Industrial & Specialty Printing - January/February 2013 - 27

Figure 1 A solar panel printed with a slump resistant Ag metallization
paste (a), in contrast with paste (b) with poor slump resistance.

Figure 2 Ruptured die
bond after shear test for a
joint formed with the use
of nano-silver ink sintered
at 270°C without use of
pressure.

imaged. For instance, a printed DuPont Fodel ink is dried for 10
minutes at room temperature, 25 minutes at 80°C, patterned by
exposure under a 1-KW UV source (450-1200 mJ/cm2) for 30-75
seconds, spray developed with 1% Na2CO3 at 30°C, rinsed clean
with water, then fired under air with a 60-minute cycle with 10
minutes at 850°C. Line/spacing resolution finer than 40 μm is a
challenge for the industry.
Using a ceramic substrate and high-temperature firing is an
expensive process. In general, the industry prefers to use low-cost,
low-temperature processes. Often, this is accomplished with the
use of polymer as a binder. Depending on the application, the ink
characteristics vary significantly, ranging from a very thin, watery
liquid to a high-viscosity paste.
Therefore, for EMI shielding on a device, a low-viscosity liquid
may be sprayed onto the casing to form a uniform thin-film coating. For circuitry-conductor formation, a thin ink may be inkjet
printed onto a plastic sheet to form the pattern with high resolution. Alternatively, a high- viscosity ink may be screen printed onto
a rigid substrate with medium to high resolution. In general, the
ink is deposited onto the object’s surface. It is then dried and cured
either through UV or thermally at a temperature below 200°C. The
polymer forms a film that adheres to the surface of the object, with
filler particles embedded. Slump-resistant ink is critical for high
print resolution. Figure 1 shows a photo of a solar panel printed
with a slump-resistant silver metallization paste and with a paste
that exhibits poor slump resistance.
The physical and mechanical properties of the cured ink are
dictated by the binder chemistry. Epoxy promises very good adhesion. However, its storage life is sensitive to the storage temperature. Two parts epoxy may be used to address the shelf-life issue.
But on site, material handling is a significant tradeoff that is not
well received. The majority of inks used by the industry are onepart inks.

Creasing resistance is important in applications requiring flexibility, such as a conductor on flexible circuitry. Silicone is very
stable at elevated temperatures and is very flexible and crease resistant. The down side is its weak adhesion. Polyvinyl is insensitive
to humidity, but it is also relatively weak in adhesion. Acrylic can
be photoimaged, thus promising high resolution. Its rigid structure results in low creasing resistance. For outdoor applications,
weathering resistance, including temperature, humidity, and UV, is
important.
Polymers with conjugated double bonds, such as polyaniline or
polythiophene, are very flexible and deliver reasonable electrical
conductivity without a filler. The downside is sensitivity toward
weathering, particularly UV aging. For many systems, the ink
can be diluted to various levels to cope with various applications,
including the deposition methods. For instance, a high-viscosity
ink formulated for screen printing can be diluted for dispensing or
flexo applications.
filler
The electrically conductive filler commonly used by the industry is
shown in Table 2, with n-hexane included as a reference point.
Silver is the most popular choice, due to its high conductivity
and stability. It is also a very solderable material. Copper is also
high in conductivity, but it’s sensitive to oxidation. Therefore, it
is mainly used in polymeric conductive ink for EMI shielding
and static protection. However, copper can be used for conductor applications for thick film, although it has to be fired under
nitrogen at 600°C. For next-generation, high-power devices such
as power amplifiers, high melting temperature and high electrical
and thermal conductivity of silver or copper are desired as a semiconductor die-attach material. Since the die-attach temperature
has to be low, the silver or copper would have to be introduced as
nanoparticle ink, which promises a reduced sintering temperature.
Here, polymer binder would also have to be avoided to achieve
high conductivity. Figure 2 shows a sheared Ag joint sintered at
270°C without pressure with the use of such Ag ink. The die shear
strength is 40 MPa, well exceeds 6.1 MPa requirement of IEC
60749-19 specification.
Nickel was popular for EMI-shielding application 20 years
ago, but it has been mostly replaced by silver. Gold is a good and
very stable electrical conductor. It is used for hybrid-conductor
applications, inner- and top-layer photoimageable thick film. Gold
is desired when Al and Au wire bonding is needed. Gold is also
compatible with alumina, LTCC. Aluminum is used as a hybrid
conductor for c-Si applications, mainly for back-side contact. Iron
is not commonly used as an electrical conductor. However, iron
particles coated with gold film allows formation of Z-axis electrical conductivity in adhesives without any pressure applied—if the
magnetic field is present at curing state.
Platinum and palladium are expensive and not very good in
conductivity. They are mainly used in silver thick film for regulating solder leaching. For instance, Ag/Pt 100:1 is used for leaching resistance, capable of tolerating two dips in 62Sn/36Pb/2Ag
at 230°C, or two dips in 10Sn/88Pb/2Ag at 340°C. Ag/Pd
january/february 2013 | 27

Industrial & Specialty Printing - January/February 2013

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