Lasers—a flexible tool for PCB and device rework
appropriate. First, line definitions achieved
Chemicals can be used to by the laser are very tight with
remove markings from elec- 1 mil traces and spaces well
tronic components. Care must within the capability spectrum
be taken to insure that the of most lasers. Second, very
chemicals do not contain any complex trace paths, which are
chelating ingredients such as very time-consuming to hand
EDTA, phosphates, soy, caus- fabricate when the traces are
tics or chlorinated solvents. added manually, are easy to
Some etchants can be overly fabricate with a laser. Finally,
aggressive and can remove the cost for rework is greatly
too much of the mask or the reduced. The cost per piece part
epoxy body of the component. for rework is a fraction of the
The parts need to be neutral- same work done manually, with
ized and cleaned so they do the difference becoming more
not interfere with subsequent pronounced as the patterns get
process steps. more complex and the volumes
There are several types of greater. In both cases these
laser sources that can be used Figure 2. Laser etching of solder mask to extend a trace. newly created traces are solder-
for marking active electronic able surfaces.
components (Figure 1). The
most common type is near infrared (NIR)
or building traces and a wire re-routing
pcB marking
laser that produces laser energy in the
method, on circuit boards.
There are cases where the PCB needs
1,064 nm range of the spectral field. The
There are several issues the rework
secondary marking as part of a rework
most common type of NIR laser used
engineer needs to be aware of when using
process. In some instances a date code
in marking is the Nd:YAG (neodymium
wires for the jumpers:
needs to be marked on the assembly more
yttrium aluminum garnet) laser. Recently,
• When a jumper wire is added prior
permanently, or the marking needs to with-
second harmonic generation or green
to part assembly, screen printing of
stand the temperature and environmental
lasers have become available for this appli-
the jumper wire is not possible.
conditions of the electronic assembly. In
cation. C02 lasers that are able to machine
• Stress relief must be incorporated
other cases board information needs to
away any organic materials such as epoxy
into the jumper wire routing
be permanently erased in order to protect
resin, underfills and plastics are also being
• Wire cannot impede the potential
intellectual property.
employed to mark these packages.
for the subsequent reworking of
In the case of PCBs, laser marking
One of the main concerns during
the PCB
and labeling are two of the most common
remarking for manufacturers of silicon- marking options along with their associ-
packaged components is the potential risk
Conversely, traces can be built or modified
ated attributes, as shown in Table 1.
of damaging the device during remarking.
right on the PCB. The benefits of using
The application of a label is a com-
With silicon-based packaging, there is no
this method are the following:
mon method for board identification in
plastic, metal or ceramic in the package
• You can solder anywhere along the
the PCB assembly industry. They are able
body to protect the silicon chip from the
trace path, thereby increasing the
to withstand rigorous manufacturing pro-
outside. With the proper laser characteris-
flexibility of further connections or
cesses, including soldering and cleaning.
tics and optimized marking parameters, the
testing
They are simple to apply, and many times
component package can be permanently
• Better aesthetic appeal
the ability to generate a new one already
marked without incurring damage to the
• The elimination of messy tape dots,
exists in the manufacturing area.
device.
tape strips, quick set adhesives, hot
Labels have several disadvantages, one
melt adhesives, hot bonding or
being per-board marking cost. The average
Jumper wires
other potential ‘foreign’ matter on
cost of a 1.00 x 0.25” preprinted polyester
Jumper wires are used to change a circuit
the PCB
label is US $0.04 per label. Individual
path, such as a BGA site, for engineering label costs increase as stock requirements
changes and modifications. Jumper wires
3
A trace modification technique can be
change from the low cost polyester labels
fall into the following categories:
used if and when a jumper wire is in close
to the high cost ESD polyimide labels.
1. Those which are considered wires
proximity to another. In this method, a
Labels sometimes can fall off, curl, shrink
and are added or installed during
replacement trace is added to the PCB
or absorb chemicals that later leach out.
the assembly
trace layout scheme by either using a
Fluxes can be entrapped on labels. Their
two-part epoxy or by applying heat and
thorough removal is essential for certain
2. Those that are added after the
pressure to a dry film-backed replacement
types including OA, RA, and RSA4. Oth-
assembly to effect a change or
trace. In another method, mask can be
erwise ionic contaminants will cause hu-
modification in the electronic
lased away to expose the copper, effectively
midity, sensitive electrical leakage currents
functionality
creating a trace (Figure 2) that has very fine
and long-term corrosion. Another side
3. Those that are added to correct a
line definition. This method
1
, while having
effect can be unwanted contaminants on
defect
engineering and set-up costs not present
the board. Widely used labeling standards
There are two common methods for
to near the degree of the addition of the
require a minimum of 50% reflectance for
placing jumper wires, including adding
manual trace methods, has several benefits.
spaces, and in effect there is a required bar-
www.globalsmt.net Global SMT & Packaging – November 2008 – 21
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