Semiconductor Packaging/Test: Marking
Marking
is the process of putting identification, traceability, and
distinguishing
marks
on the package of an IC. The device name, company logo, date code,
and lot id are examples of information commonly marked on the IC's
package. Some marks are put on the package during Assembly and some
marks are put on the package during Test.
There are
two common marking processes, namely,
ink
marking
and
laser
marking.
The most common ink marking process for semiconductor products is
pad printing.
Pad printing consists of transferring an
ink
pattern
from the plate, which is a flat block with pattern depressions that
are filled with ink, to the package, using a
silicone
rubber stamp pad. Silicone rubber repels ink, making the transfer
of the ink pattern clean and efficient. It is also resilient and
elastic, making it possible to print even on uneven surfaces.
Fig.
1. Photos of Silicone Rubber Pads used for Ink Marking
Simply
put, the pad printing process can be broken down into the following
steps: a) the stamp pad passes over the surface of the ink; b) the
surface ink sticks to the pad; c) the pad is applied to the surface
for marking; and d) the ink sticks to the marking surface as the pad
lifts. Ink marking is usually punctuated by
ink
curing.
Aside
from mark quality,
mark
permanence
is a critical aspect of IC package marking. Mark permanence
failures can be due to a lot of things such as use of inappropriate
ink, use of improperly prepared ink, inadequate ink curing, and
marking surface contamination. Good wetting of the ink on the
marking surface is necessary for good marking. For wetting to occur,
the surface tension of the ink must be less than the surface tension
of the marking surface. Thus, a careful review and interpretation
of the surface tension data of the ink to be used and those of the
intended marking surface is needed to ensure good ink marking.
Laser
marking,
as the name implies, refers to the process of
engraving
marks on the marking surface using a
laser
beam.
There are many types of lasers, but the ones used or in use in the
semiconductor industry include the CO2 laser, the YAG laser, and
diode lasers. In the semiconductor industry, lasers can also be
used in micromachining, surface processing, trimming, welding, and
cutting.
Figure
2. Example of a Laser Marker
Common
marking
failure
attributes
include the following: 1) missing
mark; 2) missing character; 3) misoriented mark; 4)
wrong
mark format; 5) misplaced mark; 6) mark permanency failure; and 7)
inadequate mark contrast.
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