ELECTRICALLY CONDUCTIVE
SILICONE ADHESIVE
Luis C. Montemayor
Dow Corning Corp.
Midland, Michigan
This paper
Was originally published in the proceedings of SMTA
International Conference, September 2002
ELECTRICALLY CONDUCTIVE SILICONE ADHESIVE
Luis C. Montemayor
Dow Corning Corp.
Midland, Michigan
ABSTRACT
As microelectronic assemblies get more complex there is an
increased need for flexibility of design. Traditional systems
are based upon attaching electronic components, e.g.
capacitors, resistors, etc., to complete circuits, primarily by
a solder based process (molten Sn/Pb alloy). Furthermore,
the application of solder has become increasingly difficult in
electronics due to the decreasing size and operational space
of circuit boards. Due to component heat sensitivity,
substrate flexibility, mismatched coefficients of thermal
expansion, proposed legislative ban on lead, processing
advantages and other design considerations, alternative
approaches to component attach are needed. One approach
that is gaining in importance is use of an electrically
conductive adhesive (ECA) to "stick" the components onto
a substrate, in effect, replacing the solder. Since the
applications are essentially replacing the conductivity and
strength of a metal with an adhesive, properties are expected
to remain relatively unchanged under environmental stress,
e.g. thermal cycling and exposure to heat and humidity. It
is here where silicones propose the best option. The
advantages of the electrically conductive silicone adhesives
include flexibility, low temperature processability, easier
assembly, and their being environmentally benign.
INTRODUCTION
Tin/Lead eutectic alloy is currently the dominant electrical
interconnect material used in the production of assemblies
for electronics industry.
However, as environmental
awareness increases, the toxicity of lead has become
increasingly important with impending bans on its use in
electronics proposed by several countries. Due to these
concerns, the electronics industry has been investigating
lead-free solder alternatives for over two decades. One
obvious approach was the utilization of lead-free low
melting point metals and alloys. However, most of these
alternatives such as mercury, cadmium, arsenic, and so
forth, are as toxic as lead, while others are too expensive or
have only limited availability. Furthermore, these lead-free
alternatives have many of the issues of Tin/Lead solder such
as cleaning, high temperatures for processing and have
unproven reliability. Another approach has been the use of
Electrically Conductive Adhesives (ECA). Typically the
ECA consists of conductive particulate randomly dispersed
in a polymeric adhesive matrix. Like solder, ECAs serve as
electrical and mechanical linkages for component
attachment. A representative assembly schematic is shown
in figure 1, where an electrically conductive adhesive
replaces Tin/Lead solder for surface mount assembly.
In addition to being an environmentally benign lead-free
alternative to solder, ECA’s also offer other advantages over
conventional solder. These include:
1. Low temperature processing. Unlike Tin/Lead solder,
which requires processing temperatures over 220 °C,
ECA’s can be cured at temperatures below 150 °C.
This is especially important for flexible circuit
substrates, such as in smart cards that cannot tolerate
high temperatures.
2. Fine pitch printing. The limit of pitch size for
conventional solder is about 15 to 20 mils due to solder
bridging issues. For a polymer based ECA it is
anticipated that stencil printing down to 1 mil
separations are achievable.
3. Elimination of flux. Conventional re-flow solders
employ organic flux that contains activators to remove
oxidation on the components and substrates. The flux
is normally removed with freon-based solvents after
solder re-flow. ECA’s did not require flux treatment
and subsequent cleaning procedures.
4. Flexibility of ECA. The greater flexibility of ECA’s
compared to solder can reduce coefficient of thermal
expansion (CTE) mismatch limitations, especially for
large surface mount components.
5. Reduction in processing steps.
Processing steps
involved in surface mount assembly could be
significantly reduced with ECA.
Component
Metalization
Conductive Trace
Electrically
Conductive
Adhesive
(ECA)
Component
Substrate
Figure 1: Schematic application of an
SILICONE MATRICES
Epoxy resins have been the most common polymeric
matrices used to formulate ECAs. However, epoxies have
shown some limitations for certain applications due their
water permeability and ionic impurities. Silicones, as a
counterpart, offer distinct advantages to ECA’s. These
include:
1. Thermal
stability.
Silicones
offer
consistent
performance over a wide temperature range. They
retain their properties from –40 to 200 °C. This is
important for devices that will be used in processes
with wide temperatures variations. Under-the-hood
applications in automotive electronics are a good
example of this tough environment, and this is an area
where silicones are often used.
2. Surface energy. Silicones have a very low surface
energy, which allows them to wet the surface extremely
well. Once cured, there are no gaps or pockets for
moisture to collect in. Thus, they maintain excellent
corrosion resistance.
3. Ionic purity. Silicones have inherent low ionic content
and very high purity. Shrinking device size and shorter
pathways between components amplify the potential
impact of ionic impurities, so this characteristic is
increasingly important.
4. Low temperature flexibility. Silicones have a low glass
transition temperature (Tg = - 120 °C). This allows
them to remain flexible, and retain a low modulus
throughout a wide temperature range. This property
minimizes the effects caused for the mismatch of the
coefficient of thermal expansion between the
component and board.
5. Water absorption. Silicones have an outstanding
moisture resistance. Moisture absorption is very low.
This quality, coupled with good adhesion, provides an
excellent corrosion resistance in harsh environments.
6. Process flexibility. Mechanisms to cure silicones are
versatile. The silicone adhesive that is discussed in this
paper is addition reaction cured, utilizing a platinum
catalyst. This cure system offers several advantages. It
creates no cure by-products that could cause corrosion
or voiding problems. It cures at the same rate
throughout the material because it does not require
exposure to air, which permits faster and more
complete cure.
ELECTRICAL CONDUCTIVITY AND FILLER
PARTICLES
The electrical resistivity of a material is determined by
measuring the difficulty of passing a current through a
sample of known dimensions. Placing a sample between
two electrodes, applying a voltage to the system and
measuring the current flowing through the sample
accomplish this. The electrical resistivity is usually reported
in units of ohm-cm.
The range of resistivity values
between good conductors and good insulators is
tremendous. A good conductor such as silver has a
resistivity of about 1.6 X 10-6 ohm-cm. In contrast,
polystyrene, a good insulator, has a resistivity of about
1X1016 ohm-cm. Table I presents the electrical resistivity
value for different materials.
Table 1. Electrical Resistivity
Material
ABS a
Polycarbonate a
Poydimethylsiloxane a
PVC b
Epoxies b
Silver c
Copper c
Aluminum c
Gold d
Steel c
Lead c
Silicon c
ρ (Resistivity) ohm-cm
1X1016
1X1016
1X1015
1X1013
1X1014
1.6X10-6
1.7X10-6
2.8X10-6
2.2X10-6
7.4X10-5
2.1X10-5
6.3X106
a)
“Materials Data Book for Engineers and Scientists”, E.R. Parker, McGraw-Hill, 1967
“The CRC Materials Science and Engineering Handbook”, J.F. Sackelford, CRC Press
Inc, 1967
c)
Physics for Students of Science and Engineering” D. Halladay and R. Resnick, John
Wiley and Sons, 1963
d)
J.C. Bogler ands.L. Morano, Adhesives Age, June, 1984, p. 17
b)
As seen in table 1, silicones (polydimethylsiloxanes) have a
very high resistivity; they are excellent insulators.
However, they can achieve an electrical conductivity by
incorporating a highly conductive filler into the polymer
matrix. A silicone ECA achieves a conductive state by
having a critical volume of this filler. This critical volume
is reached when sufficient particle-to-particle contacts have
been established, providing a conductive pathway through
the ECA matrix. The resistance to current flow through the
ECA silicone matrix is dependent upon the number and
quality of these particle-to-particle interactions. Partial
paths with dead ends do not contribute to the overall
conductivity and the resistance across each particle-to
particle contact accumulates. Volume resistivity or the
resistance to current flow for a defined volume of ECA,
expressed in units of ohm-cm, is used to compare the bulk
conductivity of ECA’s. A schematic path that contributes to
conduction (each particle in the path has contact with at
least two other particles) between component and substrate
metallization is shown in figure 2.
Due to the importance of particle-particle contact, anything
on the surface of these particles can impact conductivity.
Some potential surface contaminants besides residual
lubricant from the filler powder manufacturing process
include, metal oxide and by-product from powder
production. The choice of metal filler dictates whether
metal oxides are a concern or not. Use of gold or platinum
eliminates the problem of surface oxides but the cost would
be prohibitive for most applications. Easily oxidize metals,
such as copper and nickel have found limited use in
applications where high performance is required. For a
balance of cost, effectiveness and high performance, silver
is typically used. Advantages of silver include its highly
ductile nature, excellent conductivity and the relatively
conductive nature of its oxides.
Current Flow, I
Voltage Meassure, V
Component Metalization
Adhesive
Glass
Conductive
Particles
Matrix
Figure 3: Measurement of Volume resistivity
Conductive Trace
Figure 2: Schematic function of an ECA
In summary, the reason for the electrical conductivity in a
silicone adhesive is due to the percolation of electricity
between silver particles through an organic lubricant that
normally covers the silver surface. In other words, each
silver-particle must be in contact with at least two other
particles in a continuous network of metal-particles for
electricity to flow. Conductivity is a complex phenomenon
involving removal of the organic lubricant to facilitate
particle-to-particle contact, curing of silicone matrix to
maintain that contact, and adhesion to the substrate to
establish the conductive path between the conductive trace
and the component metallization.
MEASUREMENT OF ELECTRICAL PROPERTIES
The electrical properties of an electrically conductive
silicone adhesive, which concern most, are volume
resistivity and contact resistance. Volume resistivity, in
mohm-cm, of the adhesive is determined on a thin uniform
strip of adhesive cured on glass slide, as described in figure
3. A four-point probe is then used to pass current through
the outer probes while measuring the drop in voltage across
the inner probes. This measures the conductivity across the
volume of the adhesive.
Contact resistance measures the resistance of the electrically
conductive adhesive between two metal crossbars. The
contact resistance is a function of volume resistance and
interfacial resistance, and is strongly dependent on the
nature of the metal surface. The common metals utilized are
copper, gold and tin/lead alloy. Resistance is calculated
using Ohm’s law. (figure 4).
Accelerated ways of testing the electrical reliability of the
electrically conductive silicone adhesive is by exposing it to
temperature and environmental changes. The first is by
placing the samples in an oven (set at 150 °C) for isothermal
aging. The most severe test, especially for a silicone matrix,
and therefore one that is most used, is that of a temperature
cycling which occurs by exposing samples to a continuous
temperature cycle from –50 °C to 150 °C (this is an 80
minutes cycle that includes ten minute isothermal stages at
the end). The 85/85 test is one that exposes the samples to
85 °C and 85% humidity.
Our Silicone electrically conductive adhesives have been
evaluated under these conditions, presenting the results that
are shown on the following sections of this paper.
Voltage Measure, V
Adhesive
Current Flow, I
Ohm’s Law
V=IR
Figure 4: Measurement of contact resistance
160
0.1
0.08
140
130
Contact ResistanceTemperature(C)
0.06
VolumeResistivityTemperature(C)
120
Contact Resistance(ohm)
VolumeResistivity(ohmcm)
110
0.04
100
0.09
DSCHeat Flow
0.08
Contact Resistance(ohm)
G' (dyne/cm2)
2.0E+07
0.07
VolumeResistivity (ohmcm)
0.06
0.05
1.5E+07
0.04
1.0E+07
0.03
0.02
5.0E+06
0.01
0.0E+00
30
50
70
90
110
Temperature (C)
130
Contact Resistance and Volume Resistivity
90
G'(dyn/cm2)
2.5E+07
0.02
0.1
3.0E+07
0
150
Figure 5: A comparison of heat flow, modulus, volume resistivity and
contact resistance as a function of temperature
Curing is also a time dependent variable, it is important to
determine the optimum time to reach the best performance.
Figure 6 describes the development of electrical properties
as a function of time rather than temperature. It is important
to notice that once the temperature of 150 °C is reached the
majority of electrical properties for volume and contact
resistance have already developed. This occurred within
five minutes of curing. Curing can also take place at a
temperature of 130 °C, but the time to reach the optimum
properties will be a little longer. The typical time observed
to cure the electrically conductive silicone adhesive is 1
hour at a temperature of 150 °C. However, actual cure time
will vary, depending on the heat capacity of the total
structure and flux of the heat sources.
80
0
1
2
3
4
5
6
Time(min)
7
8
9
Contact Resistance and Volume Resistivity
150
Temperature (C)
CURE PROFILE
As mentioned before, the electrically conductive silicone
adhesives discussed in this paper are addition reaction cure
utilizing a platinum catalyst. This cure system, as properly
mentioned, offers several advantages: It creates no cure byproduct which could cause corrosion or voiding problems; it
cures at the same rate throughout the material because it
does not require exposure to air and it is reversion resistant.
It is known that time and temperature contributes to the
development of the electrical conductivity of the adhesive
while curing. Figure 5 describes the development of
properties for the conductive adhesive. All profiles were
obtained at a temperature ramp rate of 5°C/min.
0
10
Figure 6: A comparison of heat flow, volume resistivity and
contact resistance as a function of time.
VOLUME RESISTIVITY
Volume resistivity as a function of environmental aging was
recorded to provide an indication of the stability of the
electrically conductive adhesive. Environmental aging
conditions were room temperature, 150 °C isothermal, 150
to 150 °C continuous temperature cycling and 85 °C with 85
% relative humidity (85/85). The results are summarized in
table II.
Table II. Volume reistivity as a function of environmental
aging. Units for volume resistivity are in Ohm-cm
Silicone Adhesive
Environment
Initial
Aged
Room Temp.
1.2X10-3
1.1X10-3
150 °C Isothermal
1.2X10-3
1.5X10-3
85 °C / 85 % RH
1.2X10-3
4.2X10-4
-50 to 150 °C Cycle
9.9X10-4
1.0X10-3
Aging was for 1,000 hr
INTERFACIAL CONTACT RESISTANCE
The resistance to current flow through an electrically
conductive joint is a combination of the resistance to current
flow at the interface with substrate and component as well
as the resistance to current flow through the bulk of the
adhesive. A critical property for the silicone electrically
conductive adhesive is the interfacial contact resistance or
more simply contact or joint resistance. Joint resistance as a
function of environmental aging was recorded to provide an
indication of the stability of the electrically conductive
adhesive. The results are summarized in figure 7.
Contact Resistance as a Function of Environmental
Aging
% Change from initial
150
100
ADHESION STRENGTH
Since the silicone electrically conductive adhesive has to
provide both the electrical and mechanical connection
between a component and substrate, adhesion strength is
very important. Lap shear strength was used as a measure
of mechanical strength. Table III provides a summary of the
information obtained for different substrates.
Table III. Lap shear results on different metallizations
Substrate
Lap Shear (psi)
Aluminum (bare)
Aluminum (clad)
Copper
Niquel
Sn/Pb
Gold
178
385
117
124
79
252
50
0
0
10
20
30
40
50
-50
-100
Time (days)
Au (85/85)
Cu (85/85)
Sn/Pb (85/85)
Figure 7. Contact resistance as a function of aging
60
VISCOSITY
The rheological behavior of the electrically conductive
adhesive becomes important because of the desire to either
stencil or screen print. Stencil or screen-printing allows a
whole substrate to be prepared for component placement in
one pass and is the current method for laying down solder
paste. Basically it is desired a low viscosity during the
printing process (high shear rate) to facilitate filling the
voids in the stencil or screen, followed by non-slump (high
viscosity) behavior once the screen or stencil is removed
(low shear rate). Viscosity as a function of shear rate
provides an indicator of the thixotropic nature of the
adhesive. Typical values for the electrically conductive
silicone adhesive are shown in Table IV. It is important to
remark that the thixotropic ratio is reported in order to
address the shear thinning nature of the adhesive.
Table IV. Viscosity as a function of shear rate at 25 °C
Shear Rate (1/sec)
Viscosity (cp)
1
220,000
2
117,000
10
22,963
After high shear (cp)
Thixotropic Ratio (1/10)
112,000
4.88
CONCLUSION
The development of this family of products illustrates the
versatility of silicones. While they all offer the stress relief,
excellent adhesion, and temperature stability that make
silicones highly desirable in the electronics applications,
they can be formulated in many ways to meet specific
requirement. This is the case for the electrically conductive
silicone adhesives, where a silicone adhesive is filled with a
highly conductive filler to transform it from an insulator
material to an electrically conductive product.
These
adhesives can be used in a wide variety of applications
where electrical conductivity is needed. These adhesives
can provide a conductive path in applications where
traditional methods, such as solder, are not practical.
Electrically conductive adhesives are relatively new for the
electronics market and their use is not widely spread over
the industry. However, into the future, it is expected that
these adhesives will play an increasingly prominent role in
the designing and production of electronic assemblies due
the multiple advantages they can offer.