a CO2 laser
cannot cut through copper: because
of the long wavelength of CO2 light,
the strip-conductor material acts as a
mirror, reflecting most of the beam.
An equally widespread industrial laser
source is the Nd:YAG laser (neodymium-doped
yttrium aluminum garnet
laser), which provides coherent light
with a wavelength of 1064 nm in the
near infrared range. Its laser power
is less than the CO2 laser power, but
is still sufficient for cutting speeds of
up to 50 mm/s. Because of its shorter
wavelength, its energy is also absorbed
by copper, making it possible
to cut through thin copper wires, for
example those in BGA products. Unlike
pure circuit boards, BGA products
consist of several materials, such
as fiber glass, epoxy, filler and copper,
which all have a slightly different
light absorption. Consequently, visible
defects can occur, such as frayed
board edges, solder mask chipping,
and rough cutting edges. In addition,
materials with insufficient absorption
are not evaporated but merely pulverised,
which can lead to the aforementioned
black carbonisation. In this
case, an FBL can be equipped with a
cleaning system which removes the
dust immediately after cutting.
If the original 1064 nm wavelength
of the Nd:YAG laser is not sufficiently
absorbed by the material for
a processing task, doubling and trebling
the frequency by means of nonlinear
crystals in the laser resonator
very effectively produces laser beams
with shorter wavelengths.
Doubling the frequency produces
green laser light with a wavelength
of 532 nm, but with only half the
power, so the cutting speed has to be
reduced. However, the green light is
absorbed more easily by all materials
and gives a better cutting quality with
less dust formation. Cosmetically,
the cut looks almost perfect. Better
absorption by all materials, incidentally,
also means less variation in the
cutting speed for black or colored
compounds.
By trebling the frequency of the Nd:
YAG laser light, you can get a UV laser
with a wavelength of 355 nm in the
Figure 3 - Thin materials can be cut
precisely at very high speeds
ultraviolet range. Again the cutting
speed is reduced but the absorption
by all materials is sufficiently high to
allow perfectly smooth surfaces with
excellent cutting quality. However,
this quality has its price.
Mechanical influence
At the latest at this point, it becomes
clear that a compromise must be
found between maximum cutting
speed and cutting quality, from which
an optimum laser color can then result.
Furthermore, the cutting result
and throughput also depend on other
machine characteristics: for this reason,
the laser beam and board holder
of the Fico Bright Line (Figure 4)
are guided and moved by high-precision
linear motors. A camera system
checks the exact position of the largeformat
PCB so that it is positioned
exactly beneath the laser beam. This
results in very precise cuts with high
repeatability.
Figure 4 - The scope of application of the modular Fico Bright Line cutting
system ranges from high-throughput depanelizer to board singulation with
the highest-quality cutting edges
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OnBoard Technology June 2007 - page 21